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Aqueous solution behaviour and solubilisation properties of octadecyl cationic
gemini surfactants and their comparison with their amide gemini analogues
Article in Soft Matter · December 2017
DOI: 10.1039/C7SM02210G
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Cite this: Soft Matter, 2018,
14, 754
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Aqueous solution behaviour and solubilisation
properties of octadecyl cationic gemini
surfactants and their comparison with their amide
gemini analogues†
J. Woch, a J. Iłowska,a Z. Hordyjewicz-Baran, a S. Arabasz,b B. Kaczmarczyk,b
R. Grabowski,a M. Libera,b A. Dworak b and B. Trzebicka *b
Gemini surfactants 18–s–18(Et), comprised of two ethylammonium headgroups and two alkyl tails with
m = 18 carbon atoms with spacers of s = 4, 6, 8 and 10 linking the headgroups (alkanediyl-a,obis(diethyloctadecylammonium bromides)), were obtained. Their aqueous solution behaviour, including
adsorption at the interface and aggregation in solution, was followed by tensiometric, conductometric
and spectroscopic methods. The critical micelle concentration (CMC) of the surfactants decreased with
increasing spacer length. The size of 18–s–18(Et) aggregates formed at concentrations of 10 and
40 CMC measured by DLS varied with the elongation of the spacer. Visualisation of aggregated surfactant
structures at 40 CMC by cryo-TEM evidenced the formation of different morphologies depending on
spacer length. Gemini with s = 4 formed elongated, cylindrical micelles, while geminis of s = 6, 8 and 10
self-assembled into vesicles. The ability of the studied geminis to solubilise hydrophobic dye Sudan I in
Received 9th November 2017,
Accepted 13th December 2017
water was determined as a function of surfactant concentration, demonstrating their high efficiency.
DOI: 10.1039/c7sm02210g
containing an amide group placed between headgroups and tails. The significant impact of amide
rsc.li/soft-matter-journal
groups on the surface activity and aggregation properties of gemini surfactants was evidenced and is
related to hydrogen-bond formation by amide-containing compounds.
Results for 18–s–18(Et) geminis were compared with those previously obtained for their analogues
Introduction
The behaviour of cationic gemini surfactants in aqueous
solution has been the subject of scientific interest since the
1970s.1 Their specific structure consists of two hydrophilic
heads and two hydrophobic tails linked by spacers of various
chemical natures, at a level of, or very close to, the headgroups.
Their superiority over conventional surfactants, manifested by
a higher ability to reduce the surface tension of water, lower
CMCs or better solubilisation properties, has caused lasting
topicality of research into gemini amphiphiles.2,3 Adsorption of
gemini surfactants at the air–water interface and their aggregation in aqueous solution is influenced mainly by their chemical
structure.4 The very fact of having a spacer is responsible for the
unique properties of gemini surfactants. The role of length,
flexibility or hydrophobicity of the spacer is studied for cationic
a
Institute of Heavy Organic Synthesis ‘‘Blachownia’’, Kedzierzyn-Kozle, Poland
Centre of Polymer and Carbon Materials, Polish Academy of Sciences, Zabrze,
Poland. E-mail: [email protected]
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7sm02210g
b
754 | Soft Matter, 2018, 14, 754--764
gemini amphiphiles, mainly with a structure described as
m–s–m(Me), where m and s are the number of carbon atoms
in the alkyl tail and the alkyl spacer, respectively,4 and Me is the
abbreviation for methyl groups connected with quaternary
nitrogen. Only a few studies concerned m–s–m(Et),5,6 where Et
represents ethyl groups.
The chemical nature and length of the spacer are significant
features affecting the adsorption of these surfactants at the
interface, and the morphology and size of aggregates formed in
solution.4,7,8 In micellar structures formed by geminis, the
spacer protects the hydrophobic core from contact with water,
determines the distance between the two hydrophilic heads
and controls the charge distribution.9,10 A surface occupied by
one surfactant molecule attached to this surface depends on
the spatial arrangement of the spacer,4,11 and its closeness to
the micellar core influences the micropolarity of the micelle
interior.12 Gemini surfactants with short spacers (s = 2, 3, 4
methylene groups), in contrast to conventional surfactants, are
capable of forming elongated, cylindrical micelles in water without
any additives.13,14 Besides worm-like micelles, many types of nanoaggregates of geminis have been observed: for example, vesicles,15
ring shapes,16 nanotubes and planar bilayers.17 It has been
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confirmed that even slight differences in structure lead to
different self-assembly behaviour.17 So far, a considerable
amount of work has focused on the role of the spacer in the
aggregation of cationic gemini surfactants with dodecyl,18
tetradecyl19 and hexadecyl20 tails. Although some cationic
geminis with octadecyl tails have already been obtained and
studied, there are no data correlating their aggregation in
aqueous solution with the spacer length.21,22 The only research
on the aqueous solution behaviour of a series of geminis with
18 carbon atoms in the tail concerned gemini surfactants with
unsaturated, oleyl tails.23
To increase the biodegradability of cationic surfactants,
cleavable amide groups have been introduced into the alkyl
chains,24–27 which significantly affected the surfactant behaviour in water.28 Akbas et al. found that the CMC of geminis with
amide groups increased with increasing temperature.29 Pisarcik
et al. concluded that amide group containing gemini surfactants
were more densely packed at the interface than ester group
containing analogues.30
In some cases the geminis with amide groups in their
structure aggregated in lower concentrations, and into larger
assemblies, than those without an amide functionality.28
The presented article concerns the surface activity, aggregation and solubilisation properties of aqueous solutions of
gemini surfactants described as 18–s–18(Et), comprising two
ethylammonium headgroups and two alkyl tails with m = 18
and an alkyl spacer with s = 4, 6, 8, 10 (Fig. 1a). One of the
purposes of the present research was to determine the effect of
the length of the spacer on the size and shape of the aggregates
formed in solution.
