Journal of Colloid and Interface Science 335 (2009) 105–111

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Journal of Colloid and Interface Science

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Spontaneous vesicle formation with an ionic liquid amphiphile

Kulbir Singh a, D. Gerrard Marangoni a,*, Jason G. Quinn b, Robert D. Singer b,*

a Dept. of Chemistry, St Francis Xavier University, P.O. Box 5000, Antigonish, NS, Canada B2G 2W5b Dept. of Chemistry, Saint Mary’s University, 923 Robie Street, Halifax, NS, Canada B3H 3C3

a r t i c l e i n f o

Article history:Received 23 December 2008Accepted 3 March 2009Available online 5 April 2009

Keywords:Catanionic systemsIonic liquidsVesiclesPCSNOESY

0021-9797/$ - see front matter � 2009 Published bydoi:10.1016/j.jcis.2009.03.075

* Corresponding authors. Fax: +1 902 867 2414 (D5261 (R.D. Singer).

E-mail addresses: gmarango@stfx.ca (D.G. Maran(R.D. Singer).

a b s t r a c t

A simple and effective method for the formation of stable multilamellar vesicles is reported as a potentialapplication of ionic liquid materials (IL’s) and as replacements for conventional surfactants used in suchapplications. The methodology is based on the various approaches for the formation of vesicles fromoppositely charged surfactants. Photon correlation spectroscopy (PCS) and transmission electron micros-copy (TEM) have been used to estimate the size of the aggregates; the TEM studies have also revealedmorphological differences in the self-assembled systems with changing ionic liquid material. Size mea-surements from PCS indicate consistent growth of the ionic-liquid containing vesicles with increasingconcentration of added anionic surfactant. 2D NOESY NMR spectroscopy have been used to examinethe manner in which IL amphiphile self-assembles with the second surfactant in solution. A comparisonhas been made between the aggregates formed from hexylpyridinium tetrafluoroborate (½HexPy�½BF4

��Þ/sodium dodecylsulfate (SDS) and hexylpyridinium bromide ([HexPy][Br])/sodium dodecylsulfate (SDS).

� 2009 Published by Elsevier Inc.

1. Introduction

Vesicles represent one class of self-assembly formed by phos-pholipids and synthetic surfactants. Numerous reports on the gen-eration of vesicles from single surfactants or from mixtures ofanionic and cationic surfactants (referred to as catanionic vesicles)have appeared in the literature over the last several years [1–3].Vesicles have abundant applications in colloidal chemistry and inbiology. It is expected that catanionic vesicles might also be usedas vehicles for controlled delivery of drugs and as a template forthe synthesis of hollow particles [4]. The formation of catanionicvesicles is spontaneous and can be tailored to form aggregates ofvarying sizes by varying the surfactant ratio, the chain length,the nature of polar group, or by adding salt. An important, butnot mandatory condition for the formation of vesicles from singleor mixed surfactants is that the packing parameter, P, is in therange 1/2 < P < 1 [5,6]. The type of aggregates that form with ionicsurfactants can be broadly understood in terms of the subtle bal-ance between forces due to the packing properties of surfactanttails and those due to double layer electrostatic interactions. As acationic counter part, room temperature ionic liquids (IL’s) canbe used to modify catanionic systems in variety of ways in additionto those mentioned above. IL’s are composed of a charge-diffuseinorganic anion such as PF6

� or BF4� coupled with long amphi-

Elsevier Inc.

.G. Marangoni), +1 902 420

goni), Robert.Singer@smu.ca

philic chains such as N-alkylpyridinium or 1,3-dialkylimidazoliumions. As such, these basic structures immediately suggest that IL’smay self-assemble in solution, in a manner similar to ionic surfac-tants and, if mixed with oppositely charged surfactants, can lead toconditions favourable for the formation of higher order aggregates,such as vesicles. The vast majority of studies on the amphiphilicnature of IL’s has been related to the chain length of the alkyl sub-stituent on the cation, and it has been observed that the cationicalkyl chain length of IL’s has a direct role on the physical and chem-ical properties in ionic liquids, in a manner analogous to those ofcationic surfactants. As an example, some recent investigationson the aggregation behavior of 1-methyl-3-alkylimidazolium saltsin aqueous solution [7,8] have confirmed that the self-assemblydoes occur in aqueous solutions of ionic liquids. The morphologyof the aggregates thus formed is dependent on both the alkyl chainlength and the nature of the counterion. Tesfai et al. [9] haveachieved surfactant assisted metal nanoparticle formation usingIL’s demonstrating that the interaction of IL’s with surfactant isan attractive and novel area of research. Recently, Triolo et al. per-formed a molecular dynamics simulation study that identified theoccurrence of nanoscale aggregation in IL’s [10,11]. More recently,experimental evidence for the existence of nanoscale heterogene-ities in IL’s and supercooled room temperature IL’s, such as1-alkyl-3-methylimidazolium based salts, was reported [12]. Inthis paper, we demonstrate a simple and efficient method for theformation of stable vesicles and nanoscale aggregates betweentwo short chain cationic amphiphiles, hexylpyridinium tetrafluoro-borate (½HexPy�½BF4

