Aqueous-biamphiphilic ionic liquid systems: Self-assembly and synthesis of gold nanocrystals/microplates

Article abstract of DOI:10.1021/jp309717n

Biamphiphilic ionic liquids (BAILs) based on 1,3-dialkylimidazolium cation and alkyl sulfate anions ([C_nH_2n+1mim][C_mH _2m+1OSO₃]; n = 4, 6, or 8; m = 8, 12) have been synthesized and characterized for their self-assembling behavior in the aqueous medium. Effects of alteration of alkyl chain length in cation and anion on surfactant properties of BAILs have been examined from surface tension measurements. The effectiveness of surface tension reduction for BAILs has been found to be exceptionally higher as compared to single chain surface active ILs/conventional surfactants. The thermodynamics of the aggregation process has been studied using isothermal titration calorimetry (ITC) and temperature dependent conductivity experiments. Dynamic light scattering (DLS), nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM) studies showed that BAILs formed distinct aggregated structures depending upon the amphiphilic character present in the cation and anion. BAILs ([C _nH_2n+1mim][C_mH_2m+1OSO₃]) form micelles when n = 4, 6; m = 8, intermicellar aggregates when n = 4, 6; m = 12, and vesicles when n = 8; m = 8, 12. Gold nanoparticles and microplates have been synthesized in micellar and vesicle solutions of BAILs using a simple photoreduction method. The studies show the potential of BAILs for constructing micelles and supramolecular assemblies, such as bilayer vesicles, which are effective in preparation of nanomaterials of controlled size and morphology.

Full text of DOI:10.1021/jp309717n

Article
pubs.acs.org/JPCB
Aqueous-Biamphiphilic Ionic Liquid Systems: Self-Assembly and
Synthesis of Gold Nanocrystals/Microplates
K. Srinivasa Rao, Tushar J. Trivedi, and Arvind Kumar*
CSIR-Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), G. B. Marg,
Bhavnagar 364002, Gujarat, India
S
* Supporting Information
ABSTRACT: Biamphiphilic ionic liquids (BAILs) based on 1,3-dialkylimidazolium cation and alkyl sulfate anions
([CnH2n+1mim][CmH2m+1OSO3]; n = 4, 6, or 8; m = 8, 12) have been synthesized and characterized for their self-assembling
behavior in the aqueous medium. Effects of alteration of alkyl chain length in cation and anion on surfactant properties of BAILs
have been examined from surface tension measurements. The effectiveness of surface tension reduction for BAILs has been
found to be exceptionally higher as compared to single chain surface active ILs/conventional surfactants. The thermodynamics of
the aggregation process has been studied using isothermal titration calorimetry (ITC) and temperature dependent conductivity
experiments. Dynamic light scattering (DLS), nuclear magnetic resonance (NMR), and transmission electron microscopy
(TEM) studies showed that BAILs formed distinct aggregated structures depending upon the amphiphilic character present in
the cation and anion. BAILs ([CnH2n+1mim][CmH2m+1OSO3]) form micelles when n = 4, 6; m = 8, intermicellar aggregates when
n = 4, 6; m = 12, and vesicles when n = 8; m = 8, 12. Gold nanoparticles and microplates have been synthesized in micellar and
vesicle solutions of BAILs using a simple photoreduction method. The studies show the potential of BAILs for constructing
micelles and supramolecular assemblies, such as bilayer vesicles, which are effective in preparation of nanomaterials of controlled
size and morphology.
versatility in functionalization as compared to that of traditional
ionic surfactants.21,23,30 A comparatively higher solubility and
surface activity of SAILs in various solvents made them better
emulsifying agents for the preparation of microemulsions.34,35
SAILs have also been explored in other applications such as
formulation of aqueous biphasic systems for separations,36,37 as
a promotor in organic synthesis,38 or as promising capping/
stabilizing agents in the synthesis of nanomaterials.39−42
The substituents on the imidazolium cation, substitution at
the alkyl chain, and nature of the counterion have shown a
remarkable influence on the aggregation behavior of
ILs.12,19,20,24 It has been shown that the surface activity can
be enhanced and supramolecular structures can be formed by
mixing of SAILs, such as [C6Py][BF4], [C6Py][Br], or
[C12mim][Br], with a conventional surfactant sodium lauryl
1. INTRODUCTION
Ionic liquids (ILs) are liquids composed entirely of ions and
melts below 100 °C; they generally contain large (asymmetric)
organic cations with a variety of anions.1,2 The physicochemical
properties of ILs can be modified and tuned for desired
applications by simple variations in constituent ions, and hence,
these are popularly considered as designer solvents.3 Surfactant-
like ILs can be made by incorporation of amphiphilic character
in the constituent ions. In fact, the earlier studies on ILs
containing a 1-alkyl-3-methyl imidazolium moiety with a carbon
chain length of four and higher or an anion having n-octyl and
higher alkyl chain moiety have shown to behave like surfactants
and form well-defined aggregated structures in an aqueous
medium.4,5 On the basis of these preliminary studies, several
surface active ionic liquids (SAILs) were synthesized and their
aggregation behavior was investigated in aqueous and IL
media.6−33 These studies revealed that long chain imidazolium
ILs while acting as ionic surfactants show a somewhat superior
surface activity, better solubility in a variety of media, and
Received: October 1, 2012
Revised: November 15, 2012
Published: November 21, 2012
© 2012 American Chemical Society
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sulfate.43,44 In a recent study, we have demonstrated that, like
mixed cationic and anionic surfactant systems, the mixing of
two SAILs ([C4mim][C8OSO3] and [C8mim][Cl]) (mim:
methyl imidazolium) in different proportions can lead to
formation of micelles and vesicles.45 The supramolecular
structures generated from such systems were effectively used
for synthesis of hollow silica spheres and anisotropic gold
nanostructures.34,45
necked RBF and kept under reflux conditions for 3 h. Reaction
was monitored with TLC. After completion of reaction, water
was removed using a rotary evaporator and the product was
washed with ethyl acetate three times. Alkyl imidazolium
bromide thus obtained was dried and characterized using H
NMR and LC-MS techniques.