This investigation also allowed for the comparison of the solution behaviour of 18–s–18(Et) geminis with their previously studied
analogues containing amide groups placed in between headgroups
and tails,31 here described as 18(NHCO)–s–18(NHCO)(Et) (Fig. 1b).
The number of chemical bonds in the tail, counting from the
quaternary nitrogen to the last methyl group, for both
18–s–18(Et) and 18(NHCO)–s–18(NHCO)(Et) was equal. Geminis
presented in Fig. 1b with s = 4, 6, 8 and 10 aggregated into very
small assemblies, comprising only a few molecules. Such
behaviour is not common for the majority of gemini surfactants, which mainly aggregate to significantly larger objects.32
Soft Matter
The results obtained were useful in clarifying the role of the
amide groups and in identifying the differences in this behaviour caused by their presence.
Experimental
Materials
Diethylamine (499%, Acros), 1-bromooctadecane (Alfa Aesar,
497%) 1,4-dibromobutane (498%, Aldrich), 1,6-dibromohexane
(496%, Aldrich), 1,8-dibromooctane (497%, Aldrich), 1,10dibromodecane (497%, Aldrich), propionitrile (pure p.a., Aldrich),
acetonitrile (pure p.a., Chempur, Poland), ethyl acetate (pure p.a.,
Chempur, Poland), pyrene (498%, Aldrich), 1-hexadecyl-1pyridinium chloride (499%, Aldrich) and Sudan I (495%, Aldrich)
were used without additional purification.
For all experiments, double-distilled water with conductivity
k o 0.4 mS cm1 was used. For dynamic light scattering (DLS)
measurements the water was filtered through Millipore Express
Plust filters with nominal pore size 0.22 mm.
Methods
Synthesis of gemini surfactants. Synthesis of the amine
intermediate and the gemini surfactants was performed by a
method similar to that described by Zana.33 Diethylamine reacted
with 1-bromooctadecane to afford amine C18, which was further
quaternised by alkyl dibromide (Br–s–Br with s = 4, 6, 8, 10) to give
gemini surfactants.
Gemini surfactants were synthesised by the two-step procedure previously used by Zana10 (Fig. 2). Their molecules were
composed of two ethylammonium heads, two alkyl tails of m = 18
carbon atoms and an alkyl spacer of length s = 4, 6, 8, 10 and
are denoted as 18–s–18(Et). The geminis were purified using a
repeated crystallisation procedure to ensure the highest possible
purity, which caused moderate reaction yields (Table S1, ESI†). The
details of the synthesis, along with MS and NMR data, are shown
in ESI† (Tables S2, S3 and Fig. S1–S13).
NMR. The chemical structures of the obtained gemini surfactants were confirmed by 1H NMR (Table S2 and Fig. S1–S5, ESI†)
and 13C NMR (Fig. S6–S9, ESI†). Spectra were recorded on an
Ultrashield NMR 600 MHz spectrometer (Bruker). Chemical shifts
were registered using CDCl3 as solvent and presented in ppm with
respect to tetramethylsilane (TMS) as a signal standard.
MS. Mass spectra were recorded on a Sciex Q-TRAP 4000
series hybrid quadrupole mass spectrometer, equipped with an
electrospray ion source. Sprayed liquid was fed with a syringe
pump (Harvard Apparatus) at a flow rate of 10 mL min1.
Conductivity and Krafft point
Fig. 1 Schematic structure of gemini surfactants: (a) 18–s–18(Et) and
(b) their analogues 18(NHCO)–s–18(NHCO)(Et) containing amide groups
between headgroups and tails; s = 4, 6, 8, 10.
This journal is © The Royal Society of Chemistry 2018
Evaluation of the water solubility of the gemini surfactants was
performed by conductometric Krafft point (TK) determination
by the method proposed by Zana.34 Double-distilled water of
conductivity k o 0.4 mS cm1 was used for all measurements.
The experiments were carried out using a Metrohm 914 conductometer (Metrohm, Switzerland) with a conductivity cell
with c = 0.1 cm1 equipped with a Pt 1000 temperature sensor.
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Fig. 2
Paper
General path of 18–s–18(Et) synthesis, where s = 4, 6, 8 and 10.
One percent solutions of the surfactants, far more concentrated
than their CMC, were stored at a temperature of 1 1C until a
precipitate was formed (at least 48 h). The solutions and
precipitate were poured into a glass vessel with a water jacket
connected to a thermostated bath. The conductivity cell was
immersed in the stirred solution and the vessel was tightly
sealed. The conductivity of the stock solution was measured
with temperature increasing at a rate of 0.5 1C min1. The Krafft
point was taken as the temperature at which the conductivity
versus temperature plot showed a rapid break, which coincided
with the temperature at which the solutions clarified.
