New catanionic surfactants based on 1-alkyl-3-methylimidazolium alkylsulfonates, [CnH2n+1mim][CmH 2m+1SO3]: Mesomorphism and aggregation

Article abstract of DOI:10.1039/b822341f

Anionic and cationic alkyl-chain effects on the self-aggregation of both neat and aqueous solutions of 1-alkyl-3-methylimidazolium alkylsulfonate salts ([C_nH_2n+1mim][C_mH_2m+1SO ₃]; n = 8, 10 or 12; m = 1 and n = 4 or 8; m = 4 or 8) have been investigated. Some of these salts constitute a novel family of pure catanionic surfactants in aqueous solution. Examples of this class of materials are rare; they are distinct from both mixed cationic-anionic surfactants (obtained by mixing two salts) and gemini surfactants (with two or more amphiphilic groups bound by a covalent linker). Fluorescence spectroscopy and interfacial tension measurements have been used to determine critical micelle concentrations (CMCs), surface activity, and to compare the effects of the alkyl-substitution patterns in both the cation and anion on the surfactant properties of these salts. With relatively small methylsulfonate anions (n = 8, 10 and 12, m = 1), the salts behave as conventional single chain cationic surfactants, showing a decrease of the CMC upon increase of the alkyl chain length (n) in the cation. When the amphiphilic character is present in both the cation and anion (n = 4 and 8, m = 4 and 8), novel catanionic surfactants with CMC values lower than those of the corresponding cationic analogues, and which exhibited an unanticipated enhanced reduction of surface tension, were obtained. In addition, the thermotropic phase behaviour of [C₈H₁₈mim][C₈H ₁₈SO₃] (n = m = 8) was investigated using variable temperature X-ray scattering, polarising optical microscopy and differential scanning calorimetry; formation of a smectic liquid crystalline phase with a broad temperature range was observed. the Owner Societies 2009.

Full text of DOI:10.1039/b822341f

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
New catanionic surfactants based on 1-alkyl-3-methylimidazolium
alkylsulfonates, [CnH2n+1mim][CmH2m+1SO3]: mesomorphism
and aggregationw
b
b
Marijana Blesic,ab Ma$gorzata Swadz
´
ba-Kwasny, John D. Holbrey,*
´
´
´
Jose N. Canongia Lopes, Kenneth R. Seddon and Luıs Paulo N. Rebelo*
a
ab
a
Received 12th December 2008, Accepted 20th February 2009
First published as an Advance Article on the web 24th March 2009
DOI: 10.1039/b822341f
Anionic and cationic alkyl-chain effects on the self-aggregation of both neat and aqueous
solutions of 1-alkyl-3-methylimidazolium alkylsulfonate salts ([CnH2n+1mim][CmH2m+1SO3];
n = 8, 10 or 12; m = 1 and n = 4 or 8; m = 4 or 8) have been investigated. Some of these salts
constitute a novel family of pure catanionic surfactants in aqueous solution. Examples of this
class of materials are rare; they are distinct from both mixed cationic–anionic surfactants
(obtained by mixing two salts) and gemini surfactants (with two or more amphiphilic groups
bound by a covalent linker). Fluorescence spectroscopy and interfacial tension measurements
have been used to determine critical micelle concentrations (CMCs), surface activity, and to
compare the effects of the alkyl-substitution patterns in both the cation and anion on
the surfactant properties of these salts. With relatively small methylsulfonate anions
(n = 8, 10 and 12, m = 1), the salts behave as conventional single chain cationic surfactants,
showing a decrease of the CMC upon increase of the alkyl chain length (n) in the cation. When
the amphiphilic character is present in both the cation and anion (n = 4 and 8, m = 4 and 8),
novel catanionic surfactants with CMC values lower than those of the corresponding cationic
analogues, and which exhibited an unanticipated enhanced reduction of surface tension, were
obtained. In addition, the thermotropic phase behaviour of [C8H18mim][C8H18SO3] (n = m = 8)
was investigated using variable temperature X-ray scattering, polarising optical microscopy and
differential scanning calorimetry; formation of a smectic liquid crystalline phase with a broad
temperature range was observed.
