Surfactant properties of ionic liquids containing short alkyl chain imidazolium cations and ibuprofenate anions

Article abstract of DOI:10.1039/c1cp21057b

Interfacial tension, electrical conductivity, NMR self-diffusion and DLS experiments have been used to investigate the self-aggregation in water of ionic liquids associating an ibuprofenate anion and 1-alkyl-3-methylimidazolium [C_nMIm]⁺

Full text of DOI:10.1039/c1cp21057b

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PAPER
Surfactant properties of ionic liquids containing short alkyl chain
imidazolium cations and ibuprofenate anionsw
a
a
a
´ ´ ´
Corine Tourne-Peteilh,* Jean-Marie Devoisselle, Andre Vioux,
Patrick Judeinstein,b Martin Inc and Lydie Viau*a
Received 5th April 2011, Accepted 5th July 2011
DOI: 10.1039/c1cp21057b
Interfacial tension, electrical conductivity, NMR self-diffusion and DLS experiments have been
used to investigate the self-aggregation in water of ionic liquids associating an ibuprofenate anion
and 1-alkyl-3-methylimidazolium [CnMIm]+ (n = 4, 6, 8) cations. Despite the short alkyl chain
on imidazolium cations (n r 8), these ionic liquids exhibit particularly low Critical Aggregation
Concentrations (CAC), significantly lower than their parent 1-alkyl-3-methylimidazolium chloride
salts. This behaviour is attributed to the formation of catanionic pairs between ibuprofenate and
imidazolium.
apparent molar volumes and electrical conductivities.8–15 CAC
Introduction
depends on the cation alkyl chain length. For instance, long
alkyl chain imidazolium chloride ILs (n > 8) unambiguously
form aggregates in aqueous solutions whereas CAC of short
alkyl chain imidazolium ILs is very high and hard to
reach.16,17 CAC also depends on the nature of the anion.
Indeed, small polydisperse spherical aggregates were registered by
SANS for [C4MIm][BF4] for a concentration above 800 mM.18
The effect of the anions nature on CAC has also been studied
by Wang et al.19 They demonstrated that the anionic effect
correlates well with the Hofmeister series of the anions for
cationic surfactants.20,21 The position of an anion in this series
is considered to be dependent on its charge, its hydrated
radius/polarizability, hydrophobicity and bulkiness. Actually,
one of the lower CACs (12 mM) was obtained by Blesic et al.
using alkylsulfonate salts.13 In this case, the striking low CAC
value was ascribed to the catanionic behaviour of the IL.
Related to the same catanionic behaviour, long-chain imidazolium
ILs and an anionic surfactant, sodium dodecyl sulfate (SDS),
have been reported to form vesicles which were used for the
synthesis of hollow silica spheres.22
The interest in ionic liquids (ILs) is motivated by their unique
properties such as negligible vapour pressure, thermal stability
and non-flammability, combined with high ionic conductivity
and a wide electrochemical stability window.1 These properties
can be tuned by an appropriate choice of the anion/cation
combination. A new formulation concept based on the design
of active pharmaceutical ingredients in IL form has just
emerged. It limits the polymorphism and improves the
solubilization and dissolution rates.2–4 Inclusion of such IL
drug organic form in silica-based materials (i.e. ionogels) has
been recently proposed as an alternative drug delivery system.5
Ionogels were obtained by a one-step sol-gel synthesis using
1-methyl-3-butylimidazolium ibuprofenate. This method
allows loading high amounts of drug. In these ionogels, the
IL has two roles: it acts as a solubility promoting agent and as
a templating agent for the silica structuration.5 ILs have also
been studied for their assembling properties6 and mixed with
oppositely charged surfactants to obtain mesophases.7
It is now accepted that 1-alkyl-3-methylimidazolium cations
[CnMIm]+ undergo aggregation beyond a critical concentration
called Critical Aggregation Concentration (CAC).
The aim of this paper is to investigate the aggregation
behaviour in water of IL salts consisting of short alkyl chain
[CnMIm]+ cations (n = 4, 6, 8) and ibuprofenate (Scheme 1).
