J Surfact Deterg
DOI 10.1007/s11743-017-2000-6
REVIEW ARTICLE
How to Attain Ultralow Interfacial Tension and Three-Phase
Behavior with Surfactant Formulation for Enhanced Oil
Recovery: A Review. Part 4: Robustness of the Optimum
Formulation Zone Through the Insensibility to Some Variables
and the Occurrence of Complex Artifacts
Jean-Louis Salager1 • Raquel E. Antón1 • Marı́a A. Arandia1 • Ana M. Forgiarini1
Received: 23 February 2017 / Accepted: 13 July 2017
Ó AOCS 2017
Abstract In enhanced oil recovery, not only the low-tension performance, but also the robustness at optimum formulation is an important issue. The fourth part of our
review series is dedicated to robustness, defined as the
width of the zone exhibiting three-phase behavior around
the optimum formulation, whatever the scanned variable. It
is first corroborated from a screening of the available data
in the literature that the tension minimum is inversely
proportional to the square of the three-phase range in the
HLD scale. However, since there is still an inaccuracy of
about a factor 10 in the tension minimum, some significant
improvement can be attained in some cases by increasing
the three-phase behavior width in two ways. The first
approach consists of finding systems that are insensitive to
some formulation variable such as temperature, surfactant
mixture composition or concentration, and water-to-oil
ratio. The second way is to produce an artifact through
which the optimum formulation is produced twice in a
scan. If the distance between the two events in the scan is
reduced down to be zero, their corresponding three-phase
behavior zones merge and result in a wider WIII region
with a low tension. Several cases of such events are
reported: alkaline scans, anionic-nonionic and anionic-cationic mixture changes, linear change in composition in
three-surfactant mixture, partial precipitation from a surfactant mixture in a salinity scan, and excessive partitioning of polyethoxylated nonionics. More complex
& Jean-Louis Salager
salager@ula.ve
& Ana M. Forgiarini
anafor@ula.ve
1
FIRP Laboratory, Universidad de Los Andes, Mérida,
Venezuela
transitions with three effects in a single scan or three
concomitantly scanned variables show even more possibilities in practice.
Keywords Ultralow interfacial tension Three-phase
behavior Enhanced oil recovery Optimum formulation
Introduction
In the 1970s original studies on enhanced oil recovery
(EOR) showed the optimum formulation in a variable scan
that takes place when a specific physicochemical situation
is attained. As summed up in the first part of this review
[1], this situation obeys a condition originally described as
a Winsor R ratio unit, i.e., exactly equal interaction of the
surfactant(s) adsorbed at the interface with the oil and
water phases, as explained in detail elsewhere [2]. It has
been shown that this physicochemical situation at the
interface may be numerically expressed as a correlation
[3–5] to attain a zero surfactant affinity difference (SAD)
[6, 7], or its dimensionless hydrophilic-lipophilic deviation
(HLD = SAD/RT = 0) [8], which is as follows in its
simplest form:
HLD ¼ f ðSÞ kA ACN þ Cp þ f ðAÞ þ kT ðTTref Þ ¼ 0
ð1Þ
In Eq. (1), the basic formulation variables are the
aqueous phase salinity effect f(S) (ln S for ionic surfactants
and kSS for nonionics, where kS is a small positive coefficient depending on the nature of the salt), ACN (alkane
carbon number), Cp (characteristic parameter of the surfactant, also called Nmin [6], EPACNUS = r/kA
[3, 4, 9, 10], b = a - EON [5], or Cc [11]), f(A), which is
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J Surfact Deterg
the alcohol co-surfactant effect that modifies the surfactant
amphiphilic contribution [4, 5], which for simplicity can be
incorporated into the Cp term, as will be supposed in the
following, and T the temperature, with some reference Tref,
usually ambient temperature. The coefficient kT is negative
for ionic surfactants (*-0.01) and positive for
polyethoxylated nonionics (from ?0.04 to ?0.08) [8].
More general information is available as reviewed elsewhere [1, 2, 10].
This expression represents a sum of free energy contributions [6], which may be conceptually written as linear
Eq. (2), as is logical for energy relationships.
HLD ¼ Rki Xi ¼ 0
ð2Þ
where the Xi are the basic formulation variables appearing
in Eq. (1) and ki the corresponding coefficients quantifying
the importance of each variable change on the formulation
effect.
Decades of studies have reported more detailed effects
and thus more precise variable contributions, including the
non-alkane oil equivalent ACN effect (so-called EACN)
[2, 12–23], and details on the alcohol co-surfactant effect
f(A) [3–5, 24, 25] or the pressure effect [26–30], and even
the influence of molecular variation in the surfactant
structure characteristics such as the n-alkyl tail length, tail
branching, ionic head group, polyethylene oxide or
polypropylene oxide length [10, 16, 31–36], or effect of a
surfactant mixture composition [37, 38]. There is no need
to deal with all these details for the subject of this article,
so only the variables indicated in Eq. (1) will be taken into
account as a possible source of formulation change.
In a variable scan, the optimum formulation corresponds
to a minimum of interfacial tension and a maximum of oil/
water solubilization, which are equivalent performance
criteria according to the Chun Huh relationship [39, 40],
generally corroborated in the past decades. The minimum
tension (cmin or c*) is the basic parameter in EOR since it is
directly related to the capillary number and the actual oil
recovery [41]. This criterion, calculated as -log(c*) or any
equivalent experimental values from the best solubilization
or the minimum surfactant concentration to attain a single
phase system, has been called the performance index
(PERFIND) in part 2 of this review series [42]. It is
probably the most important indication of the system
‘‘quality’’ occurring at the optimum formulation for EOR
and other applications.
In EOR and practical applications such as emulsion
breaking and others [43], the optimum formulation of a
unidimensional scan takes place when the interactions of
the surfactant with oil and water phases are exactly equal,
i.e., at Winsor R = 1 or numerically HLD = 0 [1, 2]. The
performance index fades quickly away from the optimum
in the scan, when there is a slight discrepancy from the
123
optimum formulation, i.e., when HLD becomes slightly
different from zero. A tenth of the HLD unit is generally a
sufficient formulation deviation from optimum to result in
a significant increase from the minimum tension, even if
the system is still in the three-phase zone. It means that the
robustness of an optimum formulation is generally poor,
i.e., that the formulation range with a low tension is quite
narrow.
This review series [42, 44] shows that the performance
value at optimum and its range depend on the reservoir
characteristics, i.e., the aqueous phase salinity, oil EACN,
and temperature. In practice, it considerably depends on the
surfactant mixture included in the injected fluid, which has
to be selected for the process to attain an optimum formulation with a better performance index.
There are some apparent trends to doing it, as indicated
in our previous review [43], but their validity and generality are so far not yet obvious, although they may be
approximately guessed in some cases. Because the HLD
expression is based on the chemical potential [1], the
contributions of some of the formulation variables, which
are linear in the correlation Eq. (1) as a function of the Cp
contribution, like the surfactant head and tail length, seem
to have a perfectly linear effect on the performance in
simple cases [31].
Such linear variation versus the surfactant characteristic
parameter Cp is no longer the case when the surfactant–
oil–water system is complex with the possible occurrence
of synergetic phenomena, as often happens with surfactant
mixtures, leading to a deviation to linearity of CpMIX discussed previously in this review series [44]. Although the
non-linearity may be sometimes guessed as reported elsewhere [42], the exact effects are still to be verified by
experimental studies, particularly when strong surfactant
mixture interactions of different types take place at the
interface as seen in Figs. 16 and 17 in this review [42].
Moreover, the optimum attainable performance, e.g., the
ultralow minimum tension, is not the only important criterion in practice for EOR and other applications, because
in most cases one (or more) of the formulation variables
indicated in Eq. (1) is likely to change in some uncontrollable way during the process.
In EOR, the injected fluid is likely to be mixed with
connate water with a different salinity, thus producing a
change in S. In some cases, a variable salinity can occur
from place to place in the reservoir, particularly if the
previous history involved the use of different water
resources. Such an S variation is also the case when a
preflush or a salinity gradient is applied. Consequently, it
may be said that the water salinity might change in some
uncontrollable way during the EOR process.
The injected surfactant slug necessarily contains a
mixture of products because pure products are too
J Surfact Deterg
expensive or because a proper mixture with an adequate Cp
value is necessary to attain an ultralow tension as seen
previously [42, 44]. Additionally, the use of mixtures can
minimize or eliminate worrying problems, such as the
precipitation or the adsorption of some surfactants [45].
However, the use of surfactant mixtures produces an
unavoidable inconvenience by changing the formulation at
the interface. This occurs because of several phenomena
taking place as the injected fluid progresses through the
reservoir. One of them is the preferential partitioning
[46–51], and thus a different fractionation of the various
species between the oil and water phases and their interface, which are changed when both the surfactant total
concentration [3, 6] and the water-to-oil ratio changes
[3, 50, 52], as occurs in practice during the process.
Another interfacial formulation change can be produced
by the preferential adsorption of some species on the rock
surface, similar to the separation process taking place in a
liquid chromatography column [53, 54]. Local changes in
the water salinity or rock nature, as well as temperature,
can result in desorption of polyvalent cations resulting in
the precipitation or increased adsorption of some surfactant
species.
All these effects would produce changes in the composition of the surfactant mixture at the interface, thus
resulting in a change in its characteristic parameter Cp at
the interface.
The oil nature and thus its EACN characteristics can
also change from place to place, in particular if the dissolved gas content varies, because of a change in temperature and pressure during the process. It is also the case of a
temperature change in case of some stimulation or when
the fluid injection temperature is different from that of the
reservoir.
Consequently, it may be said that essentially all formulation variables indicated as basic in Eq. (1) are likely to
change in some uncontrollable way when the injected
surfactant slug moves through the reservoir. Even if some
changes could be approximately predicted and thus compensated by properly adjusting the injected fluid, it is still
very likely that the actual interfacial formulation will be
somehow altered during the process. As a consequence, the
HLD will depart from zero, thus resulting in an increase of
the interfacial tension from its scan optimum value cmin (or
c*), which would penalize the recovery efficiency.
represented in Fig. 1 with the abscissa indicating the actual
formulation scan variables, i.e., in this case the ethoxylation (a) and the temperature (b), indicated as well as the
corresponding HLD value. The HLD scale indicates the
same formulation deviation from the scan optimum,
whatever the formulation variable, and thus allows a more
accurate comparison of the effect of the variation, as seen
elsewhere with all usual scans [55]. Figure 1 indicates two
numerical characteristics. The first one is the minimum
value cmin of the tension in the scan at HLD = 0, and the
second one is the three-phase behavior zone indicated as
3/ around HLD = 0.
The original data are processed through several steps as
indicated in Fig. 2 where plot (a) indicates that the shape of
the two tension curves depends on the abscissa scale, i.e.,
on the formulation variable type. A better comparison is
available in Fig. 2b where the abscissa has the same scale,
i.e., HLD, obviously showing that system A exhibits a
higher minimum tension and a wider range of low tension
than system B. This tendency is very clear in a bidimensional scan of the lnS-ACN type reported in Fig. 7 of this
review part 2 [42], which came from known data [2, 3].
This trend appears here in Figs. 1 and 2c by indicating the
three-phase behavior range as DHLD3/. The two extremes
of this zone, often called XU and XL (upper and lower limits
in the X variable scan), correspond to the transition
between the o/w and w/o microemulsions into a bicontinuous microemulsion and a second excess phase, resulting
in a three-phase system.