widely studied. Evidence from molecular dynamics simulations6
Introduction
and small-angle X-ray diffraction7 suggests that this separation
of domains occurs even in ionic liquids with relatively short
butyl, hexyl, and octyl substituents, although the lifetimes
of some domains may be relatively small. It has also
been recently reported that 1,3-dimethylimidazolium dodecyl-
sulfonate exhibits a thermotropic liquid crystalline smectic A
phase and that the phase behaviour can be tuned by modification
of the substitution patterns on the imidazolium cation.8
The considerable interest in ionic liquid crystals is a
consequence of the wide spectrum of potential applications:
in sol-gel synthesis of macro and mesoporous silicas, hydro-
thermal synthesis of g-alumina, preparation of gold and silver
nanoparticles, or as stabilisers for suspension polymerisation.9
It has been anticipated that ionic liquid materials may have
a significant potential impact on the field of surfactants.10
Three possible directions for development and application,
namely using ionic liquids as solvents, surfactants, or as salt
additives have been discussed. Ionic liquids can be added as
co-surfactants or hydrotropes to aqueous solutions of
common surfactants.11,12 They can be used directly as media
for dissolution of many different common ionic and nonionic
surfactants,13 amphiphilic polymers14 or ionic liquids.15 More-
over, due to the obvious structural similarities to conventional
cationic surfactants, many ionic liquids exhibit surfactant-like
The term ‘ionic liquid’ refers to the liquid state of matter
exhibited by salts at, close to, or below 100 1C. Ionic liquids
are typically organic salts, of which examples with 1,3-dialkyl-
imidazolium cations have been the most extensively studied.1
Computational2 and direct experimental methods3 have
shown that ionic liquid systems tend to exhibit bulk isotropy
yet present relatively strong local structure correlations.
Structural modification of the individual ion components of
an ionic liquid by, for example, increasing the length of alkyl-
substituents in 1-alkyl-3-methylimidazolium cations can lead
to increases in the structural anisotropy of the liquid with
microbiphasic separation. In extremis, with decyl- or longer
substituents, liquid crystalline phases can be formed. For
instance, the thermotropic mesomorphism of 1-alkyl-3-methyl-
imidazolium4 and quaternary phosphonium5 salts have been
a
´
´
Instituto de Tecnologia Quımica e Biologica, ITQB 2, Universidade
Nova de Lisboa, Apartado 127, 2780-901 Oeiras, Portugal.
E-mail: luis.rebelo@itqb.unl.pt
b The QUILL Centre, The Queen’s University of Belfast, Stranmillis
Road, Belfast, United Kingdom BT9 5AG. E-mail: quill@qub.ac.uk,
j.holbrey@qub.ac.uk
w Electronic supplementary information (ESI) available: Experimental
and analytical characterisation data for 1-alkyl-3-methylimidazolium
alkylsulfonates 1–7. See DOI: 10.1039/b822341f
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Fig. 1 General structure of 1-alkyl-3-methylimidazolium alkylsulfonate ionic liquids ([CnH2n+1mim][CmH2m+1SO3]); m + n is the sum of the
number of carbon atoms in the two alkyl-substituents.
behaviour and similar aggregation phenomena in aqueous
solution.10,11 Changes in the aggregation behaviour of a range
of 1-alkyl-3-methylimidazolium halide, hexafluorophosphate
and bis{(trifluoromethyl)sulfonyl}imide salts in aqueous
solutions as a function of the alkyl chain length, concentration
and nature of the anion have recently been reported by our
group,10,11 and by others.16 The aggregation behaviour of
aqueous solutions of dodecyl-substituted pyrrolidinium,
pyridinium and piperidinium bromide salts has also been
examined and compared as a function of head group type.10
1-Alkyl-3-methylimidazolium alkylsulfonate salts (see Fig. 1)
are useful examples of ionic liquids in which it is simple to
systematically vary the structure of both the cations and
the anions by changing the number of carbons (n and m) of
the two alkyl chains. The physical properties of a range
of 1-alkyl-3-methylimidazolium alkylsulfonates with alkyl
groups containing six or less carbon atoms in either the cation
or anion and with a total of less than eleven carbon atoms in
the two alkyl chains have been investigated recently.17
We report here the results of investigations of the structural
and surfactant properties of seven 1-alkyl-3-methylimidazolium
alkylsulfonate salts (Fig. 1) from two subsets of the
[CnH2n+1mim][CmH2m+1SO3] (n + m) matrix: (a) salts with
n = 8, 10 or 12 and m = 1, and (b) those with n = 4 or 8 and
m = 4 or 8. In all cases, the total number of carbon atoms in
the two side chains, n + m, is eight or more. By investigating
the properties of these materials within this 2-D array of alkyl
chains, it is possible to know whether systematic changes to
the cation and anion structures induce serial (thus, merely
cumulative) or synergistic variations in the structure–property
relationship. In addition, the thermotropic mesomorphism of
the catanionic 1-octyl-3-methylimidazolium octylsulfonate
(7, n = m = 8) salt in the neat anhydrous state has been
investigated.
(98%), 1-butanol (Z 99.8%), 1-octanol (Z 99%), 1-decanol
(Z 99%), 1-dodecanol (Z 99%), and trimethylamine
(Z 99%) were purchased from Aldrich. Methyl methane-
sulfonate (99%) was purchased from Acros Organics.
1-Methylimidazole was distilled from sodium hydride prior
to use; all other reagents and solvents were used as received.
Doubly-distilled deionised water was obtained from a Millipore
Milli-Q water purification system (Millipore, USA). Pyrene
(Fluka, Germany, 99%) was recrystallised from benzene.