CACs of imidazolium based ILs have been determined
through various methods such as surface tension, fluorescence,
a Institut Charles Gerhardt Montpellier, UMR 5253,
`
CNRS-UM2-ENSCM-UM1 Place Eugene Bataillon, CC007,
34095 Montpellier Cedex, France. E-mail: lydie.viau@univ-montp2.fr;
´
Fax: +33 (0)4 67 14 38 52; Tel: +33 (0)4 67 14 39 72
b Institut de Chimie Mole´culaire et des Mate´riaux d’Orsay,
´
UMR-CNRS, 8182, Universite Paris-Sud 11, Bat. 410 15 rue Georges
Clemenceau, 91405 Orsay Cedex, France
ˆ
´
´
c
`
Laboratoire Charles Coulomb, UMR 5221, CNRS-UM2 Place Eugene
´
Bataillon, CC026, 34095 Montpellier Cedex, France
Scheme 1 Structure of the investigated 1-alkyl-3-methylimidazolium
w Electronic supplementary information (ESI) available: Synthesis
and characterization of [CnMIm][Ibu]. See DOI: 10.1039/c1cp21057b
ibuprofenate ionic liquids.
c
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The physicochemical behaviour of these ibuprofenate
imidazolium ILs in water is one key point to control ionogels
templating and drug release. Sodium ibuprofenate is an
amphiphilic molecule, but its aggregation behaviour has been
studied in few papers.23 Ibuprofenate was reported to form
catanionic mixtures with tetradecyltrimethylammonium bromide
(TTAB) and a sugar-based surfactant and was used in drug
formulation.24,25 In the present paper, surface activities and
CAC were determined by surface tension, conductimetry and
by NMR self-diffusion measurements and compared with data
obtained for long alkyl chain imidazolium chloride ILs.
Aggregates size was estimated by DLS.
The electric field was applied between the electrodes (inter-
electrode distance of 61 mm). The current in the sample was
measured between the same electrodes (sample volume of 750 mL).
The surface of the electrodes was 29.25 mm2.
Dynamic light scattering. DLS measurements were carried
out at 25 1C using a NanoZS, Malvern Instrument. The
NanoZS photospectrometer is equipped with a 532 nm laser
beam. The detection angle was fixed to 1731 to limit the
multiple scattering of the concentrated sample and to measure
very small particles (down to 0.6 nm). The correlation function
was analyzed via the nonnegative least-squares method
(NNLS) to obtain the distribution of diffusion coefficients
(D) of the solutes, and then the apparent equivalent hydro-
dynamic radius (Rh) was determined using the Stokes–Einstein
equation.
Experimental section
Materials
1-Butyl-3-methylimidazolium chloride [C4MIm][Cl] (98%) was
purchased from Solvionic whereas 1-hexyl-3-methylimidazolium
chloride [C6MIm][Cl] ( Z98%) and 1-octyl-3-methylimidazolium
chloride [C8MIm][Cl] ( Z 98%) were purchased from Merck
and used as received. Ibuprofen sodium salt was purchased
from Aldrich (analytical standard). Deionised water was
obtained from a Millipore Milli-Q water purification system.
PGSE-NMR. All solutions were prepared in D2O (99.95%
Aldrich). PGSE-NMR diffusion measurements were carried
out on a 9.4 T Bruker Avance 400 NMR spectrometer
equipped with a Bruker 5 mm broadband probe with a z-axis
gradient and a temperature controller. The self-diffusion
measurements were performed with the pulsed field gradient
stimulated echo and LED sequence using two spoil gradients
(PGSE-NMR).26 The magnitude of the pulsed field gradient
was varied between 0 and 40 G cmꢀ1, the diffusion time, D,
between two pulses was fixed at 200 ms, and the gradient pulse
duration, d, was set between 3 and 7 ms, depending on the
diffusion coefficient of the mobile species. This enabled us to
observe the attenuation of spin echo amplitude over a range of
at least 2 decades, leading to good accuracy (o5%) of the self-
diffusion coefficient values. These coefficients were determined
from the classical relationship ln(I/I0) = ꢀDg2g2d2(D ꢀ d/3),
where g is the magnitude of the two gradient pulses, g is the
gyromagnetic ratio of the nucleus under study and I and I0 are
the areas of the signal obtained with or without gradient pulses
respectively.