These boundaries are also called emulsification failure
limits [56] taking place when more oil or water is added to
a Winsor III ternary diagram. The literature has presented
the corresponding phenomenology in the surfactant–oil–
water phase behavior over the past 30 years with outstanding pioneering articles [57–63] presenting the main
ideas as well as basic experimental evidence. They showed
that the interfacial tension between the microemulsion and
an excess phase is related to some characteristic length n
which could be a domain size (n0) of a lamellar structure
with fluctuations or a persistence length (nj) over which the
surfactant layer remains flat. Using several theories incorporating the free energy, the dispersion entropy, the interfacial energy, and the bending energy and elasticity with
thermal fluctuations, and as discussed in [42, 44], the following simple relationship was found to be general, in
particular at the optimum formulation where the tension is
minimum and the characteristic length is maximum:
Relation Between the Interfacial Tension
Minimum and the Three-Phase Behavior Range
c n2 kT
The consequence of a formulation change on the interfacial
tension value actually depends on two aspects, which have
to do with the shape of the tension-formulation data curve
ð3Þ
Argumentation based on curvature issues indicated that
the three-phase zone is limited by the two points in the scan
at which one of the principal curvatures C1 and C2 is zero
[64].
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J Surfact Deterg
Fig. 1 Variations of interfacial
tension of SOW systems along
two unidimensional formulation
scans [ethylene oxide number
(EON) and temperature T], with
the indication of the generalized
HLD variable according to
Eq. (1) in which Cp = a EON according to the
correlation for polyethoxylated
nonionics [5]
Fig. 2 Different
representations of the variation
of the oil/water interfacial
tension in a unidimensional
formulation scan. a Abscissa
numerical values XA and XB
represent different scan
variables, e.g., salinity, ACN,
Cp, or T. The asterisk
superscript indicates the
optimum tension and optimum
variable value in any abscissa
and ordinate scale. b Abscissa
values are in the same
generalized formulation HLD
according to Eq. (1), and cmin
indicates the tension minimum
value in the same ordinate log
scale. c Same scales as in b with
the arrows indicating the threephases zone range DHLD3/
around the optimum HLD = 0.
d General scaled correlation cSC
vs. HLDSC for all systems
Elaborated theoretical studies mainly verified with pure
alcohol ethoxylate surfactants and temperature scans
[65–68] have attained a fair understanding of this concept.
123
The use of Helfrich’s pioneering work on the elastic
properties of a film in terms of curvature [69] has
allowed Strey’s group [32, 67] to scale the variation of
J Surfact Deterg
tension vs. temperature with two parameters. The tension
was divided by its minimum cmin at the optimum of the
scan, and the temperature was centered at optimum
formulation and divided by a parameter proportional to
the three-phase zone extension DT3/ = TU - TL in the
scan, where the subscripts U and L refer to the upper and
lower temperature limits for the occurrence of a bicontinuous microemulsion at equilibrium with excess oil
and excess water.
The introduction of scaling terms in the tension and
temperature allowed researchers from Strey’s group to
write the cSC vs. sSC expression as [32, 67, 70]:
cSC ¼ c=cmin ¼ 1 þ s2SC
ð4Þ
where sSC ¼ K1 T Topt =DT3/
and
2
cmin ¼ K2 =n2max ¼ K3 DT3/
ð5Þ
ð6Þ
Topt is at the center of the (TU - TL) three-phase range, and
the Ks are coefficients depending on the bending rigidity
and saddle deformation rigidity of the surfactant layer at
the interface. Equation (4) was found to perfectly match
the data from 20 systems containing pure alcohol ethoxylates in a temperature scan [32].
It is important to check whether this kind of relation
applies to the general case with any surfactant and any
formulation scan, on a similar scaling. To do that, the
correlation should be written similarly with the HLD
generalized formulation and the DHLD3/ three-phase
extension. The corresponding scaling would be:
HLDSC ¼ K4 HLD/DHLD3/ and cSC ¼ c=cmin
¼ 1 þ HLD2SC
ð7Þ
with cmin ¼ K5 DHLD23/ or log cmin
¼ log K5 þ 2 log DHLD3/
ð8Þ
Figure 3 contains the data retrieved from the literature
[6, 71–81] in which the performance index (PERFIND) is
calculated from the minimum tension (as -log cmin), or the
equivalent maximum solubilization or concentration
required to attain a bicontinuous microemulsion, according
to the relations previously proposed in this series [42]. The
corresponding three-phase behavior range DHLD is
reported in HLD dimensionless units according to Eq. (1)
for various kinds of unidimensional scan (aqueous salinity,
ACN or EACN, surfactant type such as pure or commercial
sulfates, sulfonates, carboxylates, cationics, surfactant
mixtures, polyethoxylated nonionics, as well as alcohol cosurfactant concentration or temperature).
Figure 3 clearly shows that Eq. (8) is fairly satisfied for
many systems, in particular for the two examples from
Fig. 1, indicated as square dots. The average matching
straight line fits a slope of 2 very well. Some discrepancies
around the line are found in systems containing alcohol, in
which the measurement of solubilization is somehow
inaccurate because the actual volume of the co-surfactant
in the microemulsion middle phase is often disregarded.
Other variations around the average, with a range of about
one log unit, i.e., up to a factor 10 as far as the minimum
tension is concerned, might be due to an actual performance change because of synergy or detrimental effects.
In what follows the robustness will be indicated as the
DHLD3/ range, leaving the exact cmin value as an extra
criterion of performance, which could be finely tuned by
selecting a complex surfactant mixture as discussed in part
3 [44] of this series.
The numerical matching of Eq. (8) with Fig. 3 data is as
follows:
PERFIND ¼ log cmin ¼ 1:6 2 log DHLD3/
Fig. 3 Correlation between the interfacial tension minimum cmin (in
mN/m) as equivalent PERFIND (-log cmin) and the range of threephase behavior DHLD3/ [in HLD units according to Eq. (1)]
ð9Þ
thus resulting in Eqs. (7–8) with K5 = 0.03 mN/m (±0.01)
and in K4 * 2.8, if additionally the tendency c/
cmin * 3 ± 1 when HLD = ± DHLD3//2 is assumed to
be general as proposed elsewhere [42]. The inaccuracy of
the coefficient K5 may be due to the presence of some
alcohol in the microemulsion middle phase, in particular in
anionic surfactants systems, because the co-surfactant
location is not always taken into account to calculate the
solubilization and the corresponding equivalent PERFIND.
This is why it is recommended to measure the tension
rather than the solubilization, it is done in (too) many
reports on EOR.
The generalized relation between the scaled tension and
the scaled HLD, as indicated in Eq. (10), seems to be fairly
valid with all scanned variables. It is represented in the
Fig. 2d curve with an ordinate log scale, which is usually
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J Surfact Deterg
more appropriate than the alternative parabolic curve (not
shown).
cSC 1 ¼ c=cmin 1 ¼ HLD2SC
with cmin ¼ 0:03
DHLD23/
and HLDSC 2:8 HLD =DHLD3/
ð10Þ
ð11Þ
ð12Þ
In the region around the optimum formulation where
-1 \ HLDSC \ 1 the variation of log c/cmin in Fig. 2d
may be roughly approximated by a straight line with unit
slope, i.e.,
log cSC ¼ HLDSC
ð13Þ
The data shown in Fig. 3 and in Eq. (11), as well as
impressive theoretical considerations verified on pure
nonionic surfactant systems [32, 67, 68, 70], corroborate
that the width of the three-phase region is inverse to the
attained minimum tension. This means that it is not possible to produce both an ultralow tension and its occurrence
over a wide HLD formulation range, as would be desirable
to protect a surfactant EOR process against an uncontrolled
formulation change.
However, there are actually two clever ways to go in
practice around this impossibility, and it is the purpose of
this article to review what can be done. In what follows the
main criterion will be to find the circumstances in which
the three-phase behavior region size DHLD3/ increases,
remembering that this range more or less corresponds to
the zone in which the tension is up to 3 (±1) times cmin.
The first way is to use appropriate conditions, in particular with surfactant mixtures, in which spontaneously
occurring formulation variation has no significant effect in
changing HLD. To produce such insensitivity, in Eq. (2) it
is necessary to considerably reduce (or to make null), the
k coefficient corresponding to the variable likely to spontaneously vary in an uncontrolled way.
The second tactic is to find an artifact, i.e., a physicochemical trick, in which the formulation change is able to
produce a succession of two opposite optimum transitions
one after the other. When the two transition ranges get
close together and eventually merge, they result in a double, and thus wider, DHLD3/ zone with low tension.
Insensitivity to a Formulation Variable Change
Reduced Sensitivity to Salinity
The principal effect of salinity has to do with the degree of
ionization of the surfactant head groups due to interactions
with the salt ions solubilized in the aqueous phase. The
effect is more important for ionic surfactants, where it
appears as ln S in HLD correlation (1). On the other hand,
123
since salinity is much less significant with nonionic surfactants, it is generally described as kS S, with a small
coefficient kS depending on the salt, e.g., 0.13, 0.10, and
0.09, respectively, for NaCl, CaCl2, and KCl wt% concentration as salinity [5]. Consequently, the general rule to
reduce the effect of salinity is to use nonionic surfactants of
the polyethoxylated type or other, or at least an external or
internal mixture containing some nonionic surfactant contribution with the usual sulfonate or sulfate anionics, as
discussed in part 3 [44] of this series. This is particularly
necessary in the presence of polyvalent cations such as
Ca?? and Mg??, which could result not only in much
higher equivalent salinity than Na?, but also in precipitation problems. By the way, mixing anionics with
polyethyleneoxide nonionics or with extended surfactants
containing a polypropyleneoxide intermediate tends to
significantly reduce the precipitation zone [82]. Not only
cations are important. The salt anions also alter the salinity
effect, tending to reduce it when the valence increases, in
disagreement with the ionic strength concept, as shown a
long time ago comparing the effective salinity of various
sodium salts [83].
This issue, particularly high-salinity problems, is not
discussed here since it has been extensively reported in the
literature, with many examples, although without quantitative rules [82, 84–95].
As is often done, in what follows, a log scale will be
used for the salinity effect in HLD Eq. (1), especially for
extra- and intra-molecular ionic/nonionic mixtures [37].
This will avoid a false aspect of the wide DHLD3/ zone,
especially at high salinity, when an arithmetic scale is used
for salinity.
Insensitivity to Temperature
The kT coefficient value in the HLD Eq. (1) is known to be
negative and small for ionic surfactants (*-0.01 for
anionics and *-0.02 for cationics), which become more
hydrophilic as temperature increases [10]. On the contrary,
kT is positive and with a much larger absolute value for
polyethoxylated nonionics (from 0.04 to 0.08), depending
on the temperature and the degree of ethoxylation in
opposite ways [8]. The polyethoxylated chain dehydrates
as temperature increases, to turn less hydrophilic, and
finally becomes insoluble in water at the cloud point [96].
In the presence of oil, the optimum formulation concept
was first determined a long time ago as the phase inversion
temperature (PIT) [97–101]. The PIT is the temperature at
which the surfactant transfers from W to O, depending on
the oil EACN, and aqueous phase salinity as indicated in
the HLD equation. In extended surfactants, the 3–4 first
propylene oxide units close to the head group are hydrated
and thus may be dehydrated when the temperature is
J Surfact Deterg
increased [102]. This effect on the propylene oxide nonionic part is enough to overcompensate the opposite effect
of the temperature on the ionic head, and consequently
extended surfactants present a positive kT coefficient,
which is however lower than the one found for
polyethoxylated nonionics [5]. Different nonionics such as
sucrose esters and other sugar derivatives are almost
insensitive to temperature [103–106].