1H and 13C NMR spectra were recorded at room temperature
on a Bruker Avance spectrometer DPX 300, using CDCl3 as
solvent.
Synthesis
The seven ionic liquids used in this study (Table 1) were
synthesised following the general procedure described below.
Alkyl alkylsulfonates (CnH2n+1OSO2CmH2m+1). A solution
of triethylamine (1.1 mol) and the desired alcohol
(CnH2n+1OH; n = 4, 8, 10, 12) (1.1 mol) in dichloromethane
(430 cm3) was cooled in an ice-bath. The respective alkylsulfonyl
chloride (CmH2m+1SO2Cl, m = 1, 4, 8; 1 mol. eq.) dissolved in
dichloromethane (100 cm3) was added dropwise to the cooled,
rapidly stirred reaction mixture while maintaining the
temperature at 0 1C. The reaction mixture was then stirred
for several hours at room temperature, filtered, and the solvent
removed under reduced pressure. The filtrate was purified by
fractional vacuum distillation to yield alkyl alkylsulfonate
esters as transparent, colourless or slightly yellow liquids,
except dodecylmethanesulfonate which was obtained as a
colourless solid. All products were characterised by 1H
NMR spectroscopy.
1-Alkyl-3-methylimidazolium alkylsulfonates. A solution of
freshly distilled alkylsulfonate ester (CnH2n+1SO3CmH2m+1
The salts herein investigated have an amphiphilic character
in either or both the cation and anion, and the results
presented below demonstrate how these materials have
exceptional performance characteristics which are comparable
to those of mixed cationic–anionic surfactants18 (pseudo-
catanionic surfactants), as well as gemini19 surfactants.
)
(1.1 mol. eq.) and 1-methylimidazole (1 mol. eq.) in ethyl
ethanoate (150 cm3) was heated under reflux with stirring
overnight. The crude product formed as a dense layer and
was removed, washed six times with ethyl ethanoate and any
remaining solvent was removed under reduced pressure to give
the corresponding ionic liquids [CnH2n+1mim][CmH2m+1SO3]
(Fig. 1 and Table 1).
Experimental
All the salts 1–7 (see Table 1) were characterised by mass
spectrometry and by 1H and 13C NMR spectroscopy (see
ESIw) and showed no impurities other than the presence of
residual water (detected independently by Karl-Fischer
analysis). All samples were then thoroughly degassed and
Materials
1-Methylimidazole (Z 99%), dichloromethane, ethyl ethanoate,
1-butanesulfonyl chloride ( Z 98%), 1-octanesulfonyl chloride
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Table 1 Physical data collected for the [CnH2n+1mim][CmH2m+1SO3] salts 1–7; critical micelle concentration (CMC, mM) measured by interfacial
tension, IFT, and fluorescence emission of pyrene, Fluor; absorption efficiency, pC20, effectiveness of surface tension reduction, PCMC (mN mꢁ1),
transition temperatures (1C) measured from the onset to the peak by DSC, and transition enthalpy (kJ molꢁ1
)
CMC/mM
DH/
PCMC /mN mꢁ1
Transitionc
T/1C
kJ molꢁ1
a
b
Compound
n
m
IFT
Fluor
Mean
pC20
1
2
3
4
8
1
1
1
4
220
60
14
—
65
19
—
220
1.0
1.5
2.1
0.4
29.0
28.5
28.5
—
C - iso
C - iso
C - iso
C1 - C2
C2 - iso
C - iso
C - iso
C1 - C2
C2 - SA
SA - iso
40.3
53.6
64.9
24.0
60.1
56.3
54.1
68.4
78.9
114.9
39.3
40.5
47.5
10.5
18.3
29.1
32.3
22.5
28.2
2.7
10
12
4
62.5 ꢃ 2.5
16.5 ꢃ 2.5
—
—
5
6
7
4
8
8
8
4
8
135
140
12
130
155
17
132.5 ꢃ 2.5
147.5 ꢃ 7.5
14.5 ꢃ 2.5
1.6
1.6
2.9
40.0
28.5
43.7
a
pC20 defined as the negative logarithm to the base 10 of the concentration of amphiphilic molecules required to reduce the surface tension of the
b
pure solvent by 20 mN mꢁ1
.
Effectiveness of the surface tension reduction, PCMC (defined as PCMC = g0 ꢁ gCMC, where g0 is the surface tension
c
of the pure solvent (water), and gCMC is the surface tension of the solution at the CMC). C is the crystal form, C1 and C2 designate lower and
upper crystal phases when a crystal–crystal transition is observed, SA is smectic liquid crystalline phase and iso designates the isotropic liquid state.
dried under vacuum (ca. 0.1 Pa) at moderate temperatures
(60–80 1C) for typically 24 h before use.
with the same pyrene aqueous solution. Pyrene was used as a
fluorescent probe to ascertain the onset of the aggregation of
the ionic liquids in water. Steady-state fluorescence spectra of
the pyrene-containing solutions in 1 cm quartz cuvettes were
recorded at room temperature with a Cary Varian Eclipse
Fluorescence Spectrometer collected at a 901 angle. Excitation
was set to a wavelength of 337 nm. The intensities of the first
(I1) and third (I3) vibronic bands in the pyrene emission
spectra located around 373 and 384 nm, respectively, were
measured and used to determine the ratio I3/I1.