Synthesis
The three ionic liquids used in this study were synthesised
following the general procedure described below.
Sodium ibuprofen salt (1 eq.) was dissolved in ethanol
([Na][Ibu] = 0.73 mol Lꢀ1). [C4MIm][Cl] (1 eq.) was also
dissolved in a minimum of ethanol and added slowly to the
previous solution. The resulting mixture was stirred at 70 1C
for 3 h and overnight at room temperature. The solution was
filtered on Millipore (0.45 mm) and 100 mL of acetone was
added leading to the precipitation of NaCl which was further
filtered and the solvent was removed under vacuum. Addition
of acetone was renewed until no further precipitation of NaCl
could be detected. The products were then dried under vacuum
at 343 K for 48 h. All products were obtained as a yellowish
viscous liquid showing glass transition temperatures between
ꢀ26 1C and ꢀ45 1C depending on the alkyl chain length.
Results and discussion
Surface activity
1
The samples were characterized by H and 13C NMR, ESI,
Surface activities of [CnMIm][Ibu] were investigated and
compared to those of [CnMIm][Cl] and [Na][Ibu] references
(Fig. 1). Surface tensions of the solutions versus concentration
in the logarithmic scale are displayed in Fig. 1. Short alkyl
chain chloride salts showed a poor surface activity consistent
with literature data.16,17 The surface tension of [C4MIm][Cl]
did not significantly decrease below a concentration of 100 mM.
Typically, a concentration of 0.5 M of [C4MIm][Cl] was
DSC and TGA (see ESIw).
Characterization
Interfacial tension measurements. Surface tension was
measured using a tensiometer Kruss K100 by the plate method,
¨
at 25 1C (ꢁ0.5 1C). Each measurement was averaged over
30 values recorded for 120 s (stand. dev. = 0.02 mN mꢀ1).
Samples have been prepared by successive dilutions with
MilliQ water and carefully homogenized. The MilliQ water
reference was 71.5 ꢁ 0.8 mN mꢀ1. In order to avoid any
contamination, the plate was systematically rinsed with alcohol
and flame burned. The vessel was cautiously washed with
alcohol and then dried.
needed to decrease the surface tension to 50 mN mꢀ1 16
.
[C8MIm][Cl] and [C10MIm][Cl] showed significant surface
activity, confirming that imidazolium chloride ILs were surface
active only for the long alkyl chain length (n Z 8). By
contrast, whatever the length of the alkyl chain, [CnMIm][Ibu]
aqueous solutions (n = 4, 6, 8) underwent a pronounced
decrease in surface tension followed by a plateau indicative
of aggregation. The pronounced minimum and the subsequent
positive slope observed at higher concentrations might be
ascribed to the presence of impurities in commercial IL
Conductimetry. The conductivity of the IL solutions was
measured on a NanoZS, Malvern Instrument at 25 1C.
Measurements were performed on disposable folded
capillary cells including the electrodes (Malvern, Instrument).
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Fig. 1 Surface tension isotherms at 25 1C of (a) [CnMIm][Cl] K n = 4, ’ n = 8, m n = 10, and (b) c [Na][Ibu] and [CnMIm][Ibu] J n = 4,
B n = 6, & n = 8.
batches, or to a relative surface excess of water18 or to
coulombic interactions.27 It is noteworthy that the plateau
was reached directly for n = 8 suggesting more favorable
aggregation in this case. The surface activities of [CnMIm][Ibu]
might be related to that of [Na][Ibu]. Indeed, this latter
presented interfacial properties with a plateau region close to
36 mN mꢀ1. Thus, association of ibuprofenate and imidazolium
cations could result in an ion pair amphiphile showing good
interfacial activity.