1
ii. Synthesis of 1-Alkyl-3-methyl Imidazolium Octyl Sulfate
or Lauryl Sulfate Ionic Liquids. An equimolar mixture of alkyl
imidazolium bromide and sodium alkyl sulfate salt was
dissolved in water and kept at 60 °C for 4 h. Reaction was
monitored with TLC. Water was removed from the reaction
mixture after completion of reaction using a rotary evaporator.
The product was extracted with dichloromethane (DCM), and
solid salt (NaBr) was filtered off. The product was washed with
water several times until complete removal of chloride ions
(monitored with acidic solution of AgNO3) and completely
dried prior to use. The NMR analysis of the synthesized BAILs
is given in the Supporting Information.
2.3. Methods. 2.3.1. Tensiometry. Surface tension
measurements were carried out at 298.15 K using a Data
Physics DCAT-II automated Tensiometer employing the
Wilhelmy plate method. Aliquots of BAIL stock solutions
were added to Millipore grade water by weight and stirred for
about 10 min for complete solubilization. Prior to measure-
ments, the resultant solutions were kept for at least 10 min for
equilibration. The data was collected in triplicate and was found
to be accurate within 0.1 mN m−1. The temperature of the
measurement cell was controlled with a Julabo water
thermostat to within 0.1 K.
2.3.2. Conductometry. Specific conductivity was measured
at four different temperatures in a temperature range of
288.15−318.15 K at 10 K intervals by a digital conductivity
meter (Systronics 308) using a cell with a unit cell constant.
The temperature of the measurement cell was controlled with a
Julabo water thermostat to within 0.1 K. The conductivity after
each addition was measured by adding aliquots of concentrated
BAILs aqueous solutions to a conductivity cell containing
water. Measurements were performed with an uncertainty of
less than 0.5%.
Observation of dual transitions in various physical properties
during the investigations on the aggregation behavior of 1-
butyl-3-methylimidazolium octyl sulfate, [C4mim][C8OSO3], in
aqueous medium46 prompted us to design and study the self-
assembling behavior of ILs having amphiphilic character in both
the constituent ions. Henceforth, we will term such ILs as
biamphiphilic ionic liquids (BAILs). The BAILs will behave as
catanionic surfactants with improved surface activity and
capability of forming a variety of self-assembled structures.
Recently, the BAILs based on systematic variation in alkyl chain
length of 1,3-dialkylimidazolium cation and alkyl sulfate or
sulfonate anions have been reported.47−49 Obliosca et al.47
synthesized a series of 1-alkyl-3-methylimidazolium lauryl
sulfates and characterized their absorption and photolumines-
cence properties. Santos et al.48 investigated the alkyl chain
interactions of 1,3-dialkylimidazolium cation and alkyl sulfate
ILs at the gas−liquid interface using sum frequency generation
studies and concluded that the surface is dominated by an alkyl
functional group. Mesomorphism of 1,3-dialkylimidazolium
cation and alkyl sulfonate anion ILs using X-ray scattering,
polarizing optical microscopy, and differential scanning
calorimetry has been investigated by Blesic et al.49 In the
same article, self-aggregation and surface adsorption character-
istics of synthesized ILs in an aqueous medium using surface
tension and fluorescence spectroscopy with pyrene as a probe
molecule is reported. These studies demonstrated that a
synergetic packing effect is responsible for lowering CMC
values and enhanced surface activity. However, none of these
studies shed light on the formation and stability of distinct self-
assembled structures formed by the BAILs when the
amphiphilicity of either the cation or anion is tailored.
In the present work, we synthesized a series of BAILs, based
on 1-alkyl-3-methylimidazolium cation and alkyl sulfates as
anion, and studied their self-assembling behavior in aqueous
medium. The role of the alkyl chain length of the cation or
anion on the formation of micelles, aggregated micelles, and
vesicles has been defined, and a detailed study has been carried
out to understand the superior surface active characteristics and
aggregation thermodynamics of the synthesized BAILs. Self-
assembeled BAIL media has been used to produce gold
nanoparticles/microplates through a photoreduction method.
The morphology and crystallinity of synthesized nanomaterials
has been examined through SEM and XRD techniques.