Surface activity by tensiometry
Tensiometric measurements were carried out at 50 1C using a
Kruss 100 K tensiometer (Kruss, Germany) equipped with a
Wilhelmy plate. The force required to break the plate away from
the surface of the measured solution was registered and the
surface tension calculated by eqn (1):
g¼
F
L cos y
(1)
where g is the surface tension expressed in N m1, F is the
maximum force acting on the plate in N, L is the wetted length
in m, and y is the wetting angle. Assuming that the contact
angle y is equal to 01, cos y equals 1, the plate is completely
wetted, and the surface tension is affected only by the measured
force and the wetted length.
The surface tension of a series of preheated to 50 1C aqueous
solutions of surfactants of concentration below and above the
expected CMC was measured. CMC was taken as the breakpoint
in the surface tension versus concentration curve. C20, the surfactant concentration required to reduce the surface tension of water
by 20 103 N m1, and gCMC, the surface tension at CMC, were
determined from surface tension–concentration plots.
The relationship of surface tension versus concentration was
used to calculate the surface area occupied by one molecule, a.
The value was calculated using the surface excess concentration
G derived from the Gibbs equation:
1
dg
G¼
(2)
2:03nRT d log C T
a = (NAG)1 1016
1
1
(3)
where R = 8.314 J mol K , T = temperature (here 323.15 K),
g is the surface tension expressed in N m1, NA is the Avogadro
number (6.02 1023 mol1) and n is a constant that depends
756 | Soft Matter, 2018, 14, 754--764
on the number of species constituting the surfactants and
adsorbed at the interface. Here, the value n = 2, generally used
for ionic gemini surfactants,11 was also taken.
Fluorescence spectroscopy
The emission spectra of pyrene, as a fluorescent probe, were
recorded using a FS-2 Fluorescence Spectrometer (Sinco, Korea)
equipped with a Peltier Temperature FL Controller, with an
excitation wave at 318 nm and a bandwidth of 5.0 nm. The
spectra were recorded in the range 320–600 nm at 50 1C.
CMC was determined based on the solubilisation of a hydrophobic fluorescent probe inside the aggregates of the gemini
surfactants. The dependence of the ratio of the fluorescence
intensity of the first (373 nm) and the third (384 nm) vibronic peak
(I1/I3) in the emission spectrum of pyrene on the surfactant
concentration was expressed as a Boltzmann sigmoid.35 The interception of I1/I3 versus concentration was taken as the CMC. The
fluorescence spectra of pyrene for aqueous solutions of the studied
compounds were recorded in the concentration range 0.1–100 CMC. The concentration of pyrene in each solution was 106 M.
Dynamic light scattering
DLS was used to determine the average size and size distribution of
aggregates formed in solutions. The measurements were performed using a Brookhaven BI-200 goniometer (Brookhaven, US)
with vertically polarised incident light of wavelength l = 632.8 nm,
supplied by a He–Ne laser operating at 35 mW, and a Brookhaven
BI-9000 AT digital autocorrelator. The autocorrelation functions
were analysed with the constrained regularised algorithm CONTIN.
All measurements were carried out at an angle of 901 at 50 1C. The
apparent hydrodynamic diameter of aggregates (D90
h ) was calculated
using the Stokes–Einstein equation. The dispersity of particle sizes
m
was given as 22 , where G is the average relaxation rate and m2 is
G
its second moment; both values were obtained from cumulant
analysis. Samples were prepared using double-distilled water filtered
through Millipore Express Plust filters with nominal pore size
0.22 mm. Samples were additionally filtered before measurements
using Whatman Anotop 25 Plus syringe filters with nominal pore
size 0.2 mm. The measurements were performed for the gemini
solutions at concentrations 10 and 40 CMC, which correspond to
10 and 40 times the CMC determined by fluorescence.
Cryo-TEM
Cryogenic transmission electron microscopy (cryo-TEM) micrographs were obtained using a Tecnai F20 TWIN microscope
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Soft Matter
(FEI Company, USA), equipped with a field emission gun
operated at an acceleration voltage of 200 kV and a retractable
cryo-box. The images were recorded using an Eagle 4K HS
camera (FEI Company, USA) under low-dose conditions.
All samples were vitrified on support grids with a holey
carbon film (Quantifoil R 2/2; Quantifoil Micro Tools GmbH,
Germany). Prior to use, in order to provide a hydrophilic film
surface, the grids were argon-plasma treated for 15 s using a
Femto plasma cleaner (Diener electronic GmbH, Germany). The
grids were introduced into a Vitrobot Mark IV (FEI Company,
USA), an automatic plunge-freezing device, where they were
allowed to reach equilibrium at 50 1C. The sample solution was
filtered (Whatman Anotop 25 Plus syringe filters with nominal
pore size 0.2 mm) and maintained at 50 1C for 20 min. A droplet
(3 mL) of solution was then applied to the grid in Vitrobot,
blotted with a filter paper, and immediately submerged into
liquid ethane. The vitrified samples were stored in liquid
nitrogen until they were transferred to a Gatan 626 cryo-TEM
holder (Gatan, Inc, USA) and analysed in the TEM microscope
at around 178 1C. The concentration of surfactants for cryoTEM analysis was 40 CMC.