[CnMIm][Cl], confirming the strong contribution of ibuprofenate
anions to surface adsorption. Thus, the surface tension of
short alkyl chain [C4MIm][Ibu] was equal to that obtained for
long alkyl chain [C10MIm][Cl].
The surface tension reduction was defined as PCAC = g0 ꢀ gp,
where g0 is the surface tension of water and gp the surface
tension of the solution at the plateau. In the [CnMIm][Cl]
series PCAC was nearly constant, showing that the monolayer
was essentially the same in terms of nature (imidazolium
cations) and compactness (Table 1). However, the surface
tension reductions for [CnMIm][Ibu] series were different from
that for [CnMIm][Cl]. This confirmed that the monolayer at
the water/air interface did not consist of only imidazolium
cations, but rather of imidazolium/ibuprofenate ion pairs.
Moreover, PCAC values of the ibuprofenate series dramatically
increased from n = 4 to 8, revealing an increase in molecular
packing at the film interface when increasing the chain length.
This was demonstrated by estimating the area occupied
by a surfactant molecule at the air–water interface Amin
according to:
The CAC was taken at the minimum surface tension which
was considered to be the aggregation onset.27 CAC data
determined from surface tension measurements are set out in
Table 1. CAC values turned out to be much lower for
[CnMIm][Ibu] than for [CnMIm][Cl]. As a matter of fact,
CAC obtained for [C8MIm][Ibu] was 14 fold smaller than
for [C8MIm][Cl]. More interestingly, the short alkyl chain
[C4MIm][Ibu] possessed a CAC close to that obtained for
[C10MIm][Cl]. However, as for a classical ionic surfactant, a
decrease of the CAC was observed when increasing the alkyl
chain length of the imidazolium cation.
The ability to decrease surface tension was characterized by
two other parameters: (i) the efficiency of adsorption pC20
defined as the negative logarithm of the amphiphilic molecules
concentration required to reduce the surface tension of the
pure solvent by 20 mN mꢀ1, (ii) the effectiveness of the surface
tension reduction PCAC (see Table 1). From the measurements
of pC20, two points can be underlined. First, pC20 values
increased with the alkyl chain length, which confirmed a
higher efficiency of surfactant activity upon increasing n.
Second, pC20 values were higher for [CnMIm][Ibu] than for
1
Amin
¼
ð1Þ
NAGmax
where Na is the Avogadro constant (6.022 ꢂ 1023 molꢀ1), Gmax
is the surface excess concentration calculated by applying the
Gibbs equation:28
ꢀ
ꢁ
1
@g
2RT @ ln C
Gmax ¼ ꢀ
ð2Þ
T
where R is the gas constant (8.314 J molꢀ1
K
ꢀ1), T the
absolute temperature and C the surfactant concentration.
Here adsorption of the ion pair (imidazolium + ibuprofenate)
has been assumed.
Table 1 Surface properties at 25 1C
CACa/mM
pC20
P
CAC/mN mꢀ1
Amin/A2
Amin was observed to decrease as the alkyl chain length
increased in the [CnMIm][Ibu] series. This suggested that the
interaction between imidazolium and ibuprofenate was not
only electrostatic, but also hydrophobic. This is consistent
with strong ion pairing under cooperative interactions, i.e.
Coulombic interactions, p-stacking, strong hydrogen bonding
between the carboxylate and the acidic proton on the imidazolium
ring, and van der Waals interactions when increasing alkyl
chain length on the cation.29 The strong ion pairing associated
Compound
[C4MIm][Cl]
[C8MIm][Cl]
[C10MIm][Cl]
[Na][Ibu]
[C4MIm]Ibu]
[C6MIm][Ibu]
[C8MIm][Ibu]
—
190
45
142
75
38
12
—
—
1.67
2.55
1.92
2.42
2.74
3.00
32.2
33.7
35.4
38.5
42.5
45.5
84
92
95
105
82
62
a
CAC determined at the minimum of g values.
c
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Fig. 2 Conductivities vs. concentrations at 25 1C of (a) [CnMIm][Cl] ’ n = 8, m n = 10, c [Na][Ibu] and (b) [CnMIm][Ibu] J n = 4 (onset),
B n = 6, & n = 8.
