Supplementary Material
The Schulman method of cosurfactant titration of the oil/water interface
(dilution method): A review on a well-known powerful technique in
interfacial science for characterization of water-in-oil microemulsions
Soumik Bardhan a, Kaushik Kundu b#, Gulmi Chakraborty a, Swapan K. Saha a*,
Bidyut K. Paul b*
a
Department of Chemistry, University of North Bengal, Darjeeling-734 013, India
b
Surface and Colloid Science Laboratory, Geological Studies Unit
Indian Statistical Institute, 203, B.T. Road, Kolkata-700 108, India
#Current Address: Department of Biotechnology, Bhupat and Jyoti Mehta School of
Biosciences, Indian Institute of Technology Madras, Chennai 600036, India
Authors for correspondence:
Prof. Bidyut K. Paul
Prof. Swapan K. Saha
Surface and Colloid Science Laboratory
Department of Chemistry
Geological Studies Unit, Indian Statistical Institute
University of North Bengal
203, B.T. Road, Kolkata-700 108, India
Darjeeling-734 013, India
E-mail: bidyut.isical@gmail.com
E-mail: ssahanbu@hotmail.com
Page 1 of 25
Background of the dilution method
Early in 1982 to 1987, reports on exploring the dilution method were due to Birdi et al. for
water/SDS/pentanol/benzene microemulsions [1], Singh et al. [2], followed by Kumar et al.
for water/CTAB/2-ethyl-1-hexanol/pentane or hexane or heptane microemulsions [3] and
Verhoecku et al. [4] for SDS/Pn stabilized system. Pithapurwala and Shah [5] acknowledged
modified Schulman-Bowcott model for the evaluation of interfacial composition of oilexternal microemulsion formed with sodium stearate, pentanol, brine, and linear
hydrocarbons (with > C10) as oils (viz. decane, dodecane and hexadecane). It was reported
that the molecular packing was highest at the optimal salinity which corresponded to
maximization of molar ratio of alcohol to surfactant at the oil/water interface. Later on, Singh
et al. [6] investigated the influence of chain length of alkanes (pentane to heptane and
benzene) as oil and amine (n-alkylamines and cyclohexylamine) as cosurfactant on water
solubilization behavior of cationic microemulsions (CTAB and CPC) from dilution
experiments. In 1998, Gu et al. [7] performed calorimetric investigation on the partition of npentanol (Pn) between external oil phase (heptane, Hp) and the interface of w/o
microemulsion, sodium dodecyl benzenesulfonate (DDBS)/Pn/Hp/water. The results showed
fine changes in the structure of water-in-oil emulsion and the microemulsion droplets.
Further, the alcohol/surfactant mole ratio in the interface of the droplets, and also the standard
thermodynamic functions of alcohol transition from the external phase to the interface were
derived from calorimetric data. During analysis of these results, a criticism was put
forwarded by the authors on the basics of dilution method or the relationships used in the
dilution method by affirming that the calorimetric method provided better-quality of results
compared to the dilution method. However, in the following year, Moulik et al. [8]
investigated two w/o microemulsion systems; water/cetylpyridinium chloride (CPC)/alkanol
Page 2 of 25
(1-butanol, 2-butanol, 2-methyl-1-butanol, 1-pentanol and 1-hexanol)/alkane (C6 and C7) and
same microemulsion system (as adopted by Gu et al. [7]) at different temperatures by the
dilution method, in order to collect additional data for the sake of comparison and
interpretation. According to the authors, the dilution method for the understanding of the
interfacial cosurfactant/surfactant composition and the distribution of the cosurfactant
between the interface and bulk oil phase, was found to be sound and useful. Further, it was
demonstrated that the equations involved in both the methods were, in fact, identical and that
the criticism of Gu et al. [7] was incorrect. Later, a series of papers on w/o microemulsion
systems have been reported.by employing the dilution method. Of these, a significant
contribution in utilizing this elegant and powerful method to accomplish formation of
microemulsion through consistent motivation, has been made by the school led by Professor
S.P. Moulik, Centre for Surface Science, Jadavpur University, Kolkata, India. Subsequently,
Digout et al. evaluated thermodynamic and structural parameters of w/o microemulsions
comprising water/CPC/alkanols (C4-C6)/alkanes (C8 – C10) by the dilution method [9]. In this
vein, Hait and Moulik [10] introduced a polar amphiphilic (biocompatible) oil, isopropyl