FTIR studies
Investigation of hydrogen bond formation was performed using
FTIR studies, carried out with a Jasco FTIR 6000 spectrometer
at a resolution of 2 cm1 and scan number 32. Solid samples
were analysed in dry KBr pellets. To simulate aqueous solution
behaviour, D2O was added to the solid surfactants.36 The
samples were kept in the presence of heavy water for 2 weeks
in a vacuum desiccator. The final concentration of surfactants
in heavy water was 40–50 wt%.
Solubilisation studies
Solubilisation studies were performed using the hydrophobic
dye Sudan I as a solubilisate, according to the procedure
described by Tehrani-Bagha and Holmberg.37 The calibration
curve for the suitable range of concentrations of the dye in
ethanol–water (50% v/v) was prepared. The absorbance of
Sudan I solutions at lmax = 487 nm was recorded using a Jasco
V-530 UV/VIS spectrophotometer at 25 1C. An excess of Sudan I
was added to the series of gemini solutions at concentrations
0.001–50 CMC and heated to 50 1C. The solutions were stirred
at 50 1C for at least 24 h to ensure maximum solubilisation of
dye molecules inside the aggregates. Then, the insolubilised
Sudan I was quickly removed by filtration through syringe
filters (Pureland, PVDF, 0.22 mm), the filtrate diluted with an
equal volume of ethanol and the absorbance of the solutions
measured. The concentration of the solubilised dye in aggregates was determined from the absorbance of the solution at
lmax using the previously prepared calibration curve. The solubilisation power (SP), the measure of solubilisation efficiency
defined as moles of solubilised substance per mole of aggregated amphiphile, is given as
SP ¼
Cs Cwat
C CMC
This journal is © The Royal Society of Chemistry 2018
(4)
where Cs is the molar solubility of the dye in the surfactant
solution, Cwat is the molar solubility of Sudan I in water, taken
as 0.0008 mM,38 and C is the molar concentration of the
studied amphiphile. The SP of the surfactants was determined
from the slope of the dependence Cs = f (C).
Results and discussion
Behaviour of 18–s–18(Et) gemini surfactants at the air–water
interface and in solution
Krafft temperature. The water solubility of the surfactants
was related to their Krafft temperature determined by conductometry. The dependence of specific conductivity versus temperature obtained for all surfactants exhibited clear breaks,
coinciding with clarification of the solution (Fig. S14, ESI†).
TK values given in Table 1 are relatively high. TK in the studied
range decreased with increasing s, with the exception of s = 6,
which exhibited the lowest TK. Studies performed by Zana34 for
12–s–12(Me), with s = 2, 3, 4, 6, 8, 10, 12, and 16–s–16(Me),
where s = 2, 3, 4, 6, 8, 12, also showed that TK did not change
monotonically with spacer length. Zana attributed such behaviour to the spatial arrangement of the spacer at the interface
and within aggregates formed above TK.
Surfactants exhibit their activity above their Krafft temperature;39
therefore, all further measurements were carried out at 50 1C.
CMC by tensiometry. Gemini surfactants, like all amphiphiles,
are able to adsorb at the air–water interface, which results in a
decrease in the surface tension of the solution. When the interface is completely occupied by molecules and the concentration
increases further, the amphiphiles begin to aggregate into organised assemblies. This phenomenon is marked by the discontinuity in the surface tension–concentration plot.39 For
typical surfactant solutions, after aggregation begins, the surface
tension remains constant. Fig. 3. shows variation of surface
tension versus concentration obtained for 18–s–18(Et).
The surface tension of each solution decreased with increasing surfactant concentration until the appearance of the discontinuity in the plot, taken as the CMC, which coincided with
the onset of aggregation. These breakpoints were observed for
all compounds (Table 1).
Surface tension at CMC and surface area per molecule. The
parameters of adsorption of geminis at the air–water interface
are summarised in Table 1. The value of gCMC, the surface
tension at CMC, increased with increasing spacer length for
s = 4, 6 and 8. For s = 10 gCMC was slightly lower than for s = 8.
Table 1 Krafft point, critical micelle concentration and surface area per
molecule for 18–s–18(Et)
s
gCMC 103,
TK, 1C N m1
CMCa, mM CMCb, mM C20, mM a, nm2
4
6
8
10
47.3
22.2
40.6
29.9
a
32.3
36.4
49.9
44.8
0.682
0.174
0.170
0.158
Determined by tensiometry.
b
0.498
0.185
0.176
0.150
o0.08
0.03
—
0.09
1.51
1.55
1.73
1.88
Determined by fluorescence.
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Soft Matter
Fig. 3 Surface tension versus concentration for 18–s–18(Et) gemini
surfactants at 50 1C.
The value of C20, the concentration at which the surfactant
lowered the surface tension of water by 20 103 N m1, was
determined from the surface tension–concentration curves. C20
increased with increasing spacer length for s = 4, 6 and 10. For
18–8–18(Et) under the studied conditions C20 was not determined.
Its lowest measured surface tension was 49.9 103 N m1 for
concentration 0.17 mM.
The surface area occupied by one molecule, a, a reflection of
the packing density of surfactants at the air–water interface,40
was calculated according to eqn (3) and the values are summarised in Table 1.
As presented in Fig. 4, a increased with increasing s.
A similar dependence was found for gemini surfactants
12–s–12(Me)41 and further for geminis with pyrrolidinium
headgroups and tails of m = 12.42 An explicit increase of a with
s is observed from s = 6.