In a system containing both anionic (AI) and nonionic
(NI) species at fixed oil EACN and brine salinity, the HLD
resulting from the mixture calculated as a linear mixing
rule [37] would be expressed by a characteristic parameter
as follows, where x indicates the fraction of the species.
CpMIX ¼ xAI CpAI þ xNI CpNI ¼ xAI CpAI þ ð1 xAI ÞCpNI
ð14Þ
by taking the derivative with respect to temperature T, this
equation becomes
oCpMIX
oCpAI
oCpNI
¼ xAI
þ ð1 xAI Þ
oT
oT
oT
ð15Þ
In Eq. (1), the kT coefficient for the anionic surfactant is
negative and will be called as -kTAI, whereas for the
nonionic it is positive and will be written as ?kTNI. The
variation of CpMIX with increasing temperature would be
positive (respectively negative), i.e., the optimum surfactant would become more lipophilic (respectively more
hydrophilic) if the anionic proportion (xAI) is large (respectively small).
The composition of the AI/NI mixture at which the
CpMIX does not vary with temperature is attained by setting
the derivative to zero, as indicated in Eq. (16):
oCpMIX
¼ xAI kTAI ð1 xAI ÞkTNI ¼ 0
oT
ð16Þ
The insensitivity to temperature is thus attained for the
following composition of the mixture
xNI ¼ kTAI =ðkTAI þ kTNI Þ
Fig. 4 Variation of AI/NI surfactant mixture parameter (CpMIX
indicated as lnS*) versus T (°C) for mixtures of dodecyl benzene
sulfonate and ethoxylated nonylphenols (NPEON) in a system
containing 0.5 wt% total surfactant, 3 vol% sec-butanol, and nheptane at WOR = 1 [115]
ð17Þ
These equations for the attainment of such AI/NI mixture insensitive to temperature were reported to be accurate
a long time ago [37, 107] and corroborated for many systems [103–105, 107–113]. If Eq. (14) might not be linear in
some cases because of a strong interaction between the
surfactants [44], the kT coefficients are quite constant for
each surfactant [37] and thus the principle of Eq. (16) is
correct. However, the actual mixture composition for
insensitivity to temperature calculated from Eq. (17)
strongly depends on the surfactant species.
This is seen in Fig. 4, where the variation of the optimum formulation of an AI/NI mixture is detected from the
change in optimum salinity vs. temperature, essentially
similar to the Cp derivative in Eq. (16).
Figure 4 shows that the more hydrophilic the selected
nonionic is (the higher its EON), the stronger its contribution, i.e., the higher its kTNI [8] and the lower the xNI
fraction required in the mixture to attain insensitivity to
temperature [114]. An empirical inverse relationship such
as in Eq. (18) was found between the ethylene oxide
number EON and the inverse of its required fraction to
attain insensibility with dodecyl benzene sulfonate sodium
salt and was justified by a linear AI/NI mixing rule [109]:
EON 3:7 ¼ 1=xNI
ð18Þ
An extensive study on the temperature effect on the
phase behavior of an AI/NI surfactant mixture [109, 116]
indicated that the exact phenomenology varies with the two
selected surfactants, in particular their characteristic relative parameter values. Remember that because of the kT
coefficient sign in Eq. (1), when the temperature increases
the characteristic parameter Cp tends to decrease for an
ionic surfactant and to increase for a polyethoxylated
nonionic. The different cases depend on the temperature at
which the WI–WIII–WII phase behavior transition takes
place, the so-called T*NI in an increasing temperature scan
for a nonionic and T*AI in an opposite scan for an anionic.
As seen in Fig. 5 the three-phase behavior zone indicated
as WIII (with the minimum tension at its center) exhibits a
curious shape depending on whether the optimum transition temperature for one of the surfactant is higher than,
equal to, or lower than this temperature for the other [114].
Figure 5a shows that if T*AI \ T*NI, there is a central
zone in between these temperatures in which both AI and
NI surfactants have a hydrophilic characteristic parameter,
and, unless there is a very strong interaction (not the case
here because of a high enough ethoxylation), some of their
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J Surfact Deterg
zones [116], but the principle shown in Fig. 5 still works,
and a remarkable situation can arise when case (b) is
reached by the proper choice of surfactants. This means
that the selection of the surfactants in a mixture for EOR,
which was dealt with in the previous review [44] to attain
an ultralow tension, is also important to improve the
robustness if the temperature is likely to change as seen
here and in other situations to be treated next. The cross or
hyperbolic shape of the optimum formulation line
(HLD = 0) indicated in Fig. 5 was recently verified to take
place for a value of HLD slightly away from the optimum,
with some explanations based on the deviation found in the
free energy mixture [117].
Insensitivity to the Surfactant Mixture Composition
Fig. 5 Scheme of phase behavior versus temperature and composition of an anionic-nonionic mixture at constant salinity and EACN,
depending on the order in which the WIII transition temperatures T*NI
and T*AI of the two surfactants are selected. General scheme adapted
from data [114]
mixtures are also hydrophilic, and a WI phase behavior is
observed.
If TAI* [ T*NI in the mid temperature range as in case
(c), both surfactants are lipophilic, as are some of their
mixtures, with a WII phase behavior, maybe with an extra
hydrophobicity because of the head interactions. Figure 5
shows that in two cases the WIII zone is more or less
horizontal at a temperature much lower or much higher
than the central zone.
When the two transition temperatures are equal, as in
Fig. 5b, the three-phase WIII zone has an amazing cross
shape, whose exact vertical symmetry aspect depends on
the composition of the insensitive mixture according to the
Fig. 4 results. The elongated horizontal branch WIII zone
corresponds to insensitivity to temperature along an
extremely wide range. The vertical WIII branch from top to
bottom shows an extended insensitivity to the whole AI/NI
mixture composition. Consequently, the center of a cross
exhibits a double insensitivity to temperature and to mixture composition, which could be interesting in practice for
EOR in very cold climates as well as in other applications.
The eventual interactions between the AI and NI surfactants might produce some distortion in the shape of the
123
In a surfactant mixture one of the components can separate
or be delayed with respect to the other along the process by
a preferential phenomenon, such as precipitation, transfer
to the oil phase, or adsorption/desorption on the rock surface. This is particularly the case if the surfactants have
very different molecular structures with and without an
electrical charge, a difference in tail length and branching,
a very different Cp, and strong sensitivity to polyvalent
ions or exhibit precipitation at high salinity. Even if these
effects are exactly the same for all components of the
mixture, a reduction in the total concentration or a change
in the water-to-oil ratio (WOR) will alter the partitioning
into the phases and at the interface, thus altering the mixture composition. Consequently, it is important to find a
surfactant mixture whose optimum formulation is as
insensitive to its content as possible.
As already shown [44] in the case of an AI/NI mixture, a
zone exists in which an interaction between the head
groups tends to reduce the overall hydrophilicity, i.e., the
optimum salinity will decrease. However, this insensitivity
is very dependent on the choice of surfactants as seen in
Fig. 6, where the optimum salinity is shown for a mixture
between an alkyl benzene sulfonate and nonylphenols with
different ethoxylation degrees. It is seen that with a relatively high ethoxylation (7.5 EO), the mixing rule is not
very far from linear, probably because the long length of
the polyethoxylated chain forces it to go into the water to
be hydrated and thus reduces its interaction with the ionic
head by moving it further away from interface. With the
shorter chain (as 5 EO), the nonionic is not hydrophilic
enough, and its head group tends to stay close to the
interface and to completely wrap up around the sulfonate,
thus displacing the water and resulting in a zone with a
much lower optimum salinity, i.e., a less hydrophilic surfactant mixture.
The interesting point here is that in this case (5.3 EO) a
central zone has a constant optimum salinity over a wide
J Surfact Deterg
Fig. 6 Three cases of variation in optimum formulation versus the
AI/NI surfactant mixture composition in which two cases exhibit a
zone of insensitivity to the mixture composition
range, thus resulting in insensitivity to the mixture composition. If the nonionic is less hydrophilic, e.g., EON
*4.5, just at the limit of water solubility, the effect is
stronger with an even lower optimum salinity.
In the intermediate case of an NI surfactant (6.0 EO) that
is slightly more hydrophilic than the AI alone, the optimum
salinity formulation is seen to stay at the value corresponding to the AI surfactant over the left half of the plot,
i.e., when there is less than 50% of NI, hence with a pretty
good insensitivity range. This is particularly interesting in
practice because the addition of some NI, even with a lot of
inaccuracy in the composition, would not change the
optimum formulation, even if it helps avoid precipitation
because of a high salinity. The simple explanation for this
fine-tuned case is that when more NI is added, the interaction with the AI produces an increase of the hydrophobicity of the mixture that exactly compensates the extra
hydrophilicity brought by a higher proportion of the more
hydrophilic NI component.
Effect on Formulation of the Partitioning
of the Different Surfactant Species Contained
in a Commercial Mixture
Before considering other cases of formulation insensitivity,
it should be remembered that when a surfactant is present
in a mixture, the different components are likely to be
distributed in different ways in the phases and at the
interface, as has been discussed extensively in the literature, particularly concerning what happens for each surfactant at optimum formulation [3, 6, 117–120].
In general, the most hydrophilic components in mixtures
tend to preferentially go to the water and the most
hydrophobic ones to oil, with the remaining species
adsorbed at the interface (or partitioning in the
microemulsion middle phase at optimum formulation),
where they generally result in a variation in formulation
HLD [38, 48, 49, 51, 80, 121].
The partitioning coefficient of the surfactant species
between oil and water is the critical information to explain
what happens with mixtures [122–126]. This segregation of
the species tends to turn ionic (respectively nonionic)
surfactant mixtures remaining at the interface more lipophilic (more hydrophilic) [38], i.e., the Cp tends to increase
(respectively decrease) with the respective HLD formulation change. This effect is particularly important if the
mixture contains very different species as far as their
hydrophilicity/lipophilicity balance or the characteristic
parameter is concerned. This is the case for commercial
petroleum sulfonates [6] and polyethoxylated nonionics
[48, 50], which are extensively used in EOR.
In all cases, the reduction of the concentration of the
surfactant mixture tends to increase the preferential segregation, thus increasing the magnitude of the partitioning
effect. The formulation shift due to a decrease in concentration could be considerable at the limit of microemulsion
occurrence, i.e., close to the so-called critical microemulsion concentration (cl) [127, 128], a quite low concentration, typically ten times the critical micelle concentration
(cmc).
This change in formulation with surfactant concentration
means that a surfactant concentration scan can produce an
optimum concentration, at which the interfacial formulation
corresponds to HLD = 0 [79, 95, 118, 119, 127, 129, 130].
A variation of the water-to-oil ratio (WOR) is also likely
to change fractioning and thus to alter the HLD of the
remaining species at the interface. An increase in WOR
would tend to increase the partitioning of hydrophilic
species to water and thus make the interfacial surfactant
mixture more hydrophobic than that originally introduced
in the system [3, 38, 50]. Therefore, the plots showing the
optimum formulation (any HLD variable, in particular T or
EON for NI) versus surfactant concentration (so-called
gamma or fish map) or versus the water/oil composition
(so-called X map) exhibit a tilted three-phase behavior fish
zone with a slope that could be noteworthy, as discussed
later.