myristate (IPM) for formation of w/o microemulsions based on surfactants of different charge
types, CPC, CTAB and SDS with 1-butanol as cosurfactant along with the evaluation of
structural parameters at different physicochemical environments. The results were analysed
using the dilution method. Further, Bayrak [11] reported stable microemulsion system using
two non-ionic surfactants, Triton X-100 and Triton X-405 with the variation in cosurfactant
chain length (C5OH and C7OH) and oil topology (hexane, heptane, octane, decane and
benzene) in 2004. As the dilution method (or a cosurfactant titration of the oil/water interface
proposed by Bowcott and Schulman [12]) is a simple and inexpensive method which is
accomplished by repetitively oil dilution with cosurfactant titration till attainment of stable
microemulsions, doubts are often expressed on the reliability of data obtained. In the same
Page 3 of 25
year, Palazzo et al. [13] published a paper entitled, “Does the Schulman’s titration of
microemulsions really provide meaningful parameters?” The interfacial compositions of
CTAB/n-hexane/1-pentanol/water w/o microemulsion system were measured using the
dilution method and found satisfactory agreement with high accuracy with those obtained by
using the pulsed gradient spin-echo NMR (PGSE-NMR) technique. They further pointed out
that this method finds applications in scattering and diffusion studies, because it provides
extrapolation to single-particle properties by reducing inter-particle interactions of the
microemulsions without changing its composition. Further, the same system with high
concentration of Pn was investigated to underline its influence on phase equilibria and
mesophase structure using the dilution method and PGSE-NMR technique [14]. It is worthy
to mention that Abuin et al. employed UV-Visible spectroscopy as a tool for titration
experiment involved in the dilution method for characterization of various microemulsion
systems in 2004 [15]. In the following years, Zheng et.al extended the field of the dilution
experiment through characterization of Gemini surfactant based microemulsion, C12-s-C12 or
C12-OEx-C12/n-hexanol/n-heptane for the first time [16, 17]. In 2006, the applicability of the
dilution method on mixed cationic-non-ionic surfactant microemulsions, CTAB/Brij-58/1butanol (1-pentanol)/heptane (decane) was first reported by Paul and co-workers [18].
Subsequently, Paul et al. investigated water (or, aqueous NaCl)/oil (decane and dodecane)
microemulsions stabilized by anionic (SDS), cationic (CPC) and non-ionic [polyoxyethylene
(23) lauryl ether, Brij-35] surfactants and 1-pentanol (Pn) as cosurfactant [19]. During the
period 2006-2007, a series of papers were published on the investigation of interfacial
composition, thermodynamic properties and structural parameters of anionic and cationic
surfactant (alkyltrimethylammonium halide or alkylammonium bromide and its mono-, di-,
and trihydroxyethylated head group analogues) stabilized w/o microemulsions using linear
chain alkanols (viz. C4OH to C9OH) and heptane, isooctane and isopropyl myristate (IPM) as
Page 4 of 25
oils by the dilution method [20-22]. Wang et al. [23] investigated the effect of cosurfactant of
different types with linear and branched chains (viz., n-butanol, n-pentanol, iso-pentanol, nhexanol,
n-octanol)
on
water/cetyltrimethylammonium
the
formation
chloride
of
diesel
(CTAC)/alkanol/diesel
oil
oil.
microemulsion,
The
interfacial
composition and thermodynamic properties of these microemulsions were investigated by the
dilution method. It is interesting to note that under identical physicochemical conditions, noctanol is more suitable cosurfactant than the others in formation of w/o diesel oil
microemulsions. In 2009 and onwards, another group led by Panda and his coworkers [24,
25] investigated formation of w/o microemulsions stabilized by cationic (CnTAB, where n
stands for 10, 14, 16), nonionic (Tween-20, sorbitan polyalkanoates of different alkyl chain
lengths) surfactants, 1-butanol and 1-pentanol as cosurfactants and heptane as oil by the
dilution method. Since 2006 Paul and his coworkers published a series of papers on
interfacial surfactant/cosurfactant compositions, structural and thermodynamic parameters of
single and mixed cationic or anionic-nonionic surfactant microemulsions at different
physicochemical environments by employing the dilution method [26-29, 18, 19, 30-33].