CMC by fluorimetry. By fluorescence spectroscopy, CMC was
determined, using pyrene as a probe. The intensity ratio of the
first and third peaks (I1/I3) in the emission spectra of pyrene is
sensitive to changes in the polarity of the microenvironment of
Fig. 4 Variation of surface area per molecule with spacer length for
18–s–18(Et) gemini surfactants.
758 | Soft Matter, 2018, 14, 754--764
Paper
Fig. 5 I1/I3 versus concentration plots for 18–s–18(Et) gemini surfactants,
where s = 4, 6, 8, 10 at 50 1C.
the probe.43 In the case of surfactant solutions, after reaching
CMC, the probe is captured inside micellar cores, where polarity
is significantly lower than that of water, and I1/I3 decreases. CMC
was taken as the intersection of I1/I3 versus concentration
tangents (Fig. 5). For concentrations above CMC, I1/I3 remains
constant, indicating that the environment of pyrene does not
change.
CMC values, obtained by tensiometry and fluorescence
measurements and reported in Table 1, differ as a consequence
of the method used. Similar differences were also found for
other gemini surfactants.5,44
The effect of the structure of the surfactant head on the
onset of the aggregation is revealed when comparing the value
of CMC as measured by Zana for 18–8–18(Me) (0.011 mM, by
conductivity10) with the values obtained here for 18–8–18(Et)
(Table 1). The replacement of the Me group with Et causes a
significant increase in the gemini’s CMC.
CMC values, determined by both tensiometry and fluorescence measurements, decreased with increasing spacer length
(Table 1). The same relations were also found for 12–s–12(Et)
gemini surfactants with s = 6, 8, 10 and 125,18 and for
16–s–16(Me) with s = 6, 8, 10, and 12.45 The decrease in CMC
with s is related to progressive incorporation of the spacer into
the micellar core, which makes it more hydrophobic.46
Micropolarity of the aggregates’ interior. The change in
micropolarity sensed by the pyrene molecule in its vicinity is
reflected by variation of the value of I1/I3. The polarity at the
CMC can be related to the polarity of the microenvironment
inside the surfactant aggregates. The relationship of the value
of I1/I3 with spacer length s is presented in Fig. 6. The micropolarity decreased with increasing spacer length.
Structure and size of gemini aggregates. The size of aggregates formed was measured at concentrations 10 and 40 CMC
(as determined by fluorescence) by DLS at an angle of 901 and a
temperature of 50 1C. The sizes of aggregates are expressed as
hydrodynamic diameter D90
h . Size distributions by intensity obtained
for 18–s–18(Et) gemini surfactants are presented in Fig. 7. Distributions expressed by number are shown in Fig. S15 (ESI†).
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Table 2
Size of aggregates and PDI for 18–s–18(Et) gemini surfactants
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10 CMC
Fig. 6 Micropolarity of the aggregates’ interior expressed as I1/I3 at the
CMC versus spacer length s, for 18–s–18(Et) gemini surfactants.
The D90
h values, calculated as the average of five measurements, along with PDI data, are collected in Table 2.
For s = 4 the size distribution was multimodal at concentration 10 CMC and bimodal at 40 CMC, indicating the presence of
more than one type of aggregates. For s = 6 at concentrations 10
and 40 CMC, the bimodal size distribution evidenced two populations of aggregates. The hydrodynamic diameter of particles in the
lower mode changed significantly when the concentration was
increased from 10 to 40 CMC (Table 2). The average size of particles
was related to the higher mode, which also increased in the studied
range of concentration. The most pronounced change was
observed for aggregates of 18–6–18(Et), which tripled their size.
40 CMC
S
D90
h , nm
PDI
D90
h , nm
PDI
4
6
8
10
3.9; 114
3.4; 90
200
120
0.18
0.20
0.19
0.11
30; 144
18; 290
330
145
0.22
0.06
0.05
0.15
Such behaviour is known for cationic gemini surfactants, which
increase the size of assemblies well above their CMC.46,47
Both peaks corresponding to larger aggregates were significantly
more intensive than those corresponding to smaller assemblies.
For s = 4, at concentrations 10 and 40 CMC, the size
distribution expressed by number revealed the presence of only
one peak (Fig. S15, ESI†) related to 3 nm and 27 nm, respectively.
For s = 6 the size distribution by number was monomodal and
corresponded to 2 nm and 27 nm for 10 and 40 CMC, respectively. The modes in size distribution expressed by number
correlated to lower peaks in distribution by intensity and indicated that small aggregates in solution prevailed significantly.
For s = 8 and 10, size distributions were monomodal when
expressed by intensity and by number (Fig. 7c and Fig. S3c, ESI†).
The size of aggregates was relatively large for s = 8: D90
h = 200 nm at
10 CMC, and 400 nm at 40 CMC. The aggregates were smaller for
s = 10: D90
h = 120 nm at 10 CMC and 140 nm at 40 CMC.
For s = 4, 6 and 8 the peaks corresponding to larger aggregates
are narrower for concentration 40 CMC than for 10 CMC, indicating
improvement in the uniformity of the aggregates.
Fig. 7 Size distribution of aggregates formed by (a) 18–4–18(Et), (b) 18–6–18(Et), (c) 18–8–18(Et), and (d) 18–10–18(Et) in solution at 50 1C.