Characterisation
Differential scanning calorimetry (DSC). Phase transition
temperatures were measured using a TA Instruments DSC
2920 in modulated DSC mode. Cooling was accomplished by
using a refrigerated cooling system capable of controlling the
temperature down to 220 K. Dry dinitrogen gas, with a flow
rate of approximately 20 cm3 minꢁ1, was purged through the
DSC cell. Transition temperatures were determined from the
onset points by initially heating the sample from room
temperature at a rate of 10 1C minꢁ1 to beyond the clearing
point, followed by cooling at a rate of 5 1C minꢁ1 to below the
crystallisation temperature. The cooling–heating cycle
was repeated three times. The transition temperatures and
enthalpies of the transitions obtained from the second and
subsequent cycles were reproducible.
Interfacial tension (IFT). Interfacial tensions were measured
using a Drop Shape Analysis Tensiometer (Contact Angle
System OCA, Carl Stuart Ltd) working in the pendant drop
mode at a constant temperature of 23 ꢃ 2 1C. IFT is derived
from the fit of the pendant drop profile, and care was taken to
ensure that the apparatus was calibrated with several solvents
of known IFT in the range of interest. The drops were left to
equilibrate close to the rupture point and at least three
consistent measurements per solution were recorded.
Polarising optical microscopy (POM). Microscope
observations were made using an Olympus BX50 microscope
equipped with a Linkam TH600 hot stage and TP92 temperature
controller. Neat samples were examined in the form of fine
crystals placed between thin round glass plates. Contact
experiments between the salts and water were performed using
the Lawrence penetration experiment20 by placing a few
crystals of the salt between two glass slides and introducing
a small drop of water in contact with the surfactant through
capillary action.
Results and discussion
The thermal and solution behaviour of seven
[CnH2n+1mim][CmH2m+1SO3] salts (1–7) have been investigated.
The salts comprised two sets; a series of three salts (1–3) with
methylsulfonate (m = 1) anions and different alkyl chain
lengths on the cation (n = 8, 10, and 12, respectively),
and a matrix of four salts with intermediate length of alkyl-
substituents on both cation and anion, 4 (n = m = 4), the
cross terms 5 (n = 4, m = 8) and 6 (n = 8, m = 4), and 7
(n = m = 8). Thermophysical data for two of the salts, 1 and
2, have been previously reported,17 whereas the remainder
(3–7) are new and were synthesised and characterised here for
the first time.
X-Ray diffraction (XRD). X-Ray diffraction data were
collected using a Siemens D5000 powder diffractometer
equipped with a sealed Paar heating stage with Cu Ka X-rays
(l = 1.542 A). Data were recorded from between 2 and 301 in
steps of 0.051 over the temperature range 25 to 120 1C.
Fluorescence spectroscopy. For the fluorescence measure-
ments, aqueous stock solutions of salts 1–7 were prepared
using water containing 1.74 ꢂ 10ꢁ6 M pyrene, and all studied
solutions were prepared from the stock solutions, by diluting
All salts were readily soluble in water at room temperature,
and produced surface active solutions. The CMCs of the salts
in water were determined at room temperature using both
interfacial surface tension and fluorescence spectroscopy.
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Self-aggregation of neat [CnH2n+1mim][CmH2m+1SO3] salts
and thermal behaviour
X-Ray diffraction data were collected for 7. The sample,
initially in the crystalline state at ambient temperature, was
heated to the liquid state at 120 1C; then, the temperature was
decreased to 40 1C in 10 1C steps, and equilibrated for 20 min
at each temperature. Both phase transitions iso - SA and
SA - C2 were observed with a significant degree of super-
cooling such that the iso - SA transition occurred between
110–100 1C and the SA - C2 transition between 60–50 1C.
This is to be compared with the corresponding transition
temperatures in the heating cycle of 115 and 79 1C, respec-
tively. Typical X-ray diffraction patterns for each phase region
are shown in Fig. 2a with polarising optical micrographs of the
three phases shown in Fig. 2b.
The thermal behaviour of the salts was investigated using
modulated DSC. First order endothermic transition temperatures,
enthalpies, and assignments from the onset temperatures of
the heating cycle from the second and subsequent cycles are
given in Table 1. On the cooling cycles, all salts showed
thermal hysteresis and supercooling for the liquid-to-crystal
transition. For 7, the liquid crystalline phase to crystal
transition shows an average temperature depression of
15 1C. Similar phenomena were observed in the case of
crystal–crystal transformations.