During a decade, the formulation of microemulsions has been pursued with an ionic liquid
(IL) as a substitute of water or oil or as an additive (aqueous), because of their proposed
environmentally-benign nature with many unique and attractive properties. The objective of
these studies was to combine a nanostructure of a microemulsion with the unique properties
the ILs [34, 35] IL-based microemulsion systems are of current interest [36-38]. IL-in oil
(IL/O) nonaqueous microemulsions have attracted much attention from the viewpoints of
theoretical and application prospects in various fields [39, 40]. Chaoumont et al. [41] reported
the molecular dynamics study of the interface between water and room-temperature ILs. The
first report on the investigation on the formation of w/o microemulsions involving surfactantlike IL, 1-alkyl-3-methylimidazolium bromides/alcohol/alkane/water (brine) by the dilution
Page 5 of 25
method, is due to Chai et al. [42]. Subsequently, a series of papers have been published on the
investigation of IL-based microemulsion systems by the dilution method by Chai et al. [43],
Moulik et al. [44], Panda et al. [45], Kar et al. [33], and Wang et al. [46].
Dilution study with single surfactants
The compositions of the cosurfactant and surfactant at the interface, the distribution of the
cosurfactant between the interface and the continuous oil phase, and the energetics of transfer
of the cosurfactant from oil to the interface were evaluated for cationic and anionic
surfactant-derived w/o microemulsions, water/cetylpyridinium chloride, CPC/ 1-butanol or 2butanol or 2-methyl-1-butanol or 1-pentanol or 1-hexanol/hexane or heptane and
water/sodium salt of dodecylbenzenesulfonate, DDBS/1-pentanol/heptane, respectively by
Moulik et al. [8] using the method of dilution The distribution of cosurfactant between the
interface and the oil phase was found to depend on number of factors, namely, their
hydrophobic/hydrophilic molecular nature, the nature of the oil, interfacial forces, and
interaction with the surfactant, system composition, and thermal conditions. The cosurfactant
transfer process was spontaneous for all the systems. Under identical environmental
conditions, DDBS was a more effective surfactant than CPC in transferring cosurfactant from
the oil to the interface. Such transfer process was found to be mostly exothermic, whereas
both positive and negative entropy values of the transfer process were observed. Further, the
enthalpy and entropy of the transfer process were fairly compensated each other. A
correlation between the free energy of transfer and carbon number of the alkanols was
examined and reported to be exponentially dependent [8]. Digout et al. [9] also evaluated
thermodynamic properties and structural parameters of w/o microemulsions comprising
water/CPC/alkanols (C4-C6)/alkanes (C8-C10) by the dilution method. The degree of
spontaneity depended on the chain length of both alkanol and alkane. The free energy of
Page 6 of 25
alkanol transfer process (ΔG0t) varied in an opposite manner for even-numbered versus oddnumbered alkanes. The transfer process was least spontaneous for C4 alkanol and increased
with increase in chain length of alkanols. It was suggested that the formation of
microemulsion can be more ordered or disordered depending on their chemical and thermal
conditions. In the same year, Moulik and his coworkers [47] also estimated the spontaneity of
alkanol transfer process as well as different structural parameters for water/CPC/1-alkanol
(C4 or C5)/n-hexane microemulsion systems. ΔG0t values were found to be decreased with
increase in surfactant/cosurfactant mass ratio (σ) for CPC/1-pentanol (Pn) system whereas
mild dependence of ΔG0t on σ was evidenced for CPC/1-butanol (Bu) system. The transfer
process for Pn based system was slightly more spontaneous than the Bu based system.
However, the transfer process was entropy-controlled for both systems. The water pool
diameter of the droplets were lower for Pn based system than Bu based system, whereas
reverse trend was observed for the ratio of the interfacial aggregation number. However, the
size of the droplets increased mildly with temperature for Bu based systems.