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Soft Matter
Fig. 8 Cryo-TEM micrographs obtained for samples vitrified at 50 1C at a
concentration of 40 CMC: (a) 18–4–18(Et), (b) 18–6–18(Et), the inset in
(b) with scale bar of 50 nm, shows smaller objects, (c) 18–8–18(Et), and
(d) 18–10–18(Et). Arrows with solid heads indicate larger aggregates and
those with line heads, smaller ones.
Cryo-TEM enabled direct visualisation of the morphology
and size determination of aggregates formed by the surfactants
in solution. The measurements were performed for 40 CMC.
The actual state of the microstructure of the studied solution at
50 1C was maintained by vitrification in the cryopreparation
process. Selected micrographs, obtained for 40 CMC solutions
as used for DLS measurements, are presented in Fig. 8.
The image of sample 18–4–18(Et) (Fig. 8a) reveals small,
well-dispersed, spherical objects and a few larger, but still thin,
elongated aggregates of highly variable length. The small
objects have a diameter of about 10 nm and correspond to
classical spherical micelles. The elongated structures, known as
worm-like micelles, have already been described for other
gemini surfactants with spacer s = 4.14
For the 18–6–18(Et) solution, vesicles of two distinct size
ranges were detected. The larger ones (Fig. 8b), of diameter
between 100 and 150 nm, are predominantly of a multivesicular
type. The smaller ones, not larger than 10 nm in diameter, are
shown in the inset of Fig. 8b. Large multivesicular structures,
between 300 and 400 nm, were also found in the 18–8–18(Et)
solution (Fig. 8c). For the 18–10–18(Et) solution, densely packed
vesicles of diameter around 100–200 nm were seen, as shown in
Fig. 8d.
The cryo-TEM results for all 40 CMC samples analysed
are consistent with the size distributions obtained from DLS
measurements.
Comparison of 18–s–18(Et) and their amide analogues
18(NHCO)–s–18(NHCO)(Et)
Aggregation properties of 18–s–18(Et) gemini surfactants were
compared with their amide group containing analogues (Fig. 1b)
760 | Soft Matter, 2018, 14, 754--764
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previously studied.31 The structures of the analogues consisted
of ethylammonium headgroups linked by a spacer of s = 4, 6,
8, 10, with tails with the same number of chemical bonds as
18–s–18(Et), counting from quaternary nitrogen, but with an
amide group placed between the headgroup and tail (Fig. 1b).
The previously studied 18(NHCO)–s–18(NHCO)(Et), in contrast to
the 18–s–18(Et) studied herein, had much lower TK (o0 1C).31 The
amide group and its location near the surfactant headgroup significantly increased the polarity of the 18(NHCO)–s–18(NHCO)(Et)
molecule, which caused its better water solubility. The difference
is important, as the solubility of 18–s–18(Et) is rather poor and all
measurements had to be performed at 50 1C.
18–s–18(Et) gemini surfactants had lower I1/I3 at CMC
(Fig. 6). The values of I1/I3 for 18(NHCO)–s–18(NHCO)(Et) were
1.42, 1.48, 1.47, 1.42 for s = 4, 6, 8, 10, respectively.31 The
presence of the amide group in between the headgroup and tail
affected the polarity sensed by pyrene.
The values of C20 obtained for 18–4–18(Et) and 18–6–18(Et)
(Table 1) were lower than those for 18(NHCO)–4–18(NHCO)(Et)
(0.14 mM31) and 18(NHCO)–6–18(NHCO)(Et) (0.29 mM). The
values of C20 for 18–8–18(Et), and 18–10–18(Et) were higher
than those of amide group containing analogues (C20 = 0.19
and 0.02 for s = 8 and 10 respectively). The results indicate that
for spacers longer than s = 6 the presence of amide groups leads
to more effective lowering of surface tension of water.
The values of a (Table 1) were higher for 18–s–18(Et) than for
18(NHCO)–s–18(NHCO)(Et), which were a = 0.6, 0.6, 0.6, 0.9 for
s = 4, 6, 8, 10 respectively. A smaller surface area occupied by
one molecule at the interface for amide-containing geminis
reflects more tight packing of their molecules at the interface. It
was suggested in our previous paper26 that this could be caused
by the formation of hydrogen bonds in the case of 18(NHCO)–
s–18(NHCO)(Et). Hydrogen bonds can link two amide groups
within one molecule and the amide groups of two adjacent
18(NHCO)–s–18(NHCO)(Et) molecules.
CMC values were 2–10 times higher for 18–s–18(Et) (Table 1)
than for 18(NHCO)–s–18(NHCO)(Et) (which were 0.059, 0.066,
0.068, 0.078 for s = 4, 6, 8, 10, respectively, determined by
tensiometry).31
At the same time, DLS and cryo-TEM revealed significantly
different sizes and types of aggregates of these two groups of
geminis. 18(NHCO)–s–18(NHCO)(Et), even at concentration 400
CMC, formed very small (1–4 nm) assemblies. These types of
objects resembled more hydrotropic aggregates48 or premicellar
aggregates,10,49 rather than typical micelles. Hydrogen bonds
were indicated as a cause of limited mobility of the tails and
inhibition of further aggregation of amide geminis.31
To specify the character of hydrogen bonds, FTIR studies of
18–s–18(Et) and 18(NHCO)–s–18(NHCO)(Et) surfactants were
performed (Fig. S16 and S17, ESI,† respectively).