Table 2 CAC determined by conductimetry and NMR with the
corresponding degree of ionization and free energy of aggregation
Compound
CACa/mM
a
DG0m/kJ molꢀ1
CACb/mM
[C4MIm][Cl]
[C8MIm][Cl]
[C10MIm][Cl]
[Na][Ibu]
[C4MIm]Ibu]
[C6MIm][Ibu]
[C8MIm][Ibu]
—
205
78
187
75
41
14
—
—
—
—
—
—
72
—
14
0.65
0.54
0.86
0.45
0.44
0.30
ꢀ18.7
ꢀ23.8
ꢀ16
ꢀ25.4
ꢀ27.9
ꢀ34.8
a
b
Determined by conductivity. Determined by PGSE-NMR.
to low critical aggregation concentrations is characteristic of
catanionic surfactant-systems in water.30 Catanionic systems
consist of mixtures of anionic and cationic surfactants that
reveal enhancement of their interfacial properties (compared
to the individual molecules).31
Fig. 3 DG0m as a function of the number of CH2 groups of ’
[CnMIm][Cl] and of K [CnMIm][Ibu]. Data in empty squares are
those from Jungnickel et al.17 Dashed and dotted lines represent the
expected linear variation with (DG0m;CH ¼ ꢀ2:85 kJ molꢀ1
) for
2
[CnMIm][Cl] and [CnMIm][Ibu], respectively.
Electrical conductivity measurements
Conductivities of aqueous solutions of [CnMIm][Ibu] (n = 4,
6, 8) were compared with those of [CnMIm][Cl] (n = 8, 10) and
[Na][Ibu], used as references. As it can be seen in Fig. 2, there
were two linear regimes in the concentration dependence of the
conductivity. The difference in these regimes was not well
marked for the references [CnMIm][Cl] and [Na][Ibu]. On the
other hand, the slope change was well pronounced for the
[CnMIm][Ibu] series. The slope break was assigned to the onset
of the critical aggregation concentration (CAC) due to the
decrease of free charged species in the bulk. Data obtained by
this method are collected in Table 2. The latter were consistent
with the values obtained by surface tension measurements.
Application of the pseudo-phase model of micellization
allowed determining the Gibbs energy of aggregation (DG0m)
according to the following equation:28
As expected from surface tension measurements, an increase
in the alkyl chain length on an imidazolium ring resulted in a
decrease of DG0m. The variations of DG0m with the nature of the
anion are represented in Fig. 3. As previously reported,33 DGm0
could be divided into contributions from the head group
DG0m,head group, the methylene group of the hydrophobic chain
DG0m;CH and the terminal CH3 group of the alkyl chain
2
DG0m;CH . These contributions express the fact that aggregates
3
form under the influence of both attractive and repulsive
forces. Attractive forces are associated with the poor solubility
of alkyl chains, while effective repulsive forces result from the
high solubility of head groups.
DG0m ¼ DGm0 ;head group þ DGm0 ;CH þnCH2DG0
ð4Þ
m;CH2
3
DG0m = (2 ꢀ a)RTlnwCAC
(3)
A plot of DG0m versus nCH2 should give a straight line with an
intercept equal to DG0m;head group þ DGm0 ;CH and a slope equal
3
where wCAC is the critical aggregation concentration expressed
in mole fraction, a is the degree of ionization determined by
Frahm’s method i.e. as the ratio of the slopes of the linear
fragments above and below CAC.32
to DG0m;CH . Since DG0m;CH is independent of the chain length
2
3
in a homologous series, differences in the intercept essentially
reflected the reluctance of the head group to aggregate.