1–6 melted directly to form ionic liquids at less than 100 1C;
4 (n = m = 4) also showed a reversible crystal–crystal phase
transition at 24 1C before melting at 60 1C. 7 (n = m = 8)
exhibits a crystal–crystal transition at 70 1C and then melts to
a smectic liquid crystal phase at 79 1C, clearing to the isotropic
liquid (iso) at 115 1C. Confirmation of the character of the
DSC transitions was made by comparison with observations
from hot-stage polarising optical microscopy; and, for 7, also
by small-angle X-ray diffraction to elucidate the nature of the
mesophase formed.
The initial crystal form (C1) has a highly ordered layer
structure, with a strong small-angle diffraction peak h100i at
2y = 3.421, corresponding to a layer spacing of 25.81 A. A
second set of intense peaks in the wide angle region at ca.
13.12, 16.36, and 19.641 show a high degree of structure in the
other two dimensions.
In the isotropic region, 7 shows a small residual peak in the
small-angle region at 6.51 and a characteristic broad diffrac-
tion band centred around 221 (B4 A) consistent with fully
mobile liquid alkyl chains. The SA mesophase region is
characterised by the presence of a sharp peak in the low angle
region (2y = 2–51) and a broad band centred at 221, consistent
with fully mobile alkyl chains. The change in the layer spacing
(d), determined from the position of the most intense peak in
the low angle region, as a function of temperature through the
different phase regions is shown in Fig. 3.
Upon heating a crystalline sample of 7 from ambient
temperature on the hot-stage of the polarising optical micro-
scope, a crystal–crystal transition was observed consistent
with the DSC data, followed by melting of the solid into the
enantiotropic liquid crystalline phase, which cleared to the
isotropic liquid at 115 1C. Cooling from the isotropic liquid
yielded the phase sequence, iso - SA - C2 - C1. The
mesophase forms a dark homeotropic texture under crossed
polarisers, both on heating and cooling from the isotropic
liquid, characteristic of the smectic A bilayer structure
commonly found for neat amphiphilic salts, especially for
the imidazolium-type ionic liquid crystals.4
In the SA phase, the layer spacing d decreases from 23.11 A
at 60 1C to 21.75 A at 100 1C, corresponding to an interlayer
contraction of 0.034 A Kꢁ1 consistent with the decreasing
mobility of the alkyl chains as temperature is reduced.
Unusually, the layer spacing in the mesophase is larger than
Fig. 2 Development of the smectic (SA) and two consecutive solid crystalline (C2 and C1) phases for 7 (n = m = 8) on cooling from the molten
isotropic (iso) state: (a) XRD data in which the iso spectrum is scaled by 10ꢂ; and (b) photomicrographs of 7 in the three phase regions; C1 (20 1C),
C2 (72 1C) and SA (82 1C).
ꢀc
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Fig.
4
Snapshot of
a
simulation box containing 396 ions of
([C8H17mim][C8H17SO3]) at 400 K. The application of a coloring code
enables one to identify the positively charged (blue), negatively
charged (red) polar and nonpolar (green) domains that form the
nano-segregated structure of the bulk isotropic liquid phase of
the ionic liquid. The yellow lines—separated by B20 A—are guides
to the eye that help to recognize the partially layered distribution of
the polar liquid network. Details concerning the molecular dynamics
simulations can be found elsewhere.6
Fig. 3 Change in the layer spacing (d) of 7 in the liquid crystalline
(SA, J) and two crystalline phase (C1, B; C2, &) regions as a
function of temperature, determined from the position of the peak
at the lowest value of 2y in the XRD data.
the sum of the lengths of the two amphiphiles, and suggests
the formation of a double-bilayer structure. This may be a
result of the high density packing of the surfactant head-
groups (calculated from the surface tension data, vide infra).
Further cooling leads to crystallisation from the SA to the
crystalline C2 phase, this transition being characterised by a
discontinuous increase in d (from ca. 23.11 A at 60 1C in the SA
phase to 25.96 A at 50 1C in the C2 crystalline phase,
see Fig. 3) and the appearance of well defined diffraction
peaks in the wide angle region that support the formation of
a crystalline phase. In contrast to the mesophase region, the
d-spacing in the crystalline phases shows a typical contraction
on cooling (0.015 A Kꢁ1).