Insensitivity to Surfactant Concentration
Since the surfactant concentration will diminish as the
injected slug progresses through the petroleum reservoir, it
is very important to use a formulation insensitive to such a
change. Fortunately, as seen before for the temperature, the
concentration effect is opposite for ionic and nonionic
commercial surfactants whose species are fractioning
between the phases and the interface.
123
J Surfact Deterg
As the concentration decreases, the nonionic (anionic)
species going to interface tends to be more hydrophilic
(lipophilic) [6, 48]. Consequently, a proper mixture of both
surfactants should be able to produce insensitivity to the
change in total concentration CT [48, 50].
By differentiation of Eq. (14) with respect to the total
surfactant concentration CT, an equation similar to (15) is
obtained.
oCpMIX
oCpAI
oCpNI
¼ xAI
þ ð1 xAI Þ
oCT
oCT
oCT
ð19Þ
The condition to attain insensitivity to the total surfactant concentration would be similar to Eq. (16) for temperature. In Eq. (19), the derivative for the anionic
surfactant is positive and will be expressed as kCAI,
whereas the derivative for the nonionic one will be negative and written as -kCNI.
The variation of CpMIX with decreasing concentration
would be positive (respectively negative), i.e., the optimum
surfactant will be more lipophilic (respectively more
hydrophilic) if the anionic proportion (xAI) is large (respectively small). An insensitivity to the total surfactant
mixture concentration will be obtained if the derivate of the
mixture is zero, i.e., when:
xAI kCAI ð1 xAI ÞkCNI ¼ 0
ð20Þ
However, there are different problems to solve before
going ahead along a similarity with the insensitivity to
temperature case. The first one is that the values of the kC
coefficients are quite dependent on the surfactant mixture
case, because for both kinds of surfactants, the fractionation depends on the variety of the different species. Generally, there is a more significant fractionation and a thus a
larger shift versus concentration when the distribution of
the species is wider. For instance, it was shown for a
commercial nonylphenol with an average of six ethylene
oxide groups (NPEO6) that when the total surfactant concentration was reduced ten times, the increase in surfactant
hydrophilicity at the interface was equivalent to about 0.5
additional EO group in the head. When the averaged
EON = 6 was attained by mixing commercial products
with an average EON = 2.5 and 10, the variation was
twice as much [131].
Similar trends are found in the fish diagram, this time
in the slope of optimum formulation variation, i.e., the
center line of the WIII zone, which indicates the kC
value [132–134]. It should be noted that the fractionation
of the species tends to increase as the total concentration
decreases. Consequently, for a given commercial surfactant, whether it is AI or NI, the absolute value of the
kCAI or kCNI slope generally increases with the a
decrease in total concentration, sometimes considerably
[3, 5, 48, 135–137].
123
On the other hand, since pure products do not exhibit
this formulation shift, then their corresponding kC is
essentially zero for both kinds of surfactant, a fact that
has no interest in EOR practice for cost reasons
[6, 100, 138–141].
These phenomena mean that the formulator can actually change the value of the kC coefficients by changing
the distribution of species in both AI and NI surfactant
types. This is helpful, because a similar effect may be
attained with wider or narrower species distribution in
each of the two types of surfactants. However, there is a
limit to the range width, which is that a too hydrophilic
surfactant will go only to water and a too lipophilic one
only to oil. To avoid too much surfactant loss at the
interface, it is often necessary to eliminate the extreme
species in a commercial mixture distribution, e.g., the
very low ethoxylation nonionics and double head anionics
like disulfonates.
These effects mean that the principle of insensitivity to
the total AI/NI mixture concentration is valid and that it
can be used in practice [142]. Nevertheless, no accurate
prediction can be proposed because the kC values are not
always known, and the insensitivity to the concentration
data should be found through experimental trials.
Figure 7 gives an example of such a trial and error
experimentation, which shows the variation of the position
of the interfacial tension minimum point for two different
total surfactant concentrations for various AI/NI intermolecular mixtures [131].
It is seen that when the total surfactant concentration
decreases (from 0.05 to 0.005 wt%), the optimum salinity
of the 100% AI case (respectively 100% NI case) decreases
(respectively increases), i.e., the interfacial AI surfactant
becomes more lipophilic (respectively the interfacial NI
surfactant becomes more hydrophilic).
Fig. 7 Optimum formulation points indicating the minimum interfacial tension in the salinity scan for an NI (ethoxylated nonylphenol
with an average of 6 EO) and an AI (PHL petroleum sulfonate MW
450) and their mixtures from xAI = 0.2–0.8 at two total surfactant
concentrations: 0.05 and 0.005 wt%
J Surfact Deterg
For the AI/NI mixtures (from 20 to 80% AI), it is seen
that the NI effect dominates up to about 70% of the AI
content. This corresponds to the fact that the shift due to
the NI component is about 3–4 times larger than the one
due to the AI, as seen in the 100% data in Fig. 7, e.g.,
jkCNI j 3:5 jkCAI j.
This difference in effect is likely to be related to a wider
distribution of the NI species and thus a more important
fractioning.
It is thus in good accordance with Eq. (20) that a higher
proportion of AI (*75–80%) is required to attain an exact
compensation of the AI and NI opposite shifts and thus an
insensitivity to the surfactant total concentration. It is
worth noting that this insensitive mixture is also the one
with the lowest optimum salinity, i.e., the one with the
stronger AI/NI interaction and thus the less hydrophilic
mixture, as already seen previously in part 3 of this review.
It is not known whether this is a coincidence or a general
trend.
Another way to produce an AI/NI mixture with compensating opposite effects is to use an intramolecular combination of characteristics in an extended surfactant structure,
where an alkoxylated intermediate chain is placed between
the hydrocarbon tail and ionic head. Different petroleum
companies, essentially without published studies, proposed
these surfactant types in patents in the very first years of EOR
research and development [83, 143–147].
Then, they were essentially forgotten for 40 years,
before being proposed very recently as one of the performant components in complex surfactant mixtures
[35, 44, 45, 89, 148].
These surfactants have been reinvented and studied for
other applications such as the ethoxylated sulfonates [149]
to eliminate the co-surfactant requirement for petroleum
sulfonates in microemulsions or to improve lignosulfonate
tensioactivity and salt tolerance [150]. Highly branched
Guerbert type propoxylated structures [151], as well as
surfactants for systems containing chlorinated oils [152],
were also proposed.
The most significant line of research was started in the
1990s by designing a single molecule as an intramolecular
AI/NI mixture of a surfactant with a lipophilic linker
[153–155], i.e., a lipophilic long n-alcohol type co-surfactant to extend the interaction with the oil phase and
improve the surface activity and microemulsion solubilization with polar oils, in particular triglycerides, which is
very poor with conventional surfactants [33, 34, 156].
The alkyl polypropyleneoxide sulfates and similar threeblocks amphiphiles, so-called extended surfactants in 1995,
have been extensively studied by several research groups in
the past 20 years for various different applications
[95, 102, 157–179], among them, some related to the recent
EOR ASP formulations [82, 180–184].
A recent study [131] on this kind of surfactant showed
that they are essentially insensitive to change in concentration, i.e., only an extremely small variation in optimum
formulation is found as the concentration decreases. It may
be said that this kind of AI/NI intramolecular surfactant
mixture, which probably contains a wide distribution of
species, basically exhibits an opposite fractioning effect
from its two parts, even with different characteristic
parameter values. For the three extended carboxylate surfactants (EXC) reported in Fig. 8, it is seen that a reduction
in concentration from 0.05 to 0.005% produces an
insignificant shift of optimum salinity, much less than the
shift exhibited by the kinds of NI and AI common surfactants reported in Fig. 7 to contribute to the
extramolecular mixture with similar characteristic
parameters.
The change exhibited from 0.05 to 0.005% concentration in Fig. 8 is from S* = 3.5 to 3.4% NaCl for EXC1
(C18PO14EO2COONa), from S* = 8.0 to 7.5% for EXC2
(C12PO14EO2COONa) and from 9.0 to 8.7% for EXC3
(C12PO7EO7COONa).
It is worth noting that these extended carboxylate surfactants have a quite different characteristic parameter Cp,
but its variation with the structure indicates a different
reasoning than for an external AI/NI mixture. In effect, it is
seen that the optimum salinity increases as expected when
the C18 n-alkyl (EXC1) tail is reduced to C12 (EXC2).
Now, in a change from EXC2 to EXC3, the lipophilic
polypropylene oxide becomes shorter, and the hydrophilic
polyethylene oxide becomes longer. Consequently, EXC3
is expected to be more hydrophilic, i.e., with a higher
optimum salinity. Actually, Fig. 8 shows a lower optimum
salinity. A simple explanation for this apparent contradiction is that the AI/NI interaction, by wrapping of the
polyethylene oxide around the ionic head, as seen in part 3
Fig. 8 Interfacial tension vs. concentration for three extended
carboxylate surfactants (EXC), as well as for ordinary commercial
NI and AI, i.e., a commercial hexaethoxylated nonylphenol (NPEO6)
and a petroleum sulfonate (PSHL)
123
J Surfact Deterg
and inserted in the present Fig. 6, does not apply in the
same way.
The 7-EO chain is long enough to go around the carboxylate head in the water phase, but the carboxylate head
is on average relatively far away from the interface, and
thus the nonionic chain is capable of reaching and interacting only if it is particularly longer than the average, i.e.,
only by a part of it. Therefore, some hydrophobic AI/NI
interaction takes place, but it is smaller than in an
extramolecular mixture, where the ionic and nonionic
groups are both located in water close to interface.
Besides, this surfactant with two intermediate zones in
the right order produces a lower minimum tension, i.e., a
better performance at the interface. This is certainly related
to a more continuous variation of lipophilicity to
hydrophilicity, i.e., some gradation in the surfactant, as
discussed elsewhere [42, 173, 174, 177].
How to Control the Sensitivity to WOR
When the aqueous injected fluid in EOR contacts the
reservoir connate water, both the surfactant concentration
and the WOR are likely to change. Both effects produce a
modification of the interfacial formulation because of a
change in partitioning, but the WOR variation has some
different specificities from the surfactant concentration
effect as will be discussed here.
When the generalized formulation was numerically
correlated with variables as in Eq. (1), the effect of the
WOR on the formulation was also tested and found to
produce a slight change [3]. For anionic surfactants, an
increase in WOR produced a decrease in the optimum
salinity, which could be approximated by the following
relationship in the 0.2 \ WOR \ 5 range.
lnS 0:05=WOR ¼ constant
ð21Þ
This means that when the WOR increases, the surfactant remaining at the interface becomes more lipophilic, i.e., its optimum formulation Cp increases.
According to the partitioning model for surfactant mixtures [48], this tendency is due to more hydrophilic
species going to the higher volume of the water phase
and thus a decrease in the concentration in the oil phase
and at the interface.
The first time a relationship was reported between the
formulation and WOR was in the polyethoxylated nonionic
phase behavior studies by Shinoda’s group on the phase
inversion temperature, PIT, which was the equivalent of
the optimum temperature in Eq. (1). In a temperatureWOR 2D plot, the optimum temperature was experimentally reported to decrease as the water content increased or
the oil content decreased, but with no partitioning explanation [99, 100, 185, 186]. In some cases the WOR effect
123
was almost null for an AI surfactant [100] or very small for
AI/NI mixture [187].