2.3.3. Isothermal Titration Calorimetry (ITC). Calorimetric
titration was performed with a MicroCal ITC200 micro-
calorimeter. The sample and reference cells were filled with
Millipore water and stabilized at 298.15 K. 40 μL of BAIL stock
solutions prepared in water were taken in an instrument
controlled Hamiltonian syringe, and 2 μL aliquots were added
to the sample cell containing 200 μL of water with continuous
stirring (500 rpm). The parameters like time of addition and
duration between each addition were controlled by software
provided with the instrument. The enthalpy change at each
injection was measured and plotted against concentration by
using origin software provided with the instrument.
2.3.4. Dynamic Light Scattering (DLS). DLS measurements
were performed at 298.15 K by using a NaBiTec Spectro-
Size300 light scattering apparatus (NaBiTec, Germany) with a
He−Ne laser (633 nm, 4 Mw). BAIL solutions of a
concentration above the CAC (∼10 times the CAC) were
filtered directly into the quartz cell using a membrane filter of
0.45 μm pore size. Prior to measurements, the quartz cell was
rinsed several times with filtered water and then filled with
filtered sample solutions. The temperature of the measure-
ments was controlled with an accuracy of 0.1 K. The data
evaluation of the dynamic light scattering measurements was
performed with the inbuilt CONTIN algorithm.
2. EXPERIMENTAL SECTION
2.1. Materials. 1-Bromo butane, 1-bromo hexane, and 1-
bromo octane of >98% purity were purchased from SRL, India.
1-Methyl imidazole and sodium octyl sulfate (SOS) of AR
grade were purchased from Spectrochem, India. Sodium
dodecyl sulfate (SDS) was purchased from Sigma-Aldrich,
Germany. Ethyl acetate and dichloromethane of AR grade were
procured from SD-fine chem. Ltd., India.
2.2. Synthesis of BAILs. i. Synthesis of 1-Alkyl-3methyl
Imidazolium Bromide. An equimolar mixture of 1-methyl
imidazole and 1-bromo alkane was dissolved in water in a two-
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Figure 1. Plots of surface tension as a function of concentration of BAILs ([CnH2n+1mim][CmH2m+1OSO3]) in aqueous solutions at 298.15 K.
Table 1. Critical Aggregation Concentration (CAC), Surface Tension at CAC (γCAC), Adsorption Efficiency (pC20), Effective
Surface Tension Reduction (ΠCAC), Maximum Surface Excess Concentration (Γmax), and Area Occupied by a Single Molecule at
the Air−Water Interface (Amin) of BAILs in Aqueous Medium at 298.15 K
[CnH2n+1mim]
[CmH2m+1OSO3]
n
m
CAC (mmol·kg−1
)
γCAC (mN·m−1
)
pC20
Πcac (mN·m−1
)
Γmax (μmol·m−2
)
Amin (Å2)
4
6
8
4
6
8
8
34.9
14.2
4.1
26.1
25.6
24.4
34.4
27.1
26.0
2.5
2.9
3.3
3.3
4.1
4.3
45.9
45.2
47.6
37.6
44.9
46.0
1.9
1.9
2.5
2.4
2.4
2.4
87.1
84.7
66.0
67.8
68.5
68.5
8
8
12
12
12
2.4
1.1
0.4
1
2.3.5. Nuclear Magnetic Resonance (NMR). The H NMR
spectra of aqueous BAIL solutions were recorded in a Bruker
500 MHz spectrometer using C6D6 as an external solvent.
2.3.6. Transmission Electron Microscopy (TEM). Samples
were prepared by putting a drop of BAIL solutions on the
carbon-coated copper grid (300 mesh). Residual liquid was
blotted immediately. Samples were imaged under a JEOL JEM-
2100 electron microscope at a working voltage of 80 kV.
2.3.7. Synthesis and Characterization of Gold Nanostruc-
tures. Absorption spectra of gold sols were recorded on a UV−
vis spectrophotometer (Cary 500, Varian). Scanning electron
micrographs of the gold nanostructures were taken from a LEO
1430 VP Carl Zeiss scanning electron microscope. X-ray
diffraction (XRD) patterns of the specimens were recorded by a
Rigaku MiniFlex-II Desktop X-ray diffractometer (Ni-filtered
Cu Kα radiation, k = 1.5404 Å).
good electrical charge distribution, the sulfate group shows
more propensity toward counterion binding.50 The marked
decrease in the CAC of BAILs is also due to high counterion
binding of less hydrated imidazolium counterions (an increase
of the chain length of imidazolium cation further reduces its
hydration and enhances counterion binding) which offers
effective screening between head groups.