For all samples of 18(NHCO)–s–18(NHCO)(Et) surfactants,
the spectra evidenced the bands characteristic of amide groups
(Fig. S17, ESI†), i.e., the bands in the range of 3600–3200 cm1,
corresponding to stretching vibrations of NH2 and NH groups,
and the I amide band in the range of 1670–1640 cm1, related
to stretching vibration of the carbonyl group. In these two
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Table 3
Soft Matter
Bands related to vibrations of amide groups in FTIR spectra obtained for 18(NHCO)–s–18(NHCO)(Et)
Band, wavenumber, cm1
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n of NH groups
I amide band (n of CQO)
II amide band (d of NH)
Free
Surfactant
Free
Bonded
Free
Bonded
18(NHCO)–4–18(NHCO)(Et)
3470
3389
3542
3400
3493
3425
3329
3312 sh
3356 sh
3307
3307
1667 sh
1643
1544
1657 sh
1637
1549
1661
1642
18(NHCO)–6–18(NHCO)(Et)
18(NHCO)–8–18(NHCO)(Et)
sh
sh
sh
sh
1531 sh
Bonded
1549
n – stretching vibrations, d – deformation vibration, sh – shoulder.
regions the bands arising from associated amide group vibrations appear at lower wavenumbers than those originating from
free amide groups. The II amide band, corresponding to
deformation vibrations of NH2 and NH groups, is observed at
1560–1530 cm1. In that case, bonded NH groups absorbed at
higher wavenumbers than free NH groups.
The foregoing analysis of FTIR spectra indicates that
18(NHCO)–s–18(NHCO)(Et) geminis form several types of
hydrogen bonds (CQO H–N), varying in bonding power. This
is reflected within the range corresponding to stretching vibrations of associated NH groups (3600–3200 cm1), in which
multiple bands are observed (Fig. S17, ESI†). Considering the
structures of the studied compounds, the formation of intramolecular hydrogen bonds within one tail of surfactant molecules or between amide groups of two tails can be expected.
Intermolecular hydrogen bonding between two tails of adjacent
molecules is also expected. The intensities of particular bands
in the range varied with spacer length, in the same way as their
positions (Table 3). For s = 4 and 6 the bands arising from free
NH group vibrations are present as shoulders of lower intensities, which indicates that only a small number of amide
groups are not associated.
The formation of hydrogen bonds in 18(NHCO)–s–18(NHCO)(Et)
was also confirmed by 1H NMR measurements (see ESI†).
18–s–18(Et) geminis do not contain amide groups, and the
formation of hydrogen bonds in those cases is excluded. FTIR
spectra of gemini 18–s–18(Et) are shown in Fig. S16 (ESI†).
Except the compound with s = 4, in other spectra of 18–s–18t(Et)
the bands characteristic of vibration of water molecules (a broad
band at 3469 and 3408 cm1 and a weaker one at 1623 cm1) are
observed, related to the surfactants’ hygroscopic properties.
To simulate the behaviour of gemini surfactants in aqueous
solution, D2O was added to the surfactants, as described in the
Experimental section, to give a final concentration of 40–50 wt%
of surfactant in heavy water. In contrast to H2O, the bands
attributed to D2O vibrations do not overlap with the bands
originating from amide groups.50 Examples of the spectra registered for 18(NHCO)–4–18(NHCO)(Et) and 18–4–18(Et) before and
after addition of D2O are presented in Fig. 9. Surfactants containing amide groups exhibited changes in the spectra after addition
of D2O. Example spectra of 18(NHCO)–4–18(NHCO)(Et) (Fig. 9)
show that the intensity of the band at 3329 cm1 diminished
strongly, causing the band at 3312 cm1 to be just detected. The
band at 3429 cm1 appeared, and the intensities of the bands at
This journal is © The Royal Society of Chemistry 2018
3470 and 3389 cm1 increased, similar to the band at 3550 cm1
corresponding to vibrations of free NH groups. The I amide band
shifted from 1643 to 1634 cm1 while the shoulder at 1667 cm1
disappeared, which demonstrates that the share of associated
CQO groups increases.
In the case of 18–s–18(Et), the addition of D2O did not affect
the FTIR spectra (Fig. 9b) indicating that only hydrophobic
interactions are responsible for aggregation of these geminis in
aqueous solution.
The FTIR measurements of 18(NHCO)–s–18(NHCO)(Et) surfactants in heavy water at concentrations far above their CMC
evidenced the existence of intra- and intermolecular hydrogen
bonds within aggregates. The changes in spectra confirmed that
heavy water molecules formed hydrogen bonds with CQO groups
(CQO D–O–D), which changed the distribution of hydrogen
bonds within aggregates of 18(NHCO)–s–18(NHCO)(Et). Nevertheless, the hydrogen bonds play a major role in the aggregation
of these geminis. They link two tails together (within one gemini
molecule or between molecules in the vicinity) forming an
environment of locally higher polarity. Therefore, hydrogen bond
formation limits their self-assembly in a similar way to that for
hydrotropes.51
Solubilisation studies
Surfactants are often used for solubilisation of hydrophobic
compounds in water. In this work the ability of 18–s–18(Et) to
solubilise the model hydrophobic dye Sudan I in water was studied.
The data of studies carried out previously for 18(NHCO)–s–
18(NHCO)(Et)31 were further compared to that of 18–s–18(Et).