Other studies carried out after the optimum correlations
were available for all kinds of surfactants and mixtures,
reporting the WOR effect with other formulation variables
appearing in HLD Eq. (1). It was found that when WOR
increases, for polyethoxylated NI surfactant T* decreases,
sometimes considerably, and for ionics T* increases, in
general only slightly. For a polyethoxylated mixture, the
required EON* has to increase to maintain the interfacial
EON*int so that HLD = 0. For any kind of surfactant S* or
lnS* decreases when WOR increases as reported in Eq. (2)
[38, 50, 101, 109, 132, 188–196].
All these effects mean that when the water content
increases (from left to right in Fig. 9), the surfactant mixture at the interface changes because of more partitioning
of the hydrophilic surfactant to the water phase. Consequently, the surfactant mixture HLD at the interface
becomes higher (more lipophilic) and thus a change in the
system formulation HLD (indicated in the Fig. 9 ordinate)
has to diminish to compensate. The apparent phase
behavior change in the HLD-WOR system indicates the
kind of variable change to maintain the optimum formulation 3/ behavior (along the dashed line). The slope of the
dashed line depends on the system. It is generally larger for
polyethoxylated nonionic surfactants than for ionics and
larger for surfactant mixtures with a wide distribution, thus
with high partitioning of the species.
Very close to the extreme 100% water and 100% oil, the
optimum formulation system exhibits a single-phase system triangular zone (1/) solubilizing, respectively, in
aqueous micelles or oil micelles. As the surfactant concentration increases, these 1/ zones extend to the center,
whereas the central 3/ zone is shrunk. At a high
Fig. 9 Variation of the phase behavior with WOR in systems
containing surfactant mixtures and thus partitioning of the different
species among oil, water and the interface. The HLD variation along
the dashed line corresponds to Eq. (1) with the variables describing
the ingredients contained in the system
J Surfact Deterg
concentration (equivalent to the tail of the fish diagram,
e.g., at least 10–20% surfactant), the three-phase zone
disappears and the single-phase zone replaces it from left to
right.
It is worth remarking that at the interface, the HLD is
always zero at optimum formulation and that the indicated
HLD in the ordinate in Fig. 9 corresponds to the formulation variables of the system for use so that HLD is zero at
the interface.
If all the variable-WOR plots are graphed with the same
HLD scale in the ordinate as in Fig. 9, and the oil–water
volume fraction in the abscissa, the slope of the WIII strip
(and dashed line) in the middle of the plot indicates the
relative importance of the WOR effect. This could be
expressed as the extrapolated variation DHLDOW from 0 to
100% water, using the average slope at 50% water. The
average slope has been found to vary from 0 to 10, with a
value around 1 for commercial ionic surfactants and often
4–5 for commercial nonionics. It might be close to zero for
the appropriate complex mixtures as seen next.
The fact that the temperature coefficient kT in Eq. (1) has a
different sign means that some confusion can take place in an
AI/NI mixture with the temperature change. The problem is
that the temperature is a formulation variable in the ordinate
that varies in opposite ways, i.e., upward or downward
depending on the surfactant type [109].
Figure 10 indicates the T-WOR bidimensional basic
schemes of the phase behavior for AI and NI surfactant
systems and for some of their mixtures, particularly the one
that is insensitive to temperature, typically 70% AI, as seen
in the literature [109]. All the SOW systems contain the
same oil, the same salinity and the same alcohol content to
eventually avoid precipitation at the same total surfactant
concentration. The only difference is the surfactant AI/NI
mixture composition.
For a 100% AI system increasing the temperature produces a WII [ WIII [ WI transition, with a relatively high
DHLDOW variation, whereas for the 100% NI system, the
increase in temperature results in the WI [ WIII [ WII
transition with a low DHLDOW variation. For the AI/NI
70/30 mixture, which was found to be insensitive to temperature, a change in temperature does not produce any
change in phase behavior, the DHLDOW variation is zero,
and the WIII zone is vertical. In other words, the phase
behavior exclusively depends on WOR and is WI (respectively, WII) at WOR\1 (respectively, WOR[1), with
a WIII vertical strip close to WOR * 1.
However, the phase behavior is essentially independent
of the WOR on each side. It thus may be said that the
selection of the AI/NI mixture independent of the temperature enables having a vast WOR insensitivity.
It is worth remarking that in EOR practice the WOR is
not likely to change very much during an injection, maybe
Fig. 10 Scheme of the basic phase behavior of SOW systems with
four cases of AI/NI surfactant mixtures [114]. c Corresponds to the
AI/NI mixture producing insensitivity to temperature, according to
Eq. (16)
by a factor 2, i.e., not to change the formulation much
during the process. The WOR effect is thus not very significant during the process. However, it should be
remembered that when the interfacial tension between an
oil and water phase at equilibrium is measured using a
spinning drop tensiometer, two techniques are used. The
usual quick technique is to introduce a very small droplet
of oil in the tube filled with the aqueous phase and to spin it
for at least 2 h (often more) to reach equilibration, which is
supposed to be reasonable when there is no more size
change. For this technique, the WOR of the system can be
very high (1000 or so) and different for each measurement.
It is thus not necessary for the attained equilibrium to be
the same, in particular the one assumed to occur at the
reservoir WOR condition. Incorrect data might be obtained
with such a technique.
This means that the correct method is to first equilibrate
the SOW system in a test tube at the appropriate temperature, WOR and surfactant concentration conditions that
would occur in the reservoir. Then the equilibrated phases,
i.e., the aqueous phase and an oil micro-drop, will be
123
J Surfact Deterg
extracted from the equilibrated system and placed in the
spinning drop tensiometer capillary tube, with no possible
change in partitioning and formulation.
Surfactant Concentration and WOR Effects
Together for Very Pure Nonionic Surfactants
These two previously discussed effects are both coming
from the partitioning of the different surfactant species in a
mixture and are somehow related. It might be assumed that
if the surfactant concentration or WOR is changed in a
system with an extremely pure surfactant, no such partitioning effect takes place, and thus a ‘‘good’’ system
without these effects might be available, although it would
not be interesting in EOR practice because of the pure
surfactant cost [138].
The general trend is that there is a low formulation
effect produced by the surfactant concentration and WOR
for relatively pure ionic surfactants, which have an almost
unity partition coefficient between water and oil at the
optimum formulation [140, 141, 197, 198].
It was thought that this effect would completely
disappear in the case of a single very pure surfactant.
But since extremely pure anionic products are very
difficult to produce, high accuracy tests were carried out
with extremely pure nonionic oligomers from the
ethoxylated alcohol type. Both effects (surfactant concentration and WOR) were studied at the same time in
the fish diagram, i.e., in the temperature-surfactant
concentration plots at variable WORs, for extremely
pure nonionic species.
The expected absence of such effects for super-pure
surfactants was not corroborated for a small amphiphile
ethylene glycol monobutyl ether (C4E1) [199], which is
practically an alcohol, or for a real surfactant tetraethylene glycol monodecyl ether (C10E4) [200]. In the
reported fish diagrams for both species, the optimum
formulation clearly varies with the surfactant concentration and WOR.
The very accurate studies exhibit interesting specificities
in the phase behavior, which are worth analyzing in detail.
These seem to be due to a particularity of polyethoxylated
nonionic surfactants of this type, which is that they are
quite oil soluble with a very high partition coefficient
between oil and water, e.g., about 100 instead of the unit
value presenting for ionic species [51, 121, 201, 202].
This anomaly provides the explanation for the formulation shift, considering that a large part of the surfactant
goes to the oil phase to participate as a polar oil segregated
close to the interface [203], thus resulting in a variation of
the oil EACN producing changes in Eq. (1) and a deviation
from HLD = 0 optimum formulation.
123
Artifacts Producing a Succession of Two Opposite
Transitions Through Optimum Formulation
The second way to improve robustness is an artifact consisting of a sequence of two three-phase zones when
changing a variable along a scan. After passing over the
first optimum formulation through a (forward) transition,
for instance WII [ WIII [ WI in Fig. 11a, an opposite
effect dominates and produces a second optimum formulation through a so-called retrograde transition, in this case
WI [ WIII [ WII in Fig. 11b. The double transition is
thus WII [ WIII1 [ WI [ WIII2 [ WII in Fig. 11c,
where the 1 and 2 subscripts indicate the two three-phase
behavior and low interfacial tension zones [204, 205].
When the system is selected so that the intermediate
zone, in this case WI, is reduced and then eliminated, the
double transition becomes WII [ WIII1 ? WIII2 [ WII,
with an extended three-phase region with low tension
called WIII1?2 in Fig. 11d. This merging results in
improved robustness as far as the three-phase behavior
range width is concerned.
If the double transition is in the other direction, it would
be the elimination of the intermediate WII zone that would
produce the wider three-phase region in the dual transition
WI [ WIII1?2 [ WI.
This artifact can happen if a variable monotonous
change is able to produce two opposite transitions and if
something can be done in the system selection and
appropriate adjustments to have the opposite transition
zones moving and coming together.
A sequence of two WIII zones has been found to happen
for three different formulation scans: a pH change, a surfactant mixture change because of molecular interactions,
and a salinity change producing a precipitation of one of
the surfactant species.
Double Optimum Formulation Occurrence
Produced by an Increase in Alkaline Concentration
with a System Containing Carboxylic Acids
The following case is typical of a system with crude oils
naturally containing carboxylic acids, particularly asphaltenic heavy oils, for which the so-called alkaline-surfactant-polymer (ASP) EOR technique has been proposed as a
low-cost alternative [206–213].
Available reviews provide extensive literature references [214, 215].
The physico-chemical principle of the process [216] is
the formation of a mixture of two surfactants, i.e., the
lipophilic natural acid and its hydrophilic salt resulting
from the in situ neutralization reaction with the injected
alkaline solution. At some pHs the Cp value of the acid-salt
J Surfact Deterg
Fig. 11 Principle of the sequence of two opposite transitions through WIII phase behavior along a single formulation scan. lo, lw, and lm
represent oil based, water-based and middle-phase microemulsions, which are the shaded phases in the test tubes
mixture will match the HLD = 0 condition to attain a low
interfacial tension, i.e., an optimum formulation, with low
tension and eventually three-phase behavior. In practice, an
aqueous solution containing NaOH, Na2CO3, or another
alkaline product [217–220] is injected so that the pH is
increased along a scan, and at some point the pH corresponds to an optimum formulation. In practice, this effect
decreases the CpMIX value, and a WII [ WIII [ WI transition takes place. However, at an alkaline concentration
ten times higher than the value at optimum, essentially all
the acid has been converted to salt, and adding more
alkaline has little effect on the pH and thus the CpMIX.
However, alkaline solutions contain electrolytes, e.g.,
Na? or another cation, and adding more alkalinity also
results in a salinity increase [221], which tends to produce
a retrograde transition WI [ WIII [ WII. This double
transition was noted 30 years ago [222] as a phenomenon
that was extending the WIII zone. It was fully explained
later [114, 196, 223]. This is probably the reason why some
extra robustness has been noted for this kind of system
[82].
Figure 12 shows the experimental result of a NaOH
concentration scan with a certain concentration of pure
myristic acid in heptane, starting with an original aqueous
solution with some alcohol co-surfactants and only 4 wt%
NaCl. Then, NaOH is increasingly added to the brine to
make the next systems, thus increasing their pH and
salinity. As seen previously in Fig. 11a, the first WIII zone
is reached in the pH 8–9 zone in Fig. 12, and then the
hydrophilic acid salt dominates and the WI phase behavior
is attained at pH 9. When more sodium hydroxyde is
added, the pH is high enough for essentially all the acid to
be in the form of a hydrophilic salt resulting in a WI phase
behavior. However, adding more sodium hydroxide also
changes the salinity, because of the increase in Na? concentration coming from the added NaOH. This increase in
salinity produces the typical WI [ WIII [ WII transition
at some point, as shown in Fig. 11b. The addition of more
NaOH in Fig. 12 system is stopped at a point where the pH
is 13.7, with a corresponding Na? concentration equivalent
to a salinity of 7 wt% NaCl.