In a subsequent study, Hait and Moulik [10] reported stabilization and destabilization of polar
lipophilic oil, isopropyl myristate (IPM) derived w/o microemulsions under varied amounts
of surfactant and water at different temperatures by the dilution method for the first time. To
understand the differential behavior of surfactants towards the distribution of 1-butanol (Bu)
between the interface and oil phase at the threshold level of stability, three surfactants of
different
polar
head
groups,
namely,
sodium
dodecylsulfate
(SDS),
CPC
and
cetyltrimethylammonium bromide (CTAB) were used. The efficacy of the Bu association
with the surfactants followed the order, CPC > CTAB > SDS, which reflected the degree of
favorable/unfavorable interaction of the surfactant with Bu. At a constant level of surfactant
addition, increasing water content required increasing population of Bu per mole of
surfactant. The transfer of Bu from oil to the interface was an endothermic process with
Page 7 of 25
positive entropy change and was energetically favored with increase in temperature. The
associated ΔH0t and ΔS0t values on the whole followed the sequence, (ΔH0t)CTAB < (ΔH0t)SDS
< (ΔH0t)CPC. Further, effective radius of the water droplet (Rw) and radius of microemulsion
droplet (Re) were found to be increased with increase in content of water, so does the ratio of
average aggregation number of surfactant-cosurfactant on the droplet surface (Ns/Na). At a
low surfactant stoichiometry at interface, greater amount of cosurfactant was required to
stabilize the system. At higher proportions, the compositions of surfactants and cosurfactants
tended towards equilibrium. Also, Fu et al. [48] evaluated the thermodynamic function
ΔG0o→i and structural parameters of anionic surfactant based microemulsion systems to
understand the effects of type of oils or diluents, alcohol, temperature and water content.
They used for the first time organo-phosphoric extractants and their sodium salts as anionic
surfactants, namely, Cyanex272 [sodium salt of di (2,4,4-trimethylpentyl) phosphoric acid,
NaDTMPP], Cyanex301 [di (2,4,4-trimethylpentyl) dithiophosphinic sodium, NaDTMPDTP]
and Cyanex302 [di (2,4,4-trimethylpentyl) monothiophosphinic sodium, NaDTMPTP]. The
order of - ΔG0o→i values followed the sequence, Cyanex272 < Cyanex302 < Cyanex301 and
also, the order was found to be, alkane > CCl4 > aromatics, depending on the oil type. The
free energy values also decreased with increasing water content, which revealed that the
higher water content was unfavorable to the formation of monophasic microemulsion of these
extractants. This study was proposed to be valuable for the application of these extractants
[48]. Later on, the free energy of cosurfactant transfer (ΔG0s) and the adsorption free energy
per methylene group of the alkanol (ΔG0s,
alkanol/CH2)
for water/CTAB or SDS/n-alkanol
(C4OH-C8OH)/oil (C5H-C7H) microemulsions were estimated at various temperatures (25350C) by Kumar and Kabir-ud-Din [49]. ΔG0s values were more negative for CTAB
microemulsion than SDS microemulsion which indicated stronger association between
CTAB-cosurfactant than SDS-cosurfactant. This trend was explained from view point that n-
Page 8 of 25
alkanols were slightly deprotonated which assisted the cosurfactant to associate strongly with
cationic CTAB than with anionic SDS. The values of ΔG0s, alkanol/CH2 were found to be in the
range of -500 to -850 J mol-1 and decreased with the number of carbon atoms in the alkyl
chain of the cosurfactant. It was also evident that the ΔG0s, alkanol/CH2 values were much lower
for pentane to heptane as compared to benzene and increased with the number of carbon
atoms in the alkyl chain-length of the oil phase.
All the studies, discussed in preceding paragraphs, were related to the dilution method
study involving ionic surfactants (anionic and cationic) based microemulsions. In view of
this, Guo et al. [50] evaluated spontaneity (Δc-iG0) of the alkanol transfer process as well as
structural parameters of non-ionic, Triton X-100/n-CnH2n+1OH/H2O microemulsions. It was
observed that the effective radius (Re) and radius of the water pool (Rw), the thickness of the
interfacial layer (L) and the aggregation number (ň) of the systems increased with increase in
solubilized water content whereas the total droplet numbers (Nd), the total interfacial area
(Ad) and Δc-iG0 values decreased. On the other hand, Re, Rw, L and ň increased and Δc-iG0
value decreased drastically, respectively with increasing chain length of alcohols. In
continuation, Bayrak [11] also determined the partitioning of alcohol at the interface of
water/Triton X-100 or Triton X-405/n-alcohols (C5 and C7)/n-alkanes (C6, C7, C8 and C10 and
benzene) microemulsions. The number of moles of alcohols per mole of surfactant at the
interface was increased with an increase in the oil chain length. Further, -ΔG0t values
increased with an increase in the chain length of the alcohol whereas reverse trend was
observed with an increase in chain length of oil. Overall, ΔG0t values were less negative for
Triton X-405 systems than Triton X-100 systems, which indicated that the alcohols are
preferentially associated with Triton X-405 more than Triton X-100 surfactant. Abuin et al.