The surface tension at CAC (γCAC) values obtained for
BAILs are given in Table 1. The lower γCAC values indicate a
densely packed arrangement of all BAILs at the air−water
interface. The γCAC values obtained for lauryl sulfate (m = 12)
based BAILs decreased in the following order: [C4mim] (34.4)
> [C6mim] (27.1) > [C8mim] (26.0), and are lower than their
sodium (34.9) or tetrapropylammonium (31.8) analogues.51
These values are comparable to the amino acid based ionic
liquid surfactants recently reported by our group.21,39 A similar
trend is followed by octyl sulfate (m = 8) based BAILs wherein
the order is as follows: [C4mim] (26.1) > [C6mim] (25.6) >
[C8mim] (24.4). The value of γCAC of BAILs is comparable to
the lowest reported for imidazolium based trichain anionic
surfactant [C4mim][TC]51 and analogous fluorocarbon surfac-
tants.51 The lowest γCAC value for [C8mim][C8OSO3] is due to
good alkyl chain compatibility because of the same cationic and
anionic alkyl chain length and is consistent with the previous
observation that the lowest surface tensions are reached when
the cation and anion tails are similar in size.51 Low γCAC for
BAILs as compared to the previously reported imidazolium
based SAILs are indicative of a densely packed interface. As
discussed earlier, the increased population of BAILs at the air−
water interface is due to effective screening offered by the
counterion, van der Waals interaction between alkyl chains of
the cation and anion along with existing H-bonding and
electrostatic interaction between sulfate and imidazolium
3. RESULTS AND DISCUSSION
3.1. Tensiometry. Figure 1 shows the change in surface
tension (γ) of water as a function of the concentration of
various BAILs, 1,3-dialkylimidazolium alkyl sulfate
([CnH2n+1mim][CmH2m+1OSO3]; n = 4, 6, or 8; m = 8, 12) at
298.15 K. In all the cases, γ decreases with the increase in BAIL
concentration before reaching a constant value. The break
points in the surface tension vs concentration plot give the
critical aggregation concentration (CAC). Linear fits just before
and after break points provided the CAC (Table 1). It has been
observed that, with the increase in cationic chain length from n
= 4 to 8, the CAC values decrease from 35 to 4 mmol·kg−1
(∼8-fold) when m = 8 and from 2.3 to 0.5 mmol·kg−1 (∼5-
fold) when m = 12, indicating a significant improvement on the
surface activity of BAILs with an increase in chain length of
both the imidazolium counterion and anion. Because of the
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Figure 2. Plots of specific conductivity as a function of concentration of BAILs ([CnH2n+1mim][CmH2m+1OSO3]) in aqueous solutions at 298.15 K.
Table 2. Critical Aggregation Concentration (CAC), Counterion Binding (β), Standard Gibbs Free Energy of Aggregation
(ΔGa°gg), Enthalpy of Aggregation (ΔH°agg), and Entropy of Aggregation (ΔS°agg) of BAILs in an Aqueous Medium at Different
Temperatures
[CnH2n+1mim]
[CmH2m+1OSO3]
n
m
T (K)
CAC (mmol·kg−1
)
β
ΔGa°gg (kJ·mol−1
)
ΔHa°gg (kJ·mol−1
)
TΔSa°gg (kJ·mol−1
)
4
8
288.15
298.15
308.15
318.15
288.15
298.15
308.15
318.15
288.15
298.15
308.15
318.15
38.5
35.8
35.0
36.9
23.7
22.2
22.1
23.1
5.8
0.70
0.71
0.71
0.66
0.89
0.88
0.87
0.88
0.91
0.94
0.91
0.89
−30.01
−31.28
−32.26
−32.14
−35.11
−36.49
−37.56
−38.65
−41.88
−44.32
−44.91
−45.71
22.29
0.26
52.31
31.54
11.91
7.52
−20.35
−39.66
5.55
6
8
8
8
40.66
37.08
33.52
30.25
73.41
48.36
23.25
−0.04
0.59
−4.05
−8.39
31.53
4.04
5.6
5.7
−21.66
−45.74
6.1
Figure 3. Temperature dependent conductivity profiles of aqueous-BAIL ([CnH2n+1mim][CmH2m+1OSO3]) solutions: (A) n = 8; m = 8; (B) n = 8; m
= 12.
πCAC = γ − γCAC
(2)
moieties. The adsorption efficiency, pC20, was calculated from
0
surface tension vs concentration graphs using eq 1.
where γ0 is the surface tension of pure solvent and γCAC is the
surface tension of the solution at CAC. This parameter
indicates the maximum reduction of surface tension caused by
the dissolution of surfactant molecules and hence becomes a
measure for the effectiveness of the surfactant to lower the
surface tension of the solvent. The values of pC20 and πCAC
parameters obtained for the studied BAILs are given in Table 1.
The adsorption efficiencies of BAILs with n = 4, 6, 8; m = 8 and
n = 4; m = 12 are similar or even higher than conventional
pC20 = −log C20
(1)
where C20 is the concentration required to reduce the surface
tension of pure solvent by 20 mN·m−1. A higher pC20 value is
indicative of a higher adsorption efficiency of the surfactant at
the interface and more efficiency in reducing surface tension.
Surface pressure at CAC is defined as
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Figure 4. Temperature dependence of the CAC of BAILs ([CnH2n+1mim][CmH2m+1OSO3]). (n = 8 corresponds to the right y-axis, also marked by
an arrow in parts A and B.)