The dye was selected due to the possibility of its quantitative
determination by UV-vis spectroscopy. The concentration of the
solubilised dye was calculated from the absorbance values of
the solution at lmax, using the calibration curve in a 50% (v/v)
water–ethanol mixture (Fig. S18, ESI†). The solubility of the dye
in 18–s–18(Et) surfactant solutions is presented in Fig. S19
(ESI†). Significant increases of Sudan solubility could be noticed
at a certain range of concentrations.
For s = 4, the solubilisation curve increased at concentrations 0.05–5 mM (Fig. S19, ESI†), which corresponds approximately to 0.1–10 CMC. For 10.5 mM, the amount of solubilised
dye was lower than the amount obtained with other surfactants
within the series at the same concentration. A similar behaviour
was also observed for 12–s(OH)–12(Me) gemini surfactants by
Tehrani-Bagha et al.38 The authors related it to the changes of
Soft Matter, 2018, 14, 754--764 | 761
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Paper
Fig. 9 FTIR spectrum of the dry 18(NHCO)–4–18(NHCO)(Et) surfactant (black line) compared with the spectrum of the same surfactant after addition of
D2O (blue line) (a); the same for 18–4–18(Et) (b).
microstructure of the solution as a consequence of formation of
larger, elongated aggregates, becoming more viscoelastic at
specific concentration. In such solutions, the efficiency of solubilisation is lower than for spherical micelles.38
For s = 6, 8, 10, the solubility of hydrophobic dye increased
with increasing concentration. Variation in aggregation above
CMC with spacer length evidenced by cryo-TEM (Fig. 8) caused
differences in solubilisation. The worm-like micelles were
detected for 18–4–18(Et) at 40 CMC whereas 18–s–18(Et) with
s = 6, 8, 10 aggregated mostly to vesicles. 18–6–18(Et) was the
most efficient for Sudan I solubilisation, causing an increase in
solubility of nearly 2700 times (Fig. S19, ESI†).
The values of solubilisation power determined for the concentration range at which the solubilisation curve increased according to
eqn (4) are listed in Table 4. SP increased in the order 18–10–18(et) o
18–6–18(et) o 18–8–18(Et) o 18–4–18(Et). The values of SP for
solubilisation of Sudan I by 18(NHCO)–s–18(NHCO)(Et) obtained
earlier31 are included in Table 4 for comparison. SP for 18(NHCO)–s–
18(NHCO)(Et) was 2–6 times lower than for 18–s–18(Et).
SP is strongly affected by the chemical structure of the solubilisate.52 Highly hydrophobic compounds, such as Sudan I,
762 | Soft Matter, 2018, 14, 754--764
Table 4 SP values obtained for solubilisation of Sudan I by 18–s–18(Et)
and 18(NHCO)–s–18(NHCO)(Et)
SP
S
18–s–18(Et)
18(NHCO)–s–18(NHCO)(Et)31
4
6
8
10
0.167
0.135
0.138
0.131
0.029
0.026
0.039
0.054
are expected to be better solubilised in more hydrophobic substances, forming the aggregates with less polar micellar cores.
Differences in SP determined for 18–s–18(Et) and 18(NHCO)–s–
18(NHCO)(Et) are also related to the type of aggregates formed
in solution.
Conclusions
The aqueous solution behaviour of 18–s–18(Et) geminis with
various spacer length was investigated to establish parameters
of adsorption at the interface and aggregation in solution.
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The CMC of gemini surfactants 18–s–18(Et) decreased with
increasing spacer length. The size and morphology of the
assemblies formed in solution at concentrations of 10 and 40
CMC also depended on the spacer length. The shortest spacer,
s = 4, allowed the formation of elongated, cylindrical micelles.
For s = 6, 8 and 10, the longer the spacer, the more uniform
vesicles were observed. It was confirmed that the length of the
spacer of the gemini surfactants is important in helping to
control self-organisation to specified aggregates.
Taking into account the data obtained for 18–s–18(Et), and
previously for their amide analogues 18(NHCO)–s–18(NHCO)(Et),31
it was possible to evidence the role of amide groups in the
18(NHCO)–s–18(NHCO)(Et) tail in the behaviour of the latter in
water. The exchange of only two methylene groups in the alkyl tails
of m = 18 with amide groups influences significantly 18–s–18(Et)s’
solubility, lowering TK by at least 22 1C. The presence of amide
groups strongly affects the size and morphology of the aggregates
formed in solution. Aggregation of amide-bearing geminis is
rather limited to small assemblies, even well above their CMC.
Assemblies of 18–s–18(Et) geminis were much larger, more homogeneous and organised into worm-like micelles and vesicles. It is
postulated that hydrogen bonding is responsible for the limited
self-assembly of 18(NHCO)–s–18(NHCO)(Et).
The type of aggregates had a strong impact on the solubilisation properties of the geminis, even causing a 2700-fold increase
in the solubility of Sudan I in 18–6–18(Et) water solution. The
solubilisation capacity of 18–s–18(Et) in water solution was
higher than that of the respective 18(NHCO)–s–18(NHCO)(Et).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by state funds for the Centre of Polymer
and Carbon Materials, Polish Academy of Sciences and Institute
of Heavy Organic Synthesis ‘‘Blachownia’’. The authors acknowledge Aleksandra Cegielska and Izabela Bonk-Barbara (Institute of
Heavy Organic Synthesis ‘‘Blachownia’’, Poland) for tensiometric
measurements.
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