The resulting double transition is of the type previously
indicated in Fig. 11c.
The distance between the two transitions and their
aspects may be altered in different ways, as shown in
Fig. 13 for the double transition due to the alkaline concentration increase with an SOW system containing long
chain carboxylic acid/soap surfactant mixture. Some trials
carried out with the original system scan shown in Fig. 12,
indicate the following trends in Fig. 13.
1.
2.
3.
Fig. 12 Phase behavior transitions produced by an NaOH concentration scan, i.e., a concomitant pH-salinity change
If the acid/soap surfactant tail length is increased, the
first optimum pH increases and the intermediate WI
zone decreases (Fig. 13a).
If the original NaCl salinity increases first, the
optimum pH slightly decreases and the intermediate
WI zone decreases, resulting in a lower second
optimum pH (Fig. 13b).
If a surfactant that is less hydrophilic than the soap
(less sensitive to pH than the acid/soap, but sensitive to
the salinity introduced by the alkali and requiring less
salinity because it is less hydrophilic) is added, the first
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Fig. 13 The two three-phase
zones in the scan producing a
forward and then a retrograde
WIII transition may be
displaced, approached and
eventually merged by changing
other conditions
4.
5.
transition pH increases very slightly, and the second
transition pH decreases (Fig. 13 c).
If an extended surfactant (less sensitive to salinity) less
hydrophilic than the soap, and essentially insensitive to
pH, is added, the first transition pH slightly increases,
the intermediate WI range decreases, and the second
transition pH decreases, often with an extended WIII
range (Fig. 13d).
If all previous tricks are properly considered, the two
WIII zones might become wider and merge together,
and the robustness will be improved. The best case is
thus the one indicated in Fig. 13e––an extra wide lowtension zone when the maximum tension found in
between the two minima stays low enough.
Mastering these effects might be particularly critical
when using the ASP technique in which the pH is likely to
start varying in the slug as an increasing pH/salinity scan.
Some recent comments about a better range with the ASP
process might be because of a good match with the previously discussed phenomena [82].
Case of Two Surfactants with Strong Molecular
Interaction (Two Narrow Optimum Zones
Approaching and Merging into a Wide One)
A strong interaction between two (or more) surfactants
could result in an essentially new substance with different
123
properties, in particular with a possible change in HLD
formulation. In some cases the association would drive the
system through an optimum transition twice, as discussed
previously. Additionally, the newly generated species
sometimes exhibits a synergy that could be of interest as far
as the performance is concerned [44]. Three such cases will
be discussed next.
Double Optimum in the AI/NI Mixture Composition Scan
The discussion of the lower plot in Fig. 6 shows that when
two surfactants, one anionic and the other polyethoxylated
nonionic, have about the same optimum formulation for an
oil–water system, then their associated mixtures become
more lipophilic because of the wrapping of the
polyethoxylated nonionic chain around the ionic head
group. As mentioned in the Fig. 3 discussion of our previous review part 3 [44], the non-linearity variation of the
characteristic parameter CpMIX of an AI/NI mixture versus
its composition might produce a double solution for the
HLD = 0 optimum formulation equation. In such a case,
when the interfacial tension is measured along a mixture
composition scan, at some fixed salinity, an interfacial
tension double minimum is found as shown in Fig. 14a.
At this Fig. 14 system salinity, the two tension minima
are far away in the left plot (a); thus, there is a wide
problematic high-tension zone between the two optima. A
J Surfact Deterg
Fig. 14 a Occurrence of two interfacial tension minima in an AI/NI mixture composition scan at a very low concentration (0.01 wt% total
surfactant). b Evolution of not only these two minima seen in (a), but also of a third one as the total concentration increases
simple way to reduce this detrimental intermediate zone is
to lower the system salinity so that the distance between the
two solutions is reduced, and the intermediate high-tension
region gets narrow and finally disappears. If the system
salinity cannot be changed, then the surfactant(s) have to
be adjusted to slightly more hydrophilic species, which
would result in a higher S* curve as shown in the inserted
plot in Fig. 14a, and thus closer minimum tension compositions. This can be easily done using surfactants with
shorter tails, more hydrophilic heads or lower interaction
between AI/NI head groups. When the intersections of the
horizontal 1 wt% salinity line with the curve get together,
i.e., when the optimum formulation curve is almost tangent
to it, the two tension minima will almost merge, resulting
in a wider low-tension zone, i.e., a larger robustness.
Figure 14b shows that a change in the total surfactant
concentration is another way to displace these minima.
Figure 14a shows that aside from the two very visible
minima at about 10 and 93% of nonionic, a very small
minimum seems to occur at 70% NI when the total surfactant concentration is 0.01 wt%. The question is whether
it is an infinitesimal minimum or an experimental discrepancy. The answer is easy, since Fig. 14b shows that
when the total surfactant concentration is increased to 0.05
wt%, this protuberance becomes a very significant third
minimum, slightly displaced to 65% NI surfactant. It is
worth remarking that the tension maximum between the
second and this new third minima around 80% NI is not
very high.
Grabbing our attention even more for the purpose of this
article is the outstanding fact that at a higher total concentration (0.1 wt%) this third minimum appears to have
merged with the extreme right one, resulting in a wide lowtension zone from 75 to 90% NI.
In addition to this significant robustness improvement, it
should be noted that the first minimum located at low NI
composition moves to the right as the total concentration
increases, thus approaching the others.
These complex events deserve more analysis. According
to what has been discussed previously about the opposite
effects of the surfactant concentration on the two kinds of
amphiphiles (AI and NI), it should be remembered that
when the total concentration increases, the interfacial NI
becomes more lipophilic and the AI more hydrophilic. On
the right side of the inserted plot in Fig. 14a where the NI
proportion is high, the interfacial NI surfactant has a lower
average EON and consequently the hydrophobic mixing
effect due to the wrapping of the polyethoxylated chain
around the ionic head decreases. If the AI/NI surfactant
mixture is less hydrophobic, its optimum salinity increases
and the S* curve moves upwards mainly at the center and
the right side of the inserted plot in Fig. 14a.
Additionally, when the interfacial AI is becoming more
hydrophilic, its effect on the S* curve is also to move it
upwards, chiefly in the middle and on the left side where
there is a higher AI proportion.
Consequently, when the total surfactant concentration
increases, the S* curve tends to move up, probably higher
at the center where both effects are accumulated, thus
producing some kind of maximum bump (as seen in the
plotted S* curve). As this S* curve moves upward, it would
make the two extreme minima closer, indicated as black
circles in Fig. 14a. When the S* curve central maximum
bump attains the 1 wt% salinity horizontal line, a third
tension minimum appear at about 65% NI in Fig. 14b. At
some higher total concentration, this third minimum would
probably split into two separated minima. At 0.1 wt%
surfactant the left minimum of the split does not clearly
appear as a minimum close to 65% NI, but only as some
perturbation. Notwithstanding, the right minimum of the
split is found to have merged with the right extreme minimum, where the S* curve essentially coincides with the
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constant salinity line at 1 wt% NaCl over the relatively
large zone of mixture composition, i.e., from 75 to 90% NI.
This analysis indicates how important the knowledge of
the shape of the S* optimum formulation curve is vs. the
AI/NI mixture composition and its possible evolution as
the total surfactant concentration changes.
It is evident that with such amazing possibilities, a good
understanding of the phenomena, together with a strong
ingenuity, and maybe some luck, should allow the expert
formulator to find many practical solutions.
Double Optimum in an Anionic/Cationic (AI/CI) Mixture
Composition Scan
In the case of a mixture containing anionic and a cationic
surfactants, the molecular interactions are extremely strong
because of the electrostatic heads with opposite signs,
which results in the spontaneous formation of a bimolecule,
the so-called catanionic (CAI) surfactant [224]. This
catanionic species has a low charge because the two original ones are partially or completely canceled out, producing an almost nonionic double head [225].
Additionally, this catanionic new substance has a double
tail. There are thus two reasons for the catanionic surfactant to be much less hydrophilic than its original components, in many cases being non-water soluble, and
precipitating when their mixture composition is close to
50–50% as discussed in pioneering articles on these mixtures [226–229] and reviewed elsewhere [230].
The optimum formulation mixing rule is far from linear,
and it exhibits a ln S* vs. composition plot of the type
indicated in Fig. 15 originally published 20 years ago
[114, 231], which is conceptually similar to the present
Fig. 6 lower curve case, with a very simple difference due
to an extremely strong one-to-one intermolecular
association.
The left and right sides of the Fig. 15 plot demonstrate
the mixing rule shown as ln S* vs. mixture composition
exhibits a linear variation, as was discovered a long time
ago in anionic surfactant mixtures [37]. However, the linear mixing rule is not the dashed line between the AI and
the CI representative points in Fig. 15, but between the
catanionic representative point CAI and the point representing the surfactant in excess in the mixture, i.e., the AI
on the left or the CI on the right. In other words, there are
two linear segments in the mixing rule.
As seen in Fig. 15, the catanionic surfactant exactly
corresponds to the middle of the composition, i.e., to an
equimolecular 1:1 compound, whose ln S* is very low,
meaning that it is quite lipophilic, and whose Cp increase
can be estimated from the ln S* value and the use of
Eq. (1), as equivalent to about ten additional carbon atoms
in the tail of an ionic surfactant [231]. Since the surfactants
123
Fig. 15 Optimum formulation mixing rule of anionic (sodium
dodecyl sulfate) and cationic (dodecyl trimethyl ammonium chloride)
surfactants. Adapted from previous data [114]
in the Fig. 15 mixture have an average of 13 carbons in the
tails, this means that there is still a small residual charge in
the CAI association.
At a given salinity (indicated as selected S), a horizontal
line indicating a constant ACN-T-S-f(A) formulation in
Fig. 15 intersects the two mixing rule branches and thus
produce two optimum formulations in the mixture scan,
with minimum tension (as seen in the inserted plot) and
three-phase behavior [232–234]. The lower the selected
salinity line is, the closer the minimum tension zones, but
also the more likely the precipitation.
As seen in Fig. 15, the central zone of the mixture,
typically when there is more than 30–40% of one of the
components, contains a catanionic precipitate. This means
that it is impossible to approach and merge the two optimum formulation occurrences to obtain a wider threephase zone and an extra robustness. However, this inconvenience can be reduced or eliminated by having a more
hydrophilic catanionic surfactant, e.g., a higher optimum
salinity for each of them in the Fig. 15 case. This may be
attained by reducing the tail length of the AI and CI
components [231] or by increasing their branching and
avoiding a close contact between the head groups [233]
with different additional tricks such as hiding the charge
inside a structure, changing the pH [228], adding cosurfactants such as alcohols or a third surfactant of the nonionic type whose polyethylene oxide or sugar head
introduces disorder [229, 235].
In recent work the use of some nonionic internal components, either separated or in an ethoxycarboxylate
J Surfact Deterg
Fig. 16 Interfacial tension
variations along linear scans in a
three-surfactant mixture
anionic, has allowed the using a water-soluble catanionic
for EOR purpose. In such cases, two close minimum tensions and a relatively wide three-phase zone take place
close to the AI/CI 1:1 mixture condition [236–239].