Table 3. Critical Aggregation Concentration (CAC), Counterion Binding (β), Standard Gibbs Free Energy of Aggregation
(ΔGa°gg), Enthalpy of Aggregation (ΔH°agg), and Entropy of Aggregation (ΔS°agg) of BAILs in an Aqueous Medium at Different
Temperatures
[CnH2n+1mim]
[CmH2m+1OSO3]
n
m
T (K)
CAC (mmol·kg−1
)
β
ΔGa°gg (kJ·mol−1
)
ΔHa°gg (kJ·mol−1
)
TΔSa°gg (kJ·mol−1
)
4
12
288.15
298.15
308.15
318.15
288.15
298.15
308.15
318.15
288.15
298.15
308.15
318.15
2.9
2.6
2.8
3.1
1.1
0.9
1.0
1.3
0.4
0.4
0.4
0.4
0.69
0.69
0.68
0.67
0.80
0.82
0.79
0.75
0.77
0.84
0.86
0.84
−39.96
−41.66
−42.53
−43.19
−46.62
−49.76
−50.10
−49.36
−50.11
−54.31
−56.80
−58.06
14.36
−2.65
−18.55
−33.46
67.44
54.32
39.01
23.98
9.73
6
8
12
12
114.06
53.65
−5.44
−61.88
143.21
99.62
57.42
16.79
3.88
−55.54
−111.24
93.09
45.31
0.61
−41.27
single chain surfactants or imidazolium based SAILs, whereas
the BAILs with n = 6, 8; m = 12 show a somewhat higher
adsorption efficiency which is comparable to amino acid ionic
liquid surfactants.8,15−21,52,53 Similarly, πCAC of BAILs is
comparable to the amino acid ionic liquid surfactants, whereas
it is quite higher than conventional single chain surfactants or
imidazolium based SAILs.8,15−21,52,53
The maximum surface excess concentration, Γmax, and the
area occupied by a single surfactant molecule at the air−water
interface, Amin, of BAILs which are mainly dependent upon the
size of the hydrophilic headgroup were estimated by applying
the Gibbs adsorption isotherm to the surface tension data.54,55
Fitting of γ values below the CAC to a second degree
polynomial equation provided the slope required for estimation
of Γmax. The values of parameters derived from γ are reported in
Table 1. Effective screening of like charge between head groups
by the imidazolium counterions results in lowering of Amin
which in turn causes an increase in Γmax. The Γmax values
observed for BAILs are higher than SAILs8,15−21 and
conventional anionic surfactants having small as well as bulky
organic counterions, like tetra alkyl ammonium ions.56
(Table 2) obtained for the BAILs from κ measurements are
fairly in good agreement with the values obtained from γ values.
Plots of the temperature dependence of κ vs concentration for
the representative BAILs, [C8mim][C8OSO3] and [C8mim]-
[C12OSO3], are shown in Figure 3A and B, respectively,
whereas the plots for other BAILs are provided in the
Supporting Information, Figure S1. Similar to the conventional
surfactants57 and imidazolium based SAILs,58,59 a U-shape
change in CAC was observed with an increase in temperature
from 288.15 to 318.15 K for all studied BAILs (Figure 4). The
ratio of the slope of linear fragments above and below the CAC
gives an estimate of counterion binding, β. Estimated values of
β for BAILs are given in Tables 2 and 3. Similar to the tetra
alkyl ammonium salts, high values of β are observed for
BAILs.60 Expectedly, an increase in β has been observed with
the increase in alkyl chain length of imidazolium cation because
of enhanced hydrophobicity. Even higher values of β observed
in the cases of [C8mim][C8OSO3] and [C8mim][C12OSO3]
are due to the involvement of alkyl chains of both cation and
anion in the aggregated structures. The standard Gibbs free
energy of aggregation, ΔGa°gg, was calculated using the
equation61
3.2. Conductometry. Conductivity measurements were
performed at four different temperatures in the range 288.15−
318.5 K. Plots of specific conductance, κ, as a function of BAIL
([CnH2n+1mim][CmH2m+1OSO3]) concentration at 298.15 K
are shown in Figure 2. The intersection of straight lines fitted
for κ before and after slope change depicts the CAC. The CACs
°
ΔGagg = (1 + β)RT ln XCAC
(3)
where XCAC is the mole fraction of BAILs at the CAC. Negative
ΔGa°gg indicates spontaneity of the process, Table 2. ΔG°agg
becomes more negative with the increase of the alkyl chain
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Figure 5. Calorimetric profiles of BAILs ([CnH2n+1mim][CmH2m+1OSO3]) in aqueous solutions at 298.15 K. (A, B) Representative raw titration data
of heat flow versus time, and (C, D) enthalpy of dilution as a function of the concentration of BAILs.