c
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Data obtained for [CnMIm][Cl] series were compared with those
described by Jungnickel et al.17 They followed the same linear
Table 3 Monomer and micellar diffusion coefficients obtained by
PGSE-NMR at 25 1C
variation with DGm0 ;CH ¼ ꢀ2:85 kJ molꢀ1 (dotted line in
Ibu a
Im
a
Ibu a
Im a
D
agg
2
Compound
CAC/mM
D
D
D
mon
mon
agg
Fig. 3). The DG0m;CH value is generally close to ꢀ3.0 kJ molꢀ1
2
[C4MIm]Ibu] 71
[C8MIm][Ibu] 14
5.2 ꢁ 0.2 7.4 ꢁ 0.2 0.8 ꢁ 0.6 3.6 ꢁ 0.4
5.5 ꢁ 0.1 6.5 ꢁ 0.2 0.3 ꢁ 0.3 1.2 ꢁ 0.2
since the transfer of a CH2 group from water to the micelle is
independent of the head-group structure.28 The dashed line in
10ꢀ6/cm2 sꢀ1
.
a
Fig. 3 represented the variation (DG0m;CH ¼ ꢀ2:85 kJ molꢀ1
)
2
for [CnMIm][Ibu] series. [C6MIm][Ibu] and [C8MIm][Ibu] followed
the same linear variation of DG0m, while [C4MIm][Ibu] showed
a slight deviation from it. The intercept corresponding to
diffusion coefficients obtained from different protons were the
same within ꢁ 5%.
DG0m;head group þ DGm0 ;CH was equal to ꢀ0.6 kJ molꢀ1 for
Fig. 4 depicts a plot of the measured diffusion coefficients
3
Ibu
meas
Im
and D
meas
[CnMIm][Cl] series and to ꢀ10.6 kJ molꢀ1 for [CnMIm][Ibu].
This confirms that imidazolium and ibuprofenate salts aggregate
as ion pairs and it is consistent with the low ionization degree
observed for these salts as compared to the parent chloride
salts (Table 2).28
D
of [C4MIm][Ibu] and [C8MIm][Ibu] as a
function of the reciprocal total surfactant concentration. This
representation was chosen to analyze the results according to
the pseudo-phase model.
In the low concentration range, the surfactant molecules
were in dynamic equilibrium between monomeric and micellar
states. If the exchange kinetics between the monomeric and
micellar states was fast on the diffusion time scale, the
measured diffusion coefficient Dmeas could be expressed as:34
PGSE-NMR
Pulsed gradient stimulated echo NMR (PGSE-NMR) was
used to determine self-diffusion coefficients of [C4MIm][Ibu]
and [C8MIm][Ibu] in aqueous solutions. Interestingly, the
CAC
Dmeas ¼ Dagg
þ
ðDmon ꢀ DaggÞ for Ct4CAC
ð5Þ
Ct
presence of protons on both the cation and the anion allowed
Ibu
measuring the diffusion coefficients of both species D
Im
for
meas
Dmeas = Dmon for Ct r CAC
(6)
the anion and D
for the cation. For each species, the
meas
where Dmon and Dagg represent the monomer and aggregates
diffusion coefficients respectively, and Ct is the total surfactant
concentration. Below the CAC, the pseudo-phase transition
model accounted well for the fact that the measured diffusion
coefficient was independent of the concentration and equal to
the diffusion coefficient of monomers. Above the CAC, for
concentrations sufficiently close to CAC, it was supposed that
the free monomer concentration remained constant.35 According
to eqn (5), variation of Dmeas against 1/Ct consisted of two
straight lines and the point of intersection of these two lines
was 1/CAC. CAC values obtained from the diffusion measure-
ments for the cation and the anion were identical, confirming
that they were both participating in the aggregates formation
(Table 3). Furthermore, the CAC values were in good agree-
ment with those obtained by other techniques (Table 2). As
expected, below the CAC, diffusion coefficients appeared
constant within the experimental error.