[15] investigated the effect of alcohols, n-hexanol, n-decanol, 2,4-dimethyl-3-pentanol and 3ethyl-3-pentanol on water-in-oil (for example, n-hexane, n-dodecane, cyclohexane and 2,2,4-
Page 9 of 25
trimethylpentane) microemulsions stabilized by tetradecyltrimethylammonium bromide,
TTAB through refinement of the dilution method by using UV-Vis spectroscopy as a tool to
monitor the titrimetric analysis. It was reported that the efficiency of alcohols in stabilizing
the microemulsion system followed the order, n-alcohols < cyclohexanol < branched
alcohols, and that of solvent (oil) topology had minimal role to play in the stabilization
process. In a concluding remark, it was accounted that the critical amount of alcohol at the
interface was mainly dependent on the alcohol topology and, to a lesser extent, on the alcohol
size, and the solvent topology. This methodology was adopted the same principal as
envisaged by Moulik et al. in 2000 [8].
Interfacial,
thermodynamic
properties
and
structural
parameters
of
w/o
microemulsions comprising of water/CTAB or SDS/n-alkanol stabilized in IPM was further
studied by Mohareb et al. using the dilution method [20]. The distribution of n-alkanol (with
varying chain length from C5 to C9) between bulk oil to the oil/water interface vis-à-vis their
energetics as a function of chain length of the co-surfactants and temperature were evaluated.
The results indicated that, both the alkanol transfer process of CTAB (C16) based systems was
more spontaneous, and overall droplet size (Re) of CTAB stabilized microemulsions was
more enlarged than SDS (C12). The chain length of cosurfactant governed the degree of
spontaneity of microemulsion formation. The standard Gibbs free energy of the alkanol
transfer process turn out to be progressively negative up to C7 (heptanol), i.e., progressed
spontaneously, but fell off at C8 and C9. The alkanol transfer process that guided the
formation of microemulsions was entropically driven. The trend of ΔH0t values followed the
order: ΔH0t (CTAB/C4OH) < ΔH0t (SDS/C4OH). Moreover, the ratio of average aggregation
numbers of surfactant-cosurfactant on the droplet surface (Na/Ns) was higher for SDS based
microemulsions than CTAB.
Page 10 of 25
Investigation on the formation of quaternary ammonium Gemini surfactants in apolar solvent,
by the dilution method was reported by Zhao and his coworkers for the first time [16]. In this
contribution, they illustrated the prominent influence of spacer length of alkanediyl-α,ωbis(dimethyldodecylammonium bromides), C12-s-C12.2Br (where, s = 2-6) on the interfacial
composition of n-hexanol in water/C12-s-C12.2Br/n-hexanol/n-heptane microemulsion. The
ratio of moles of cosurfactant, n-hexanol and Gemini surfactant (C12-s-C12.2Br) on droplet
surface initially increased with increasing water content up to a certain limit and thereafter, a
decreasing trend was observed. However, a regular increasing trend of same was reported
with hike of spacer length(s) from 2 to 6. A spontaneous formation of microemulsions was
evidenced for all systems. The dilution experiments showed that the effective radius of the
water droplet (Rw) and effective radius of droplet in the solution (Re) varied by 0.83 nm and
1.57 nm to 5.82 nm and 7.19 nm with increase water content (ω) from 10 to 50, respectively.