(in green). The red polar regions are assigned both to the
imidazolium and sulfonate headgroups, with the green
non-polar areas corresponding to the alkyl chains. The relative
sizes and distributions of the two regions, compared with other
previously studied ionic liquids,2,6 are unusual. Due to the
existence of two rather long alkyl chains in [C8H17mim]-
[C8H17SO3], the polar network occupies a relative volume
smaller than that of the non-polar regions and seems to be
constrained to form thread-like or lamellar-like structures,
some of which becoming almost parallel and showing a layer
spacing of ca. 20 A. This value is commensurate with that of
d = 21.75 A for the high temperature portion of the SA phase,
suggesting a pre-aggregation of the ionic liquid forming local
structures, i.e., anticipation of the formation of a liquid
crystal at lower temperatures. In other words, the simulations
demonstrate that the tendency of 7 to form a structured liquid
crystal mesophase is related to the nano-segregated nature of
the ionic liquid already present in the bulk isotropic liquid
phase, and to the interplay between the sizes and distributions
of the two segregated domains (polar network versus
non-polar region).
The C2 - C1 transition was not observed directly when
samples were cooled in situ on the diffractometer hot stage.
This is, in part, due to an absence of active cooling in the
diffractometer which limits the effective minimum temperature
that could be achieved in a cooling cycle to ca. 40 1C. In order
to demonstrate that the C2 - C1 transformation is enantio-
tropic, a sample of 7 was heated externally until molten, then
allowed to cool to room temperature and was then left to
stand at room temperature overnight. The powder diffraction
pattern subsequently measured was consistent with that
collected on the initial crystalline sample prepared.
Self-aggregation of [CnH2n+1mim][CmH2m+1SO3] salts in
aqueous solution
Self-aggregation of neat [C8H17mim][C8H17SO3] (7) as
predicted by molecular simulations
Contact experiments20 (introducing a droplet of water into
contact with the [C8mim][C8SO3] salt over the temperature
range 25–80 1C) revealed a slight swelling of both the crystal-
line solid and the supercooled mesophase domains on addition
of water, rapidly followed by dissolution to form a fluid
homogeneous solution as the contacting aqueous phase
penetrated the salt. Thus, 7 showed no tendency to form
lyotropic mesophases, despite the formation of a neat,
anhydrous smectic (lamellar) thermotropic mesophase.
Preliminary simulation results at 400 K of [C8H17mim]-
[C8H17SO3] (performed specifically at this temperature to match
the bulk isotropic liquid phase of 7) reveal a nano-segregated
structure (Fig. 4) similar to that found in other ionic liquids
with significant (large) low-charge density regions:21 a polar
network (coloured red for the positively charged, and blue for
negatively charged groups), permeated by non-polar regions
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Self-aggregation of the salts in water over the lower
concentration regimes was studied by surface tension and
fluorescence spectroscopy with pyrene as a probe molecule.
The air–water interfacial tension of aqueous solutions of the
salts 1–7 was measured as a function of concentration.
Surfactant aggregation in solution leads to a decrease in the
surface tension with increasing concentration, followed by a
change to almost constant surface tension as the CMC is
reached and the concentration of surfactant molecules in
solution remains static. Data for the salts, 1–3, 6 and 7, are
shown in Fig. 5a and b. The surface tension data were
used to determine the CMC from the intersection of a
linear extrapolation of the two differing regions. The two
adsorption parameters: efficiency of adsorption, pC20,
(defined as the negative logarithm to the base 10 of the
concentration of amphiphilic molecules required to reduce
the surface tension of the pure solvent by 20 mN mꢁ1) and
show the pyrene response (I3/I1) for the aqueous solutions of
the salts, 1–3, 6 and 7, as a function of concentration. The
CMC is determined from the intersection points between that
plateau and the descending part of the fluorescence profile.
Experimental CMC values are presented in Table 1,
calculated from both interfacial tension and fluorescence
experiments. The data from the two approaches are in good
agreement.
The decrease in the CMC of aqueous solutions of 1–3
containing a methylsulfonate anion is a consequence of the
increased alkyl chain in the cation and follows the empirical
rule that the CMC decreases by a factor of two for each CH2
group added to the alkyl chain of a single chain ionic
surfactant, Fig. 5a. The dependencies, and CMC values, found
here for salts 1–3 are almost identical to those reported
for the corresponding 1-alkyl-3-methylimidazolium chloride
systems in water.11 This fact indicates that the chloride and
methylsulfonate anions are equally efficient in decreasing
the electrostatic repulsion between head groups and,
consequently, have very similar values of adsorption efficiency
and effectiveness of the surface tension reduction.
the effectiveness of the surface tension reduction, PCMC
,
(defined as PCMC = g0 ꢁ gCMC, where g0 is the surface tension
of the pure solvent (water), and gCMC the surface tension of the
solution at the CMC) were calculated from the data.22
Fluorescence spectroscopy, using pyrene as a molecular
probe, was used to investigate the polarity of the probe
domains in the solutions as a function of surfactant concen-
trations. By comparing the intensities of the first, I1, and third,
I3, vibronic bands of the pyrene emission spectrum, changes to
the local environment of the probe molecule, and hence the
bulk solution, can be monitored. The intensity ratio I3/I1
varies as a function of the polarity of the pyrene environment
and increases with decreasing solvent polarity. Fig. 5c and d
The solutions of the short alkyl chain ionic liquid 4
(n = m = 4) showed reduced surface tensions with increasing
concentration of ionic liquid. However, at concentrations up
to 2 M in solution no aggregation behaviour was observed
(data are not shown).