length of the cation and anion. When compared to conven-
tional ILs/surfactants, the magnitude of ΔG°agg for BAILs was
found higher, indicating that the aggregation process is more
feasible in these compounds.19,22,24,33,57,60
The enthalpy of aggregation, ΔG°agg, was derived by applying
the Gibbs−Helmholtz equation using the calculated free energy
of aggregation
BAILs (n = 4, 6; m = 8), whereas only endothermic changes
were observed for the BAILs (n = 8; m = 8, n = 4, 6, 8; m = 12)
throughout the dilution process (the representative plots are
given in Figure 5A,B, and plots of other BAILs are provided in
the Supporting Information, Figure S2). BAILs with n = 4, 6; m
= 8 followed type B enthalpograms, wherein a positive slope
change in ΔH values was observed before the CAC and
thereafter the ΔH acquired a plateau region, Figure 5C. Positive
slope change in ΔH values for these BAILs is due to solute−
solute interactions because of high CACs (35.2 and 17.9 mm,
respectively). BAILs with n = 4, 6, or 8; m = 12 and n = 8; m =
8 exhibited two plateau regions in enthalpy values before and
after the CAC (Figure 5D) which correspond to the type A
enthalpograms.62 Type A enthalpograms are known as textbook
examples which exhibit constant ΔH changes before and after
aggregation with a sharp change at the CAC. The CAC values
obtained by the calorimetric method for all BAILs are reported
in Table 4, and are in fairly good agreement with those
obtained from conductivity and surface tension experiments.
The observed ΔH°agg was found to be positive for BAILs with n
= 4; m = 8 and n = 6; m = 8, while it was negative for the other
BAILs and the magnitude increased with the increase in
substituted alkyl chain of imidazolium cation or alkyl sulfate
anion following the order n = 8; m = 8 < n = 4; m = 12 < n = 6;
m = 12 < n = 8; m = 12. Thermodynamic parameters (ΔGa°gg,
ΔHa°gg, and ΔSa°gg) obtained from ITC are reported in Table 4.
The thermodynamic parameters obtained from conductivity
and calorimetric techniques differed in magnitude. The
discrepancy arises due to contribution of a different
physiochemical process other than the aggregation in
calorimetric determination of enthalpy change.63 The dynamic
nature of the aggregation process and the counterion binding of
aggregates are considered to influence the ΔH°agg of the process
°
∂(ΔGagg/T)
°
ΔHagg
=
∂(1/T)
(4)
It has been observed that the changes in ΔG°agg/T with 1/T are
nonlinear, and therefore, the slope was calculated through
nonlinear second order curve fitting. Like conventional
surfactants, the obtained ΔH°agg was found to be positive at
lower temperature and negative at higher temperature for all
the studied BAILs (Tables 2 and 3).
Calculated values of ΔG°agg and ΔHa°gg were used to estimate
the standard entropy of aggregation from the following relation
°
°
(ΔHagg − ΔGagg)
°
ΔSagg
=
(5)
T
Similar to ΔHa°gg, ΔSa°gg decreased with the increase in
temperature for all the BAILs. For all the BAILs, a high
magnitude of ΔS°agg at low temperatures indicated that the
aggregation process is mainly entropy driven, whereas it is
enthalpy favored at high temperature.
3.3. Isothermal Titration Microcalorimetry (ITC).
Isothermal calorimetric titration (ITC) was performed to
measure the enthalpy changes upon aggregation, ΔH°agg, for all
BAILs ([CnH2n+1mim][CmH2m+1OSO3]) at 298.15 K. Differ-
ential power (dP) values with time show exothermic and
endothermic changes before and after CAC, respectively, for
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shown in Figure 6. DLS results show the formation of micelles
in the aqueous solutions of [C4mim][C8OSO3] and [C6mim]-
[C8OSO3], with Rh = 2.9 and 9.1 nm, respectively, whereas the
aqueous solutions of [C4mim][C12OSO3] and [C6mim]-
[C12OSO3] formed aggregates of micelles of Rh = 260.1 and
260.9 nm, respectively. TEM images (Figure 7A−D) also
confirm formation of micelles and intermicellar structures. The
higher hydrophobicity of lauryl sulfate along with a sufficiently
longer alkyl chain of cation is perhaps the driving force for
aggregation of micelles. The intermicellar aggregates are formed
by bridging of imidazolium hydrophobic chains between
individual micelles of lauryl sulfate similar to that reported
for lauryl sulfate micelles by TBA+ ions, via hydrophobic
interactions between butyl chains of the ions.65 These
intermicellar hydrophobic bridges could be easily broken with
the addition of a simple electrolyte, such as NaCl. The effect of
addition of NaCl on Rh of intermicellar aggregates of
[C4mim][C12OSO3] and [C6mim][C12OSO3] is shown in the
inset of Figure 6C. Aggregated micelles of [C4mim][C12OSO3]
(Rh ∼96 nm) and [C6mim][C12OSO3] (Rh ∼113 nm) acquired
individual micellar sizes of ∼5 and 20 nm at 100 and 20 mM
NaCl concentration, respectively (Figure 6C), indicating a
higher propensity of the longer alkyl chain cation of the latter
to participate in the aggregated structure. We speculate that the
interactions of Cl− ions with imidazolium cation in the palisade
layer of micelles reduce the possibility of bridging of
imidazolium hydrophobic alkyl tails resulting in separation of
individual micelles. However, further investigations may be
required to understand the exact mechanism of breaking of
intermicellar aggregates under the influence of electrolytes.