A Possible Wide Optimum Zone in a Linear Scan Change
Inside a Three-Surfactant Ternary Mixture Diagram
A three-surfactant mixture adjusted to optimize the formulation with three different species, each having an
advantage, could improve the situation from having a
double optimum to produce a widely stretched optimum
zone with low tension and three-phase behavior.
The selected system for this example is shown in Fig. 9
of part 3 of this review [44], with the following surfactants:
S1 = C12 ethoxylate, S2 = linear C16 alkyl benzene sulfonate (with only 20% of 2/C16S oligomer), and
S3 = Guerbet C12 extended sulfate.
In this case, there is an optimum AB almost linear curve
from the optima attained with the proper S1–S2 and S1–S3
binary mixtures. These binary optima are found on the
sides in the triangular diagram in Fig. 16a, in which S2 and
S3 have similar characteristic parameter values on the
lipophilic side of the optimum, and S1 is a hydrophilic
species to compensate for the two others in order to attain
the optimum.
If the formulation scan occurs almost perpendicularly to
the AB line, as indicated by the dashed arrows starting on
the S2–S3 side and pointing toward S1, there is, as shown
in Fig. 16b, a very deep and very narrow low interfacial
tension zone with no strong interest because of the extreme
accuracy requirement.
On the contrary, if the formulation scan path takes place
close to the AB line, as for instance by changing the S3
Guerbet extended surfactant proportion at a constant S1/S2
ratio, an eventually excellent robustness can occur with
respect to the S3 proportion.
If for instance the proportion of the S3 is increased, with
the scan starting from an S1/S2 original mixture close to
point B, and going toward the vertex S3, which is close to
A in this diagram, three tension variation cases could occur
as indicated in Fig. 16c.
In the scan starting in B1, there is a narrow minimum
tension where the AB line is crossed. In the other scans
starting in B2 and B3, a wider range of low tension is
attained because the scan path is close to the AB curve
over some range. In the B3–S3 path, the minimum tension zone is quite wide with respect to the B1–S3 case.
However, the best case is found in the B2–S3 path
because it features two wide minimum zones on both
sides of a quite wide intermediate region with a low
tension. In practice, this situation results in an extremely
stretched optimum region.
Consequently, in practice it would be extremely clever
to analyze the three-surfactant diagrams to discover what
the paths are through which a wide optimum zone can be
found, which is, according to the previous discussion,
related to the AB bi-optimum line in the surfactant ternary
diagram.
Sometimes an almost miraculous path may be found. In
part 3 of this review [44], Fig. 15 showed a ternary diagram of the same surfactants, with tension values close to
the optimum at different salinities to cover the case of a
salinity variation during the process.
In such ternary diagrams, it was seen that if the nonionic
surfactant proportion is concomitantly changed to exactly
compensate for the HLD variation produced by the salinity
alteration, an amazing coincidence occurs. The interfacial
tension remains very low along a straight path corresponding to an S2/S3 ratio of 4 in the ternary mixture.
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The interfacial tension along this path actually remains
lower than 0.001 mN/m from 10% nonionic at 0.7% TDS
salinity to more than 80% noionic at 10% TDS, i.e., over an
extremely wide range in practice. Additionally, the data
also show some fair robustness perpendicularly to the path,
i.e., concerning the accuracy of the S2/S3 ratio to keep a
tension lower than 0.001 mN/m.
Of course, there is no guarantee of attaining such
amazing control of the salinity change all the time, but this
exceptional formulation adjustment in a very complex
system indicates the great value of a proper understanding
of all phenomena likely to occur.
Case of Retrograde Transition Occurring Because
of the Precipitation of Some of the Mixture
Components as Salinity Increases
Anionic surfactant precipitation generally occurs when the
salinity increases as the solubilization limit in brine is
attained. When the surfactant is a mixture of species with
different solubility levels, a partial precipitation can start
with the less hydrophilic species. This is the case with nalkyl benzene sulfonates with the sulfonated benzene ring
at different positions along the linear tail.
The surfactant characteristic parameter and water solubility of such species depend on two structure specificities.
The first is the branching of the molecule, i.e., the
position on the sulfonated benzene ring, which varies
because of the alkylation mechanism with long alkenes
from the second carbon to the one at the center of the tail,
which for instance are noted as 2/C12S and 6/C12S for a
dodecyl tail.
The effect of the benzene position was studied a long
time ago [16]. It was shown that the more symmetrical the
two parts of the tail are, the more lipophilic the oligomer
and the more soluble it is in brine. On the other hand,
species with unequal tail segments, e.g., 4/C12S, are often
Fig. 17 Interfacial tension
versus salinity for commercial
linear alkyl benzene sulfonate
sodium salts, with a large
proportion of two phenyl
isomer. Crude characteristics
are as follows: C1 from Canada
(14°API, EACN 5.3), C2 from
Iraq (33°API, EACN 6.5), and
C3 from Norway (34°API,
EACN 6.8)
123
the most performing surfactant as far as low tension is
concerned.
The second is the length of the linear tail. With no other
structural change, the longer the tail is, the more
hydrophobic the surfactant (i.e., the higher its Cp) and the
less soluble in water.
As a consequence, in commercial linear alkyl benzene
sulfonate products such as those made for detergents (C12
linear average tail) or other applications with longer tails
like EOR, the less water soluble species are the ones with
the benzene ring in the second carbon of the tail (so-called
2/CNS) and the longer tail (higher N). They will be the
first species to precipitate for one of the two reasons.
In Fig. 17, two linear alkyl benzene sulfonate mixtures
are used. All have a distribution in the tail size and a significant proportion of the 2/CNS species. The surfactant
C15LAS has an average of 14.8 carbon atoms in the tail
with a large distribution of alkyl length from 14 to 17
carbons and 56% of the 2/CNS oligomers. C18LAS has a
longer tail (C18 average from the manufacturer) but a
much lower proportion (*20%) of 2/CNS oligomers. It is
thus more lipophilic from the tail length, but with fewer
oligomers with high asymmetry in the benzene position.
In all cases, a wide salinity scan is carried out over a
range in which the precipitation occurs as determined by
the appearance of a turbidity or at least a haziness in the
water phase. This occurrence is indicated by a vertical
dashed line in the Fig. 17 scans.
The two plots in Fig. 17 show that the variation of the
interfacial tension with heptane exhibits the same behavior,
i.e., a first minimum, and then a maximum and afterwards a
second minimum before a final increase. As the surfactant
tail is increased (from Fig. 17a, b) and as its Cp parameter
increases, the first minimum happens at a lower optimum
salinity in the scan as expected from relation (1). It is worth
remarking that this first minimum appears without any
phase separation in the system, and it is thus the normal
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optimum formulation event in the typical WI [ WIII [
WII phase transition produced by an increase in salinity.
The second minimum seems to happen after and probably as a consequence of the precipitation, whose following
explanation is proposed according to the evidence.
The most likely species to precipitate in Fig. 17a are the
2/CNS, which are quite insoluble in brine. Since they are
in a large proportion, the surfactant mixture that remains
available to go to the interface has lost a large number of
hydrophobic species and thus has become quite hydrophilic. The new Cp value of the remaining mixture is much
lower and thus turns the HLD negative.
The result is a retrograde transition with a return to a WI
phase behavior. The increase of the salinity might produce
more precipitation of the less soluble species, but mostly
results in a dominating salinity term, which turns the HLD
positive again in a second phase behavior transition
WI [ WIII [ WII.
Figure 17a, b shows that the optimum salinity for the
second transition is equally shifted as the first by the surfactant Cp variation.
In Fig. 17b the phenomenon is the same but the explanation slightly different. In this case, there is still a noteworthy amount, i.e., 20%, of the 2/CNS species, but it is
much less than in the previous surfactant in Fig. 17a, and it
could be important but obviously less. On the other hand,
the tail length is significantly longer, and this results in
species with more than 16 carbons, with a much lower
optimum salinity for the first minimum and more water
insolubility, even at a low salt concentration.
There are thus two reasons for the earlier precipitation
and the occurrence of the double transition at lower
salinity.
As seen in the other examples provided in Fig. 17, the
same behavior takes place if a crude oil, from light to
heavy, is used as the oil phase instead of the n-heptane. The
two minimum tension sequences are even more evident
with some crude oils, often with better performance that
justifies this kind of surfactants in EOR.
It is worth remarking that there is a small shift in the
optimum salinity with crudes with respect to heptane
according to the EACN-S relation in Eq. (1), i.e., the
higher the crude EACN, the higher the corresponding
optimum salinity.
This double transition phenomenon occurs in many
cases, but only the first transition is generally reported
since the second one occurs in a region with precipitation
problems. This double transition happens not only with the
anionic surfactant type, but also with the association of two
or three surfactants, which are generally used for the reasons discussed previously [44].
For instance, in the presence of a nonionic surfactant
that is quite insensitive to salinity, the double transition can
also be produced, maybe with the possibility of changing
its characteristics.
Figure 18 indicates two cases in which a surfactant
similar to the previously reported anionic surfactant
(C16LAS) is mixed with only 10% of nonionic to improve
its resistance to salinity effects. Two nonionic surfactants
are added, one only slightly hydrophilic (C12EO5) and
another quite hydrophilic (C12EO8). Both result in an
increase in optimum salinity and a higher salinity limit for
precipitation, as well as a shift for the first and second
tension minima.
Case of Forward-Retrograde Merged Transition
Without an Intermediate Produced by Excessive
Partitioning of a Polyethoxylated Surfactant
N-pentanol as co-surfactant has been reported to combine
with a nonionic surfactant to produce a retrograde transition [204], which is opposite to the one previously
Fig. 18 Double optimum transition with low-tension values with crude oil and a wider optimum zone produced by adding a small amount of
nonionic (two cases) to typical anionic surfactant
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Fig. 19 Forward-retrograde
merged transition produced by
an increase in n-pentanol
content a widely distributed
polyethoxylated surfactant
system [204]
discussed. As seen in the literature [5], the role of the npentanol as a lipophilic co-surfactant is to contribute to
producing the WI [ WIII [ WII transition through the
f(A) alcohol effect in Eq. (1). In this case, the first transition corresponds to Fig. 11b variation. Then, the Fig. 11a
variation is likely to result from the partitioning of the low
ethoxylation oligomers into oil, so that the remaining surfactant going to the interface becomes hydrophilic [38, 50]
and the WII [ WIII [ WI retrograde transition takes
place. However, if the second transition starts to occur
when the first one is not yet completed, then a single forward-retrograde transition merging takes place as
WI [ WIII [ WI [204] as indicated in Fig. 19 with a wide
WIII range.
A similar forward-retrograde merged transition occurs if
the formulation variable results from adding more and
more benzene in a mixture with heptane [205]. In this case,
after a first WI [ WIII change produced by a decrease in
EACN, the increase in aromatic oil content considerably
favors the partitioning of the low ethoxylation oligomers to
the oil phase, resulting in a more hydrophilic surfactant at
the interface and a phase behavior WIII [ WI change
similar to the Fig. 19 case.
Another example of a forward-retrograde merged transition with polyethoxylated nonionics is when the formulation variable is the temperature. As the temperature
increases, the dehydration of the polyethylene oxide chain
tends to produce the WI [ WIII [ WII transition. However, in this case the temperature composition diagram
shown in Fig. 20 exhibits a three-phase behavior region,
which is extremely tilted, as found in many instances with
a widely distributed ethoxylation degree [101]. In addition,
there is a considerable effect of the water/oil ratio on the
very high partitioning of the low EON species into the oil
phase [50, 240].