The influence of increasing alkyl chain length in the anion
on the CMC is shown in Fig. 5b comparing the 1-octyl-3-
methylimidazolium salts 1, 5 and 7. A non-linear dependence
is found. For comparison purposes, the surface tension of the
Fig. 5 Determination of CMC by surface tension (a) and (b), and fluorescence (c) and (d) for the alkylsulfonate salts: 1 (n = 8, m = 1; m), 2 (n =
10, m = 1;
), 3 (n = 12, m = 1; K), 6 (n = 8, m = 4; J), and 7 (n = m = 8; ’), and for comparison [C10H21mim]Cl (&).11
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aqueous solution of 6 (n = 4, m = 8) as a function of
concentration was measured as well and showed almost
identical aggregation behaviour to the corresponding
cross-term 5 (n = 8, m = 4).
the final value of PCMC between them. This tendency was
previously described for classical cationic surfactants.22
In contrast, it was observed that both pC20 and PCMC
increase with the sum of the cation and anions alkyl-
substituent carbon atoms, for 5, 6 and 7. So that, on increasing
the alkyl-chain in the cation and the anion in unison,
substantial surface tension reductions are observed, see for
example, Fig. 5b.
Fig. 5a and b clearly illustrate that (i) increasing the alkyl
chain length in the cation is the most efficient way to decrease
CMC values, and (ii) increasing the alkyl chain length in the
anion (along with long alkyl chain in the cation) leads to
lower CMCs and, more significantly, to higher effectiveness in
surface tension reduction.
In order to explore whether the two structural variations in
the salts, namely changing the lengths of the alkylsubstituents
on the cation and anion, lead to any obvious cooperativity
effects on the surface activity, the CMC, pC20 and PCMC data
were examined as a function of the sum of the number of
carbons on the two alkyl chains, n + m. The trends are shown
in Fig. 6. The CMC values show as distinct sets for the
cationic and catanionic surfactants, with the CMC varying
by approximately the same amount (slope of the line) with
n + m for the two sets. pC20 varies proportionally with n + m
irrespective of the relative lengths of the two alkyl-chains
contribution to the parameter. The PCMC parameter also
shows two separate sets of data: for the (i) cationic surfactants,
PCMC is invariant with n + m, and for the (ii) catanionic
systems, PCMC increases with n + m.
It should be noted that each molecule of catanionic
surfactant yields two surface active monomers on dissolution.
This is the reason why, for the same concentration of
surfactant, a much lower value of surface tension and
higher absorption efficiency, pC20, were obtained for 7 in
comparison to 1.
PCMC, the effectiveness parameter,22 is important for
surface activity and reflects the detergency, foaming, emulsi-
fication, and wetting properties of surfactants. In the case of
single chain ionic surfactants, the repulsion between polar
head groups inherently establishes a limit to the dense packing
of amphiphiles at the interface. In contrast, the surface
tension reduction is dramatically increased for those surfactants
containing both amphiphilic cations and anions. Since both
the cations and anions incorporate hydrophobic alkyl chains,
they both take part in the surface monolayer formation. The
In common with the previously reported fluorescence
studies on 1-octyl-3-methylimidazolium chloride in water,11
the I3/I1 ratios for solutions of 1 show the typical increase
with increasing surfactant concentration in solution indicating
the formation of hydrophobic domains; however, a plateau is
not reached. This may be due to the progressive dense packing
of monomers in elongated aggregates without the formation of
a well defined micellar structure. For 7 (n = m = 8), the
plateau region of the I3/I1 ratio is significantly higher than
that of the single chain surfactants (1–3) and implies the
formation of micellar aggregates with a different structure,
probably cylindrical or lamellar rather than spherical. Further
investigation by light scattering and cryo-TEM techniques are
required in order to clarify whether dense packed micelles, or
other type of aggregates are formed.
Surface adsorption of surfactants
The lowering of the surface tension g is a consequence of the
increased concentration of surfactant at the air–water
surface. At concentrations greater than that of the CMC, the
interfacial composition and consequently the chemical
potential change only slightly.23 Values for the efficiency of
adsorption at the surface (pC20) and effectiveness of surface
tension reduction (PCMC) calculated in each case from the
change in, and absolute values of, surface tension as a function
of surfactant concentrations are listed in Table 1. The salts 1–3
(with methylsulfonate anions) show an increase in pC20 with
increasing alkyl chain length, with only a small variation in
Fig. 6 CMC (left, with error bars arising from the two results derived from IFT and fluorescence measurements), pC20 (middle) and PCMC (right)
plotted as a function of n + m, the total number of carbons in the two alkyl-substituents in the 1-alkyl-3-methylimidazolium alkylsulfonate. Data
for the methylsulfonate systems (m = 1) are shown as circles and the remainder (m a 1) as squares.