Whereas, Re values were decreased from 3.02 nm to 2.15 nm with increasing spacer length
(s) from 2 to 6. Conversely, with single surfactant analogue systems (water/CPC or CTAB/nbutanol/IPM), the susceptibility towards water was comparatively low [10]. The individual
packing parameter (P) was higher in C12-s-C12.2Br compared to CPC, indicating formation of
larger aggregates. Further, P values decreased with increasing spacer length (s = 2→6), which
indicated that larger amount of cosurfactant (herein, n-hexanol) were present on the droplet
surface. Zheng et al. extended the dilution method study of Gemini surfactant w/o
microemulsions, water/C12-EOx-C12.2Br (x = 1-3)/n-hexanol/n-heptane by incorporating
oxyethylene group as spacer [17]. Compared to C12-2-C12.2Br based systems, C12-EOx-C12
based systems showed higher population of n-hexanol (nai/ns) on the droplet surface rather
than in oil, due to relatively larger size of the head group. The interfacial compositions of nhexanol in these microemulsions with different oxyethylene groups as spacer were evaluated.
With increase in spacer-length of C12-EOx-C12.2Br, (nai/ns) increased from 3.90 to 5.16, while
Page 11 of 25
the effective radius of the water droplet (Rw) showed gradual decreasing tendency. Another
important feature of these microemulsions was evidenced its variation in droplet size with
spacer chain length and provided a potential application for the synthesis of nanoparticles
with small size.
The characterization of cationic w/o microemulsions comprising cetylammonium
bromide (C0) and with mono-(C1), di-(C2) and tri-(C3) hydroxylated head group of surfactant
by employing the dilution method was reported by Mitra et al. [21]. The transfer of n-hexanol
from oil (isooctane) to the droplet interface was exothermic with negative entropy change.
Further, it was reported that droplet number varied inversely with temperature, and
microemulsion with C3 revealed larger droplet number than C2 analogue at comparable
conditions. The population of surfactant and cosurfactant on the droplet interface was
comparatively lower for C3 than C2 analogues. C3-derived microemulsion demonstrated to be
more convenient for enzyme accommodation at the interface to augment better catalytic
activity.
Maiti
et
al.
[22]
also
reported
w/o
microemulsions
comprising
of
water/octadecyltrimethylammonium bromide (C18TAB)/n-butanol/n-heptane. They inferred
that the enthalpy and entropy of butanol transfer process were independent of temperature at
water to surfactant molar ratio (ω), 10→25 i.e. with zero specific heat. Further, the transfer
process of butanol from bulk to the interface progressed isoenthapically above ω (= 10). They
also showed that at a given temperature and [surfactant], the droplet sizes increased with
increasing water content whereas, growth had a decreasing trend with increasing temperature
at fixed [surfactant] and water content. Paul et al. [19] explored evaluation of interfacial
composition and thermodynamics of formation of surfactant of different charge types, viz.
CPC, SDS and polyoxyethylene (23) lauryl ether (Brij-35) derived w/o microemulsion
systems in presence of 1-pentanol (Pn) as cosurfactant and decane (Dc) and dodecane (Dd) as
oil under varied conditions of molar ratio of water to surfactant (ω), salt (NaCl)
Page 12 of 25
concentrations and temperature. The results revealed that cationic CPC was more efficient
than anionic SDS regarding formation of stable microemulsion. The head group size of CPC,
which could modify the relative adjustment pattern of amphiphiles (or molecular interaction
between CPC and Pn) at the interface, was one of the factors that decide the packing of the
amphiphiles at the droplet interface. In addition, delocalization of the charge as well as less
charge shielding due to the presence of pyridine ring in CPC might contributed the factor that
influence the accommodation of Pn at the droplet surface compared to other surfactant
systems. The presence of NaCl favored the transfer of cosurfactant from oil to interface in