Ibu
Below CAC, D
of [C4MIm][Ibu] and [C8MIm][Ibu] was
Im
mon
nearly identical, whereas D
Im
mon
of [C8MIm][Ibu] was lower
mon
than D
of [C4MIm][Ibu] as expected from the increase in
the alkyl chain length. Micellar coefficients Dmic were obtained
by introducing the values of CAC and Dmon in eqn (5). Fig. 4
reveals that diffusion coefficients of imidazolium and ibuprofenate
decreased similarly upon aggregation, suggesting the formation
of mixed micelles. It should also be noticed that the slowing
down was more pronounced for [C8MIm][Ibu], demonstrating
easier aggregation in this case, due to the alkyl chains
interactions.
The fact that Dagg differed from one species to another
(it was higher for Im than for Ibu) suggested that the residence
time of the Ibu anion in the micelle was larger than that of the
Im cation. This means that the composition of the micelles was
richer in Ibu than in Im.
Ibu
Fig. 4 Variation of self-diffusion coefficients (KD
meas
Im
, ’ D
meas
) of
[C4MIm][Ibu] (top) and [C8MIm][Ibu] (bottom) as a function of the
reciprocal concentration at 25 1C.
c
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Conclusion
The surfactant properties of 1-alkyl-3-methylimidazolium ILs
(n = 4, 6, 8) containing an ibuprofenate anion have been
investigated using various techniques. The CAC values turned
out to be very low compared to the parent imidazolium
chlorides. This behaviour was ascribed to the surfactant
activity of ibuprofenate anions, through the formation of
catanionic pairs between ibuprofenate and imidazolium ions.
Actually, determination of surface parameters, Gibbs energy
of aggregation and diffusion coefficients allowed concluding
that both ions were involved in the aggregates. These aggregates
were richer in ibuprofenate anions. The here observed catanionic
behaviour took place with short alkyl chains on the imidazolium
cation, even though it was shown to be reinforced on increasing
the alkyl chain length. This result is all the more interesting as
catanionic pairing is usually observed only with long alkyl
chains (n > 8). Further studies as SANS analysis are needed to
determine the exact shape of the aggregates. The use of these
ILs as structuring agents in ionogels synthesis is currently
under investigation.
Fig. 5 Hydrodynamic mean radius of
’
[C4MIM][Ibu],
K [C6MIM][Ibu] and m [C8MIM][Ibu] with concentration above CAC.
Diffusion coefficients Dagg arising from NMR were used to
determine the hydrodynamic radius Rh according to Stokes–
Einstein equation:
kT
Dagg
¼
ð7Þ
6pZRh
Acknowledgements
where T is the absolute temperature, k the Boltzmann constant,
Z the solvent viscosity (Z(D2O) = 1.0511 cP at 300 K).
For [C4MIm][Ibu], Rh calculated from DIbu was 2.7 nm,
whereas that calculated from DIm was 0.6 nm. For [C8MIm][Ibu],
Rh calculated from DIbu was 7.5 nm, whereas that calculated
from DIm was 1.8 nm. These results show that the exact
hydrodynamic radius of our complex aggregates can be hardly
assessed from NMR data.
This work was supported by The Agence Nationale de la
Recherche ANR-10-JCJC-0802.
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the [CnMIm][Ibu] series above the CAC. Fig. 5 displays the
variation of the hydrodynamic radius with concentration. At
concentrations close to the CAC, two populations were observed
for [C4MIm][Ibu], a very small one with Rh E 0.5 nm and
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[C8MIm][Ibu], which ranged from 6 to 16 nm. The evolution
of the hydrodynamic radius is typical for micelles growing
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aggregates. However, as pointed by Leaist et al.,36 DLS
measurements have to be considered cautiously in the case
of ionic surfactants. Actually, this technique measures surfactant
mutual diffusion coefficients, including both contributions
from micelles and relatively mobile free surfactant monomers.
Due to the catanionic behaviour of [CnMIm][Ibu] solutions,
complex aggregated structures are expected, such as mixed-
micelles and vesicles.37 This could also explain the difficulty of
correlating these results with those arising from PGSE-NMR.
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