It is seen along the arrow in Fig. 20 that the temperature scan results in a very wide WIII range on the
WOR \ 1 side of the diagram, in between two regions
with a WI phase behavior, just as in Fig. 19. Note that
the complete WI [ WIII [ WII phase behavior transition rather occurs in this diagram by changing the
WOR, i.e., by moving from left to right, because of its
considerable effect on the previously seen surfactant
partitioning.
123
Fig. 20 Forward-retrograde merged transition during a temperature
variation for a polyethoxylated nonionic mixture with a wide
ethoxylation degree distribution, adapted from a doctoral thesis [240]
Theoretical Possibilities with Even More Complex
Transitions
Apparent artifacts are actually based on theoretical complexities, which could be even more elaborated in the
future, particularly using higher dimension spaces, e.g.,
starting with three effects or three variables. To give some
tips to continue in this direction, two possible aspects will
be discussed next.
The first is a single formulation scan with three
sequential roles and effects: alcohol as a co-surfactant/as a
solvent producing low EON surfactant partitioning/as a
polar oil for interfacial segregation and changing EACN.
The second example is of a complex mixture of effects
in a three-dimensional space with three variables likely to
change the phase behavior: temperature/mixture composition/surfactant total concentration.
Combination of Three Sequential Effects Along
a Single Variable Scan
The previously presented case of a double transition, in
particular the one resulting from increasing the n-pentanol
concentration [204], may be extended to a triple forward/
retrograde/forward transition. With some proper and clever
trials, it is probably feasible to find such an astonishing
case by adding a third effect produced by a lipophilic
alcohol when it reaches a quite high concentration
(5–10%). The third effect is due to its high migration to oil,
J Surfact Deterg
Fig. 21 Possible forward/
retrograde/forward (a–c) triple
transition and its merging likely
to generate in (e) an extra-wide
WIII zone
particularly in the oil segregated close to the interface
[203], and consequently with the concomitant decrease of
the EACN.
Figure 21 shows the three transitions that could take
place at increasing concentrations of a lipophilic alcohol
like n-pentanol, n-hexanol, or 2-hexanol, eventually mixed
with very low EACN substances such as chlorinated
alcohols or wonder solvents in the Hansen solubility
approach [241, 242] like the N,N-dimethyl 9-decenamide.
By adjusting the three effects in the Fig. 21a–c sequence
so that they overlap slightly, an extremely wide WIII zone
could be attained, as shown in the lower right Fig. 21e
scheme. It is not known whether reaching such an amazing
formulation situation could be useful in EOR, but it is
probably worth analyzing.
Combination of Three Single Effects Produced
by Three Variables in a 3D Space
In the previously presented examples discussing the
eventual improvement of the optimum formulation zone
robustness, some cases considered a single variable scan
along a one-dimensional (1D) path, such as a variation of
the temperature, salinity, surfactant characteristic parameter Cp, surfactant mixture composition, and total surfactant
concentration. In such 1D space cases, the robustness
essentially depends on the length of the WIII zone along a
path.
Other previously discussed cases indicated that there are
robust zones in a 2D phase diagram, with two independent
variables, e.g., the temperature and surfactant mixture
composition, or the temperature and surfactant total concentration. In these cases, the robustness was estimated by
the extension of the 2D area occupied by the WIII phase
behavior, in some representation looking like a fish, in
another as a cross resulting from the overlapping of two
perpendicular strips.
Since Winsor’s pioneering studies in the 1950s
[243, 244], the WIII phase behavior body has been studied
in the Surfactant–Oil–Water-Formulation (SOWF) prism,
i.e., in a 3D space.
A very simple basic representation has been proposed
and used by many people for a system with a ternary of
supposedly pure SOW components. Independently of the
different and more or less elaborated aspects
[113, 139, 187, 245, 246], they all represent the same, i.e.,
the evolution of the SOW ternary diagram along the three
typical diagram sequences proposed by Winsor [2] as the
formulation is changed. In the range of the considered
formulation variables in which a three-phase behavior
occurs, the WIII body is generated by a changing triangle
with vertices representing the microemulsion middle phase
and excess phases [247–250].
In this very simple representation, the surfactant concentration and water/oil ratio are the usually used as two
independent variables in the constant formulation SOW
ternary diagram cut in the prism. The third independent
variable, i.e., the formulation, may practically be any of the
variables in the HLD Eq. (1), i.e., salinity, oil EACN,
temperature, surfactant parameter, as well as more elaborated formulation contributions such as alcohol type and
concentration effect, amphiphilic mixture composition, oil
mixture EACN, or even more complex ones.
The point is that the extreme simplicity of the HLD
expression allows for discussion of the general phenomenology, which is expected to take place in a 3D
surfactant–oil–water-formulation (SOWF) prism, and it is
extremely important in practice because the effects can be
represented in a single figure. The addition of an alcohol
component (or a second surfactant) in a 4D tetrahedron
SAOW (or S1/S2/O/W) instead of the SOW 3D triangle
has been intended and has allowed more information to be
shown [24, 133, 192, 251–255], but unless a computer
simulation is used to change the formulation as the time
123
J Surfact Deterg
Fig. 22 Different 2D cuts of
the WIII body in a 3D space
with variables: temperature (T),
composition of a two-surfactant
S1/S2 mixture (Mix), and total
surfactant concentration (Conc)
in a S1/S2/O/W quaternary
system with selected
components
elapses, so that 3D WIII body variation may be clearly
exhibited to change, it is not really useful.
In any case, the main problem is that the various formulation variables have been found to alter the shape of the
WIII body differently. The effects of the change of any of
the variables directly appearing in the HLD equation
(temperature, S, ACN, single surfactant Cp) on the WIII
body of a pure component SOW ternary are fairly understood through 2D representations, but not at a higher
dimension. The computer simulation models, which are in
progress, contain few variables and are often experimentally verified only by changing salinity [30, 256–259].
Additionally, this is still a too simple approach, not enough
to easily deal with a real system containing commercial
surfactants, and even surfactant mixtures, crude oil, and
reservoir brine. In practice, such systems require modeling
the effect of ten or more variables; among them, some are
very complex to handle, like the temperature, which
influences practically anything—the surfactant mixture
composition and surfactant concentration.
In addition, and as seen previously in the present article
examples, the robustness of the WIII body occurrence
depends on very complex phenomena, which are not
clearly understood, like the coincidental compensation of
opposite effects to freeze the interfacial formulation. Even
if some basic studies have shown the nature of the phenomena, such as the selective partitioning of the components in a mixture or the precipitation of some surfactants,
the actual explanation for the effects of the variable
changes is mostly guessed through the examination of a 2D
or 3D cut in a multidimensional space.
123
At the end of this review and with the purpose of
transferring some optimism after such rather hopeless
diagnostics, Fig. 22 is proposed to show a way to represent
the WIII body in the 3D space of temperature (T), AI/NI
surfactant mixture composition (Mix) and total surfactant
concentration (Conc), according to the approach suggested
by the clever reasoning of outstanding theoretical
researchers [260–262].
According to the available data, and with some imagination, it may be said that the WIII body with an AI/NI
surfactant mixture may be represented as two more or less
flattened fish swimming in this 3D space, either separately
or overlapping, i.e., with one of them engulfing the other
totally or partially as a phagocyte.
The few represented 2D cuts in Fig. 22 do not come
from a single existing system. They just show some very
different possibilities that were mentioned for different S1/
S2/O/W cases. The two conc-mix cuts (c, d) indicate the
possibility of merging the two 2D WIII fish-like zones as
surfactant concentration and mixture composition change.
The two T-Mix cuts (e, f) illustrate the merging of the two
2D WIII strip-like zones into a double insensitivity cross
when the surfactant mixture and temperature are properly
changed.
It is obvious that in both cases the appropriate change
can widen the WIII optimum zone 3D body and improve
the robustness, eventually more in some directions than in
others. The (b) cut comes from a study dealing with
another kind of AI/NI mixture (dioctyl sulfosuccinatebutyleneglycol) [260]. This kind of multidimensional
approach shown in Fig. 22 is probably a way to
J Surfact Deterg
experimentally look for some extra-robustness in spite of
the general inverse relationship between the minimum
tension and three-phase range.
5.
6.
Conclusion
7.
The general inverse relationship between the minimum
tension performance in a formulation scan and the range of
the three-phase behavior zone was corroborated to exist
with some error margin of a factor about 10 in the minimum tension, i.e., one unit in the logarithmical scale of the
performance index PERFIND. Such a significant margin
indicates the possibility of somehow improving the
robustness of an optimum formulation case.
This fourth review article was dedicated to discussing
the two ways likely to improve the robustness estimated as
the width of the WIII and low tension zone in a scan.
The first essential feature is to use a system in which
there is some insensitivity with respect to a change of one
or various formulation variables in the HLD equation.
The second method consists of producing a double (forward/retrograde) phase behavior transition in a scan to
generate a succession of two WIII zones and to merge
them by adjusting the system, particularly through unusual artifacts.
To have a chance of being successful, both approaches
require a high level of understanding of the many phenomena, particularly the complex ones dealing with surfactant mixture partitioning between the phases and
interface, including partial precipitation.
Acknowledgements The authors thank CEPSA for providing various
linear alkyl benzene sulfonate mixture samples indicated as x/CNLAS and SASOL for providing a specially prepared extended surfactant of the branched-dodecyl polypropoxylated (20PO) sulfate type
indicated as Guerbert C12 PO20 S. The authors thanks researchers
Aram Quijada and Mairis Guevara for their measurements of interfacial tension in special surfactant mixture systems.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
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Jean-Louis Salager earned a B.Sc. in chemistry and a B.Sc. in
chemical engineering from the University of Nancy (France) as well
as an M.Sc. and a Ph.D. from the University of Texas at Austin (USA)
in enhanced oil recovery formulation. For the past 45 years he has
been involved in teaching and research at the University of the Andes
123
(Mérida-Venezuela) where he is the founder and former director of
the FIRP laboratory. He is currently an emeritus professor and
consultant in surfactant science and technology with applications in
petroleum production, health and personal care, as well as detergent
products.
Raquel E. Antón earned a B.Sc. and M.Sc. in chemical engineering
from Orient University in Puerto-la-Cruz (Venezuela). She received
her Ph.D. from University of Pau P.A. (France). Since 1980 she has
been involved in teaching and research at FIRP Laboratory at the
University of the Andes (Mérida-Venezuela), where she has been
working in the phase behavior of surfactant–oil–water systems,
particularly with complex surfactant mixtures, for producing
microemulsions and corresponding macroemulsions. She is currently
a retired professor.
Marı́a A. Arandia earned her B.Sc in chemical engineering, her
M.Sc. in analytical chemistry, and her Ph.D. in Applied Sciences at
University of the Andes (Mérida-Venezuela). She has worked as a
junior researcher in FIRP Laboratory for 5 years in microemulsion
and low-tension on attainement for enhanced oil recovery. She is
currently living in Panama.
Ana M. Forgiarini earned a B.Sc. in chemical engineering from the
Technological Institute in Barquisimeto (Venezuela) and an M.Sc. in
chemical engineering from University of the Andes (MéridaVenezuela). She received her Ph.D. from University of Barcelona
(Spain) and spent a year as a postdoctoral fellow at North Carolina
State University (USA). Over the past 30 years she has been involved
in teaching and research at the University of the Andes, where she is
currently an active retired professor, deputy director of FIRP
Laboratory, and head of the micro- and nanoemulsions research and
development group, particularly with applications to improved
petroleum production.