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net electrostatic attraction between the positively and
negatively charged amphiphile head groups, facilitated by
van der Waals attractions between hydrophobic alkyl chains,
enhances the dense packing of monomers in the monolayer
and, consequently, leads to lowering the surface tension in the
plateau region. The magnitude of the reduction reflects the
efficiency of packing at the interface. From the surface tension
data, by assuming that for these low concentration regimes
the interfacial structure at the surface is a monolayer, the
minimum area per ionic liquid molecule, a0, can be calculated
using the well known Gibbs equation.22 A very low value of
0.7 nm2 per ionic pair was calculated for solutions of 7,
demonstrating that, at the interface, the amphiphiles are
extremely close-packed.
Conclusions
The surfactant properties of 1-alkyl-3-methylimidazolium
alkylsulfonate ionic liquid surfactants have been investigated.
In general, the low melting points and high solubility in
water combine to give materials with excellent surfactant
characteristics. The ionic liquids with methylsulfonate anions
(1–3, n = 8, 10, and 12) behave as conventional cationic
surfactants, showing CMC values comparable with those
of the corresponding imidazolium halide salts and having
equivalent surface activity, reducing water–air interfacial
tension from 72.5 to a minimum of ca. 44 mN mꢁ1
.
In contrast, when an amphiphilic character is imparted into
both the cation and anion, a synergistic packing effect appears
to lead to the formation of novel catanionic surfactants
with both CMC values lower than anticipated, and enhanced
surface activity. The large effect on the surface tension is
interpreted in terms of a cooperative effect with both cations
and anions forming the interfacial layer.
Closer packing increases the coherency of the interfacial film
and this usually results in superior emulsifying and foaming
properties. Additionally, for these catanionic surfactants, since
they contain both cationic and anionic amphiphiles, they
will be adsorbed at both negatively and positively charged
surfaces, similar to zwitterionic surfactants. Much lower
values of surface tension were obtained in the plateau region
for 7 as compared with that for 1,3-didodecylimidazolium
bromide,24 which illustrates the dominance of the electrostatic
effect over the combination of hydrophobic and van der
Waals effects.
Two features of these ionic liquids, low melting points and
high water solubilities, were combined with a synergistic
surfactant effect. This enhancement of properties obtained
from the combination of two relatively poor surface active
amphiphiles (1-octyl-3-methylimidazolium and octylsulfonate)
provides great opportunities to develop new, molecularly
simple yet functionally complex, modern high performance
ionic surfactants that outperform gemini and zwitterionic
surfactants. The presence of an amphiphilic structure, in which
both the cation and the anion contain a hydrophobic chain,
leads to significant surface tension reduction, as compared
with structures in which both chains are appended to the same
ion, or where there is only a single chain. An additional
interesting feature in systems such as these is that the alkyl
chains on the individual ions are relatively small which may
improve biodegradability and lower the overall environmental
and toxicological impact.
During the reviewing process of the current manuscript we
became aware of the work by Santos and Baldelli25 concerning
1-alkyl-3-methylimidazolium alkylsulfates in the neat state.
These are also potentially interesting, catanionic ionic liquids,
even though their thermal and chemical stabilities are inferior
to those of their alkylsulfonate-based counterparts.17
Nonetheless, it is interesting to note that at high surfactant
concentrations, i.e. above the CMC, the interfacial surface
tensions measured here (shown in Fig. 5) approach those
reported in ref. 25 for the structurally similar sulfate-based
ionic liquids. This also suggests bulk aggregation of the
surfactants at the air–liquid interface at high concentration.
Gemini surfactants,19 containing two, or more, hydrophilic
head groups covalently connected, have been shown to
have superior properties to traditional cationic or anionic
surfactants with significantly reduced surface tensions at low
concentrations. Gemini surfactants have many applications,
for example as solubilisers of water-insoluble materials and as
agents for removal of pollutants from water.22 7 displays a
high effectiveness parameter for reduction of surface tension,
and has a CMC which is lower than those reported for
comparable gemini surfactants with identical alkyl chain
lengths.26 Consequently, there is real potential to explore the
design and preparation of new surfactants with lower CMCs
and even greater efficiencies.
Acknowledgements
This work was supported by the Fundac¸ ao para a Ciencia e
Tecnologia (FC&T), Portugal (Projects POCTI/QUI/35413/
2000 and POCI/QUI/57716/2004). M.B. thanks FC&T for a
PhD grant (SFRH/BD/13763/2003) and Marie Curie Fellow-
ships for Early Stage Research Training (EST No55613).
K.R.S. thanks the EPSRC (Portfolio Partnership Scheme,
grant no. EP/D029538/1).
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