Table 4. Isothermal Titration Microcalorimetry Derived
Critical Aggregation Concentration (CAC), Standard Gibbs
Free Energy of Aggregation (ΔG°agg), Enthalpy of
Aggregation (ΔH°agg), and Entropy of Aggregation (ΔS°agg) of
BAILs in an Aqueous Medium at 298.15 K
[CnH2n+1-
mim]
[CmH2-
m+1OSO3]
CAC
ΔGa°gg
ΔHa°gg
TΔSa°gg
n
m
(mmol·kg−1
)
(kJ·mol−1
)
(kJ·mol−1
)
(kJ·mol−1
)
4
6
8
4
6
8
8
34.7
18.8
5.4
−18.3
−20.1
−33.5
−37.1
−39.7
−48.7
1.6
0.7
19.4
20.8
30.2
32.8
34.6
38.9
8
8
−3.3
−4.3
−5.1
−9.8
12
12
12
2.6
1.1
0.4
which are usually not rigorously considered in conductivity data
treatment. In the direct determination of the ΔHa°gg by
calorimetry, the consequence of the above effects is, on the
other hand, included in the measurement. ITC and κ
measurements revealed that, similar to the conventional
surfactants63 and imidazolium based SAILs,64 the aggregation
process for the studied BAILs is entropy driven at 298.15 K.
3.4. Characterization of Self-Assembled Structures.
Self-assembled structures of BAILs ([CnH2n+1mim]-
[CmH2m+1OSO3]) in aqueous medium were examined at a
concentration 10 times to that of CAC. The hydrodynamic
radii, Rh, of the aggregated structures measured from DLS are
Figure 6. Size distribution of BAIL ([CnH2n+1mim][CmH2m+1OSO3]) aggregates in water at a concentration 10 × CAC (A, B). Size distributions of
−1
□
○
[C4mim][C12OSO3] ( ) and [C6mim][C12OSO3] ( ) in 20 and 100 mmol·kg aqueous NaCl solutions, respectively (the inset shows the
variation in Rh at varying NaCl concentration) (C). Size distributions of [C8mim][C8OSO3] and [C8mim] [C12OSO3] in 20 mmol·kg−1 aqueous
NaCl solutions (D).
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are shown in Figure 8, and the changes for protons of the alkyl
chain of both the cation and anion are shown in the Supporting
Information (Figures S4 and S5). There is a clear peak
broadening in aqueous solutions of [C8mim][C8OSO3] and
[C8mim][C12OSO3] after the CAC, while in the other cases no
change in peak shape is observed, indicating the vesicle
formation in these BAILs (also evidenced from DLS and
TEM). The broadening indicates the involvement of a cation
along with an anion in the self-assembly. The balanced
interactions between head groups, alkyl chains, and compati-
bility of alkyl chains of both of the ions (cation and anion) are
responsible for the vesicle formation. The BAILs, [C4mim]-
[C12OSO3] and [C6mim][C12OSO3], though they formed
bigger structures, which are even bigger than the vesicles
formed by [C8mim][C12OSO3], no broadening of NMR peaks,
no turbidity in their aqueous solutions after the aggregation
concentration, and reversal to individual micellar sizes after
addition of an electrolyte unlike in the aqueous solutions of
[C8mim][C8OSO3] or [C8mim][C12OSO3] rule out vesicle
formation and confirm that these structures are intermicellar
aggregates.
3.5. Synthesis of Gold Nanoparticles/Microplates in
Aqueous BAIL Solutions. The aqueous solutions of BAIL
micelles and vesicles were used as a medium for synthesis of
gold nanostructures. 1 mM HAuCl4 aqueous-BAIL solutions of
varying BAIL concentrations ranging from just above the CAC
to well above the CAC were prepared. Light (in micellar
medium) and dark (in vesicular medium) yellow color aqueous
solutions of HAuCl4/BAILs were kept under UV lamp for
photochemical reduction. Gold nanoparticle formation was
monitored by visual observation of color change and confirmed
by taking UV absorbance of reduced Au3+ ion solutions. The
UV spectra of irradiated solutions of low and high BAIL
concentrations are shown in Figure 9A and B, respectively.
Absorbance peaks around 550 nm are indicative of the
formation of gold nanostructures. It is observed that Au3+
ions were reduced to Au (0) in 10 min in the micellar medium,
whereas a quite longer period of 4 h was required for the
reduction process in the vesicular medium. SEM images
(Figure 10A−D) revealed that nearly spherical nanoparticles
of ∼200 nm were formed at lower BAIL concentrations in all
the solutions except that in the solutions containing [C8mim]-
[C8OSO3], where some anisotropy was observed. With the
increase of BAIL concentration in the solution, the propensity
to form microplates increased and uniformly distributed
microplates could be obtained at certain higher concentrations
of all the BAILs except [C4mim][C8OSO3] and [C6mim]-
[C8OSO3]. Very thin microplates were formed in a micellar
medium, whereas microplates of nearly 70 nm thickness were
formed in vesicular solutions, Figure 10E−H. Gao et al.
provided a probable mechanism of gold nanoparticle formation
in imidazolium solutions wherein the imidazolium cation of
Figure 7. TEM images of micelles, aggregated micelles, and
unilamellar vesicles formed by BAILs ([CnH2n+1mim]-
[CmH2m+1OSO3]) in water. (A, B) n = 4, 6; m = 8, (C, D) n = 4, 6;
m = 12, and (E, F) n = 8; m = 8, 12.