case of CPC and SDS systems whereas, the effect was opposite for Brij-35 system. With
increase in temperature, the transfer process was exothermic with more organized interface
for CPC systems whereas, endothermic with less organized interface was observed for Brij35 and SDS stabilized systems. The effect of temperature in presence of NaCl showed similar
trends. In this study, Re and Rw values were found to be 3.43 and 2.27 nm and 7.20 and 6.33
nm, respectively, for CPC/Dc system, and the corresponding values for CPC/Dd system were
3.43 and 2.23 nm and 6.31 and 5.32 nm at water to surfactant molar ratio (ω) 10 and 50,
respectively. Re and Rw values were found to be 4.65 and 3.20 nm and 7.72 and 6.48 nm,
respectively, for SDS/Dc system and the values were found to be 4.58 and 3.12 nm and 7.13
and 5.87 nm for SDS/Dd system at ω = 10 and 40, respectively. The Re and Rw values showed
a decreasing trend while decane was replaced by dodecane for CPC and SDS stabilized w/o
microemulsion systems. In contrary, the observed values were increased while Dc was
replaced by Dd for Brij-35 systems. Since dilutability of microemulsions is a major concern
for their use in the drug delivery systems, Mehta et.al [51] reported characterization of a Utype microemulsion system, water/Brij-96 (polyoxyethylene 10 oleoyl ether)/butanol or
pentanol or hexanol/ethyl oleate (EO) with respect to variation in lipophilicity of different
cosurfactants, chemical structure of oil, water solubility, water solubilization capacity and
Page 13 of 25
temperature from dilution measurements and instrumental techniques. The results revealed
from the thermodynamics of the transfer of alcohols from bulk to the interface that Gibbs free
energy of the alkanol transfer process was spontaneous for all these systems at all
temperatures. The standard enthalpy and entropy changes of the alkanol transfer process were
mostly negative. The changes in the microstructure, state and dissolution behavior of the
added water of these microemulsions were also analyzed using conductivity, UV–visible
spectroscopy and FT-IR. Report on characterization of different charge types of w/o
microemulsions, for example, anionic SDS or cationic didodecyltrimethylammonium
bromide, DTAB or nonionic polyoxyethylene sorbitan monolaurate, Tween-20/n-butanol or
n-pentanol/n-heptane by the dilution method was reported by De et al. [24]. The spontaneity
of formation of these microemulsions was shown to be comparable for both DTAB and SDS,
whereas for Tween-20 based systems spontaneity was about 30% lower. The effective order
of microemulsion formation was DTAB > SDS > Tween-20, irrespective of type of alkanol
used. ΔH0t values were all negative (exothermic) for DTAB/butanol/ or pentanol system at
the studied temperature range. But SDS/butanol system showed exothermicity of the transfer
process at higher temperatures. All other systems were endothermic in nature. Tween-20
based microemulsions were more endothermic than SDS based microemulsions. The DTAB
derived systems were enthalpy driven, whereas the rest (with one or two exceptions) was
entropy driven. At a fixed water content, the difference between Re and Rw values were
continuously decreased with temperature for DTAB based systems, whereas both SDS and
Tween-20 derived systems upheld constancy. Such tuneable physicochemical parameters
were attributed to the variation in the head group of three surfactants. In a subsequent study,
they employed the dilution method to derive different physicochemical parameters for the
formation of water/surfactant + n-butanol/n-heptane w/o microemulsions where both cationic
and nonionic surfactants of two series were used [25]. The cationic surfactants used were
Page 14 of 25
alkyltrimethylammonium bromides (CnTAB, n = 10, 14 and 16) while the non-ionic
surfactants were polyoxyethylene (20) sorbitan monoalkanoates (polysorbate), viz., palmitate
(PS 40), stearate (PS 60) and oleate (PS 80). The spontaneity of formation of microemulsions
was increased with the surfactant chain length for both cationic and nonionic surfactants.