��Þ, hexylpyridinium bromide and ([HexPy][Br])and the anionic surfactant, sodium dodecylsulfate (SDS). Photon

106 K. Singh et al. / Journal of Colloid and Interface Science 335 (2009) 105–111

correlation spectroscopy (PCS), and transmission electron micros-copy (TEM) have been used to confirm the presence of vesicles.The size variation has been observed by changing the compositionof the cationic anionic surfactant mixture. 2D NOESY NMR spec-troscopy has been used to examine the manner in which the catan-ionic systems self-assemble in solution. Significant differences inthe morphology of the self-assembled systems were observed fromTEM when the IL material was the cationic counterpart in themixed aggregate over the more conventional amphiphilic counter-ion. The differences between the shapes and sizes of the aggregatesformed in these solutions are suggested to be related to the pecu-liar solution behaviour of the ionic liquid system.

2. Experimental

2.1. Materials

Sodium dodecyl sulfate (SDS) was purchased from Sigma Al-drich and was used as received. [HexPy][BF4] was synthesizedaccording to a modification of a method reported elsewhere [13].1-Bromohexane (10.39 mL, 74 mmol) was added to pyridine(4.92 mL, 62 mmol) in an oven dried round bottom flask andheated in water in a microwave (25 � 1 min.at 320 watts) whilestirring between microwave intervals. The resulting mixture waswashed with ethyl ether (3 � 20 mL) to remove excess reagentsand placed under high vacuum for 8 h to remove residual solventto afford a quantitative yield of N-hexylpyridinium bromide. N-hexylpyridinium bromide (3.96 g, 16 mmol) and NaBF4 (2.1 g,19 mmol) were dissolved in dry acetonitrile and stirred for 3 days.The resulting mixture was decolourized with charcoal and filtered.Concentration in vacuo then afforded N-hexylpyridinium tetrafluo-roborate (2.7 g, 67.5%).

[n-HexPy][Br]: 1H NMR (DMSO d6): d 9.4 (m, 2H), 8.4 (m, 1H), 8.0(m, 2H), 4.8 (t, J = 7.5 Hz, 2H), 1.8 (m, 2H), 1.1 (m, 6H), 0.6 (t,J = 7.0 Hz, 3H); 13C NMR (DMSO d6): d 145.1, 145.0, 128.4, 61.6,31.8, 30.9, 25.4, 22.1, 13.7.; MS (ESI, positive mode): m/z 164.1 (100).

[n-HexPy][BF4]: 1H NMR (DMSO d6): d 9.16 (m, 2H), 8.7 (m, 1H),8.3 (m, 2H), 4.7 (t, J = 7.3 Hz, 2H), 2.0 (m, 2H), 1.3 (m, 6H), 0.9 (t,J = 6.5 Hz, 3H); 13C NMR (DMSO d6): d 146.9, 146.0, 129.5, 62.4,32.1, 32.0, 26.5, 23.3, 15.; MS (ESI, positive mode): m/z 164.1(100) ½M�BF4

��þ; MS (ESI, negative mode): m/z 86.9 (100) BF4�.

2.2. Methods

2.2.1. Photon correlation spectroscopyBefore mixing, the IL solution and surfactant solution both were

filtered through an Anotop membrane filter of pore size 0.02 lm.All measurements were carried out using a Zetasizer� Nano (NanoZS), from Malvern instruments, UK. The instrument was calibratedbefore each set of measurements. A quartz cell (with lid) was usedfor all measurements. The test solution was taken in the quartz celland measurement was performed. During all experiments theinstrument was set on automatic attenuation, high resolution,and was allowed to seek optimum position for the measurement.The IL was prepared at a stock concentration of 0.155 mol L�1;the surfactant-IL mixtures were prepared by titrating in the appro-priate quantity of a stock SDS solution (0.0600 mol L�1) into a por-tion of the IL containing solution and mixing. Photon correlationspectroscopy experiments measure the intensity correlation func-tion (G2(s)) of light scattered from a sample:

G2ðsÞ ¼hItItþsihIti2

ð1Þ

where It is the intensity of scattered light at time t and It+s is theintensity of scattered light after the delay time, s. The intensity cor-relation function can be expressed in terms of the field correlation

function (G1(s)) via the Siegert relation (for an optical field obeyingGaussian statistics):

G2ðsÞ ¼ 1þ jG1ðsÞj ð2Þ

In the case of clearly monomodal distributions, the diffusion coeffi-cient (D) was determined from the peak position, which indicatesthe characteristic decay time, sc:

C ¼ Dq2 ¼ 1sc

ð3Þ

where q is the wave vector defined as q = (4pn0/k) sin(h/2), with thescattering angle h, wavelength of scattered light k and solventrefractive index n0.