The aqueous solutions of [C8mim][C8OSO3] or [C8mim]-
[C12OSO3] were turbid and formed self-assembled structures
with Rh = 328 and 187 nm, respectively, at the concentrations
∼10 CAC (Figure 6), suggesting the formation of bilayer
structures, such as vesicles. The vesicle size of [C8mim]-
[C8OSO3] was found to grow as we increased the
concentration above the CAC (Figure S3, Supporting
Information). Alkyl chain compatibility in the case of C8C8
may be the reason for growth in vesicle size with the increase in
concentration. Unlike intermicellar aggregates of [C4mim]-
[C12OSO3] and [C6mim][C12OSO3], the structures formed in
the aqueous solutions of [C8mim][C8OSO3] or [C8mim]-
[C12OSO3] did not collapse with the addition of NaCl,
suggesting the formation of vesicles in these BAILs, Figure 6D.
Unilamellar vesicles formed by these BAILs were imaged from
TEM (Figure 7E,F). The images recorded around the CAC
showed nearly similarly sized spherical shaped vesicles for both
[C8mim][C8OSO3] and [C8mim][C12OSO3]. The self-as-
sembled structures can be distinguished from NMR peak
shapes of the cationic and anionic protons involved. Formation
of compact and higher self-assembled structures, like vesicles,66
leads to peak broadening because of restricted movement and
−
BAILs interacts with gold precursor AuCl4 ions through H-
−
bond formation between ring protons and chlorine of AuCl4
ions. The CC olefinic bond of imidazolium formed in the
process is readily oxidized into an epoxide, alcohol, and finally
−
to a ketone by AuCl4 into aqueous medium which are then
adsorbed on the surface of Au particles and guiding to
formation of nanostructured microsheets.67 Figure 11 displays
the X-ray diffraction (XRD) patterns of the representative
samples prepared in a vesicular solution of low and high BAIL
concentration. The peaks of the scattering angles (2θ) exhibit a
1
shorter relaxation times of nuclei. Therefore, H NMR spectra
of aqueous BAIL solutions were recorded before and after the
CAC. The changes in peak shapes of imidazolium ring protons
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Figure 8. Changes in peak shapes of imidazolium ring protons, (A) 2H and (B) 4,5H of BAILs ([CnH2n+1mim][CmH2m+1OSO3]) before (left) and
after (right) aggregation. 1−3 (n = 4, 6, 8; m = 8) and 4−6 (n = 4, 6, 8; m = 12). Peak broadening in 3 and 6 is indicative of vesicle formation.
Figure 9. UV−vis/NIR spectrum of UV irradiated 1.0 mmol chloro auric acid solutions containing BAILs ([CnH2n+1mim][CmH2m+1OSO3]). (A)
∼CAC and (B) ∼200 mmol·kg−1 (for n = 8; m = 12, the concentration was ∼70 mmol·kg−1).
highly crystalline nature of gold nanostructures with a face-
centered cubic (fcc) packing arrangement.
superior surface active properties than SAILs and conventional
surfactants. The process of aggregation is found to be entropy
driven at lower temperatures and enthalpy driven at higher
temperatures for all the BAILs in the investigated temperature
range. BAILs formed distinct aggregated structures depending
on the amphiphilicity of the cation or anion. BAILs with butyl
or hexyl substitution in imidazolium cation and having octyl
sulfate as anion formed spherical micelles of Rh = 2.9 and 5.1
nm, whereas large intermicellar aggregates of Rh ∼260 nm were
formed when the anion was replaced by lauryl sulfate. The
4. CONCLUSIONS
Biamphiphilic ionic liquids (BAILs) based on 1,3-dialkylimida-
zolium cation and alkyl sulfate anions ([CnH2n+1mim]-
[CmH2m+1OSO3]; n = 4, 6, or 8; m = 8, 12) have been
synthesized and characterized for self-assembling behavior in
their aqueous solutions. Synthesized BAILs have shown
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Figure 10. SEM images of gold nanoparticles and microplates synthesized by the photochemical method in aqueous BAILs ([CnH2n+1mim]-
[CmH2m+1OSO3]) solutions of lower (A−D) and higher concentrations (E−H), respectively. (A, E) n = 4; m = 12, (B, F) n = 6; m = 12, (C, G) n =
8; m = 12, and (D, H) n = 8; m = 8.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mailme_arvind@yahoo.com; arvind@csmcri.org.
Phone: +91-278-2567039. Fax: +91-278-2567562.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.S.R. is thankful to CSIR for a senior research fellowship. The
authors are thankful to Department of Science and Technology
(DST), Government of India, for financial support for this
work (Nos. SR/S/PC-55/2008 and SR/S/PC-04/2010). The
Figure 11. Representative X-ray diffraction pattern of gold
nanostructures prepared in (A) low [C8mim][C12OSO3] concen-
tration and (B) high [C8mim][C12OSO3] concentration solutions.
analytical division of CSMCRI is also acknowledged for helping
in sample characterization. The authors thank Dr. Tejwant
Singh for helpful discussions.
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