Nonionic surfactants with bulkier head groups required a higher amount of n-butanol
compared to cationic surfactants. The transfer of butanol from oil to the interface was
endothermic with positive entropy change. The effective size of the droplets (R e) increased
linearly with the surfactant chain length for both types of surfactants. For cationic surfactants
a 50% reduction in droplet size with temperature was noted while for nonionic surfactants, it
was only 17%. Surfactants with longer hydrocarbon tails were advocated to produce a better
surface coverage compared to the shorter analogues. Very recently, Mandal and his coworkers evaluated physicochemical and thermodynamic parameters of both anionic (SDS)
and cationic (CTAB) microemulsion systems from dilution experiments, in view of their
suitability in oil recovery process [52]. They explored the effect of alkane carbon number
(ACN) on the maximum water solubilization capacity of microemulsion, and also,
corroborated the Bansal, Shah, O’Connell (BSO) equation and the effect of chain length
compatibility by dilution method. The influence of branching in cosurfactant chain on the
water solubilization limits of microemulsion systems composed of surfactants with different
polar head groups, SDS and CTAB was investigated. Dilution experiments were carried out
in these systems, using four alkanes (hexane, heptane, decane and dodecane) as oil phase and
pentan-1-ol or 3-methyl-1-butanol as cosurfactant, at different temperatures (303, 313 and
323K) to determine the interfacial compositions and thermodynamic parameters of the
microemulsions. The mole fractions of cosurfactant in the interfacial region (Xics) and the
bulk oil phase (Xocs) were higher for SDS microemulsions than CTAB microemulsions, and
was discussed on the basis of the hydrophobicity as well as exposure of the charge of the
Page 15 of 25
head group of two surfactants. Gibbs free energy change of cosurfactant transfer obtained
from dilution experiments confirmed that the formation of these microemulsions was
spontaneous. The degree of spontaneity of their formation depended on chain length of oil,
Higher the chain length of oil, less spontaneous was the formation process, i.e., the negative
values of the change in Gibbs free energy of cosurfactant transfer decreased with increase in
oil chain length. The partitioning of cosurfactant at the interface and the oil phase increased
with increase in oil chain length. CTAB showed higher maximum solubilization of water than
SDS system, irrespective of type of cosurfactant used. ΔH0t showed that the cosurfactant
transfer process occurs with absorption of heat (endothermic) or release of heat (exothermic),
depending on the temperature.
Recently, the dilution method was also carried out in water-in-diesel oil
microemulsion systems, using diesel oil as oil phase, cetyltrimethylammonium chloride
(CTAC) as surfactant and butanol, pentanol, iso-pentanol, n-hexanol and n-octanol as
cosurfactants by Wang et al. [23]. The solubility of all alkanols was found to be decreased in
diesel oil with increasing temperature, whereas an opposite trend was followed for n-octanol.
On the other hand, interfacial populations were higher and lower for n-butanol and n-octanol,
respectively. The diesel oil microemulsions were formed spontaneously no matter which
alkanol used for formation of these microemulsions. The transfer process was found to be
endothermic for all alkanols, except n-octanol caused the release of heat (exothermic). It was
reported that the interface and its surroundings constituted by n-octanol and CTAC showed
more orderly. Finally, it was concluded that n-octanol was more suitable cosurfactant than
others in forming water-in-diesel oil microemulsion under similar environmental conditions.
In continuation, they also measured interfacial composition and thermodynamic parameters
of CTAC and diesel oil based microemulsions stabilized by cosurfactants (as used in previous
report) in presence of aqueous HCl (mass fraction 20%) by the dilution method [53]. A large
Page 16 of 25
amount of cosurfactant was required to form aqueous HCl containing microemulsions
compared with aquo-microemulsions. However, former set of microemulsions were found to
be more spontaneous than latter set of microemulsions. Comparing the structural parameters
of the two microemulsion systems (i.e., aquo- and aqueous HCl based systems), the Re and
Rw values in aqueous HCl based microemulsions at high HCl content in water pool were
smaller than that of aquo-microemulsions with 2-methyl-1-butanol, 1-hexanol and 1-octanol
as cosurfactants. Similar to aquo-microemulsions, 1-octanol was proposed to be more
suitable cosurfactant than others for aqueous HCl based systems.
Page 17 of 25
Table S1. Variation of K, I, ∆Gc→i and D with surfactant, cosurfactant and temperature*
Variation of Surfactant for water/surfactant/1-Hexyl alcohol/heptane microemulsion at 308 K
K
I
∆Gc→i
D (nm)
(kJ/mol)
NP-9
0.072
6.41
6.32
60.9
CTAB
0.063
5.21
6.56
45.2
AS
0.043
5.12
7.70
56.3
Variation of Cosurfactant for water/AS/alcohol/heptane microemulsion
1-Butyl alcohol
0.065
6.77
6.58
63.4
1-Amyl alcohol
0.051
3.41
8.76
51.5
1-Hexyl alcohol
0.023
6.16
7.12
53.8
1-Heptyl alcohol
0.017
5.29
9.70
47.1
Variation of Temperature (K) for water/AS/1-Hexyl alcohol/heptane microemulsion
298
0.023
6.16
7.12
53.8
308
0.043
5.12
7.70
56.3
318
0.037
5.01
8.32
61.3
328
0.021
4.67
10.04
65.6
*Taken from He et al. [54] with permission from Elsevier.
Page 18 of 25
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