Diffusion coefficients (D) were subsequently used to calculatethe hydrodynamic radii (Rh) of the complexes using the Stokes–Einstein relationship:

D ¼ kBT6pg0Rh

ð4Þ

where kT is the thermal energy and g0 is the viscosity of thesolution.

2.2.2. Transmission electron microscopyNanovesicle dispersions were negatively stained with 1% uranyl

acetate solution and deposited on a carbon-coated copper grid. Thesamples were examined with a transmission electron microscope(Philips 410 transmission electron microscope with a working volt-age of 100 kV) at 293 K.

2.2.3. NMR experiments1D and 2D NMR experiments (1H, gradient COSY, NOESY with

and without solvent suppression) were performed on a BrukerAVANCE-II 400 MHz spectrometer at StFX University. The mixingtimes and the delay times for the NOESY experiments were esti-mated from the spin-lattice relaxation times of the surfactantdetermined in separate experiments. In all cases, an acquisition de-lay of�3 � T1 and a mixing time of�1 � T1 were used to obtain theNOESY spectra. For the COSY experiments, the data were zero filledonce and multiplied by a sine-bell window in both dimensionsprior to the 2DFT. In the gradient NOESY experiments, 256 tran-sients of either 2 or 4 scans over 512 complex data points were ac-quired. All acquisitions were done in phase sensitive mode, withand without the saturation of the water resonance at �4.65 ppm.The data were zero-filled twice in dimension 1 and multiplied bya squared sine function in both dimensions before 2DFT.

3. Results and discussion

3.1. Photon correlation spectroscopy

We have carried out PCS measurements for two systems start-ing from the pure cationic amphiphile ([HexPy][Br] or [Hex-Py][BF4]) material and for various compositions of the cationicamphiphile and SDS. In the case of the pure, short chain cationicamphiphiles, no evidence of aggregation in aqueous solution wasdetectable from the PCS experiments in aqueous solution up to aconcentration of 1.00 mol L�1. However, when we added0.100 mL of a 0.0600 mol L�1 SDS solution to 3.0 mL of a0.155 mol L�1 solution of the cationic amphiphile [HexPy][Br],the formation of aggregates in the solutions in the nanometer sizerange was detected. Similar results were obtained for the additionof the same volumes of 60.0 mmol L�1 SDS to a 0.155 mol L�1 solu-tion of the ionic liquid material. We note that all the solutions wereclear. The stability of the aggregates was confirmed by the re-mea-surement of some of the solutions after a 2 week period; there

Table 1Hydrodynamic radii (Rh) corresponding to various concentrations of SDS in binarymixtures of short chain cationic amphiphiles [hexylpyridinium][BF4] IL and SDS.

CSDS (mmol L�1) [HexPy][BF4]Rh (±2 � 10�9 m) [HexPy][Br]Rh (±2 � 10�9 m)

1.94 22 592.86 25 753.75 31 844.62 31 915.54 32 1017.06 38 1048.57 44 113

10.0 49 11710.0a 49 118

a Measurement was obtained after allowing the solution to stand 2 weeks.

K. Singh et al. / Journal of Colloid and Interface Science 335 (2009) 105–111 107

were no appreciable changes in the sizes of aggregates in any of thesolutions after sitting for 2 weeks, indicating that size of the aggre-gates is relatively invariant with time (more measurements wouldhave to be carried out to confirm this). The results from all the sizemeasurements from the PCS experiments are listed in Table 1. Wenote that from Table 1, a steady increase in the hydrodynamic ra-dius of aggregates formed between the short chain cationic amphi-philes and SDS is observed as the concentration of the anionicsurfactant was increased. In all cases, we note that the behaviourof correlation function was monomodal at all SDS concentrationsfor both cationic amphiphile pairs; this monomodal variation of

Fig. 1. 2D NOESY spectrum for the system 0.040 M [

correlation function implies the existence of aggregates of uniformhydrodynamic radii. From the size of aggregates and the chemicalnature of various constituents, it is most likely that the aggregationis close to that of vesicles of very small size.

Mixtures of oppositely charged surfactants can form a widerange of microstructures at various mixing fractions [1,14–16]. Anumber of catanionic systems have been shown to yield richthermotropic mesomorphism [15]. In fact, the rich diversity ofmorphologies of catanionic systems has lead to their use as struc-ture-directing agents in the synthesis of various mesoscopicallyordered materials [4,17–19]. The strong attractive interaction be-tween the oppositely charged amphiphiles results in the releaseof the counterions from both surfactants as they self-assemble intoaggregates of various shapes. The driving force for the stability ofthese aggregates is the release of counterions from the aggregatesurface formed between the two oppositely charged amphiphiles,which leads to a large entropic contribution to the entropy of mix-ing. A gradual addition of one of the surfactants of a catanionic pairto a solution of the other will lead to the formation and growth ofmixed micelles, vesicle formation and finally precipitation when anequimolar mixture of the two components is present [20–22]. Forsystems that deviate slightly from equimolarity, the formation ofvesicles and their fundamental colloidal nature is an important,yet often unexplored feature of the self-assembly properties ofcatanionic systems. A key consideration for the formation ofvesicles from single or mixed surfactants is that the Israelachvili

hexylpyridinium][BF4] mixed with 0.045 M SDS.

108 K. Singh et al. / Journal of Colloid and Interface Science 335 (2009) 105–111

packing parameter, P, be in the range of 1/2 < P < 1. For a single sur-factant, that condition can be satisfied by choosing a doublechained surfactant; for mixed surfactant systems, a pseudo-doublechain surfactant can be obtained via the formation of an ion-pairbetween the anionic and cationic surfactant. In addition, the valueof parameter P can also be optimised by varying head group areavia salt addition or mixing chain length or temperature [6]. Thetype of aggregates that form with ionic surfactants can be broadlyunderstood in terms of the subtle balance between forces due tothe packing properties of surfactant and those due to double layerelectrostatic interactions. In the case of the IL based amphiphile,the catanionic mixture prepared here provides an interestingexample of the influence of the counterion of the constituentamphiphiles on the properties of the mixtures formed. It is clearthat the size of the mixtures is dependent on the nature of thecounterion of the cationic amphiphile; the morphology of thesesystems will be evaluated below.

3.2. 2D NOESY experiments

NMR NOESY experiments were performed for the system0.0450 M of sodium dodecylsulfate, mixed with a 0.0400 M solu-tion of either cationic amphiphile. 2D-NOESY experiments havebeen shown to give excellent insights into the nature of the self-assembly process in a number of aggregated systems [23–25].The two NMR NOESY spectra are shown in Figs. 1 and 2, respec-tively. In both cases, we observe strong cross peaks between the

Fig. 2. 2D NOESY spectrum for the system 0.040 M

protons in the headgroup regions of the both components in thecatanionic aggregates. Additional cross peaks are observed be-tween the terminal methyl group of the both cationic surfactantsand the n-methylene protons of the SDS and the headgroup pro-tons. These cross peaks signify the spatial proximity (within 5 Å)of these two surfactants in the self-assembled catanionic aggre-gates. When we examine the NOESY spectra of the two catanionicsystems, we observe very little difference in the nature of the crosspeaks in both systems, indicating that the manner in which thetwo sets of surfactants self-assemble is fundamentally identical.Any differences in aggregate sizes (see the PCS results above) andmorphology (TEM results below) must be due to a difference inthe counterion for the cationic amphiphile. When we examine bothspectra, we observe cross peaks between many of the chain pro-tons of the cationic surfactant and the anionic methylene chains,consistent with a strong interaction between the anionic surfactantmolecules and the cationic amphiphiles in the self-assembledstructures. Indeed, if we examine the aromatic resonances ataround 8.7–8.9 ppm, we see the existence of cross peaks betweenthe aromatic resonances and the first few chain protons in the an-ionic surfactant; we do not observe a cross peak between the aro-matic protons and the x-CH3 protons in the SDS molecules.

From the NOESY spectra of both catanionic systems shown here,we observe strong correlations between the a-protons of the SDS(d � 4.0 ppm) and the head group protons of the cationic amphi-phile (d � 8.8 ppm). We also observed a strong correlation betweenthe methyl protons of the cationic surfactants (d � 0.9 ppm) and

[hexylpyridinium][Br] mixed with 0.045 M SDS.

K. Singh et al. / Journal of Colloid and Interface Science 335 (2009) 105–111 109

the SDS chain protons (d � 1.4 ppm). We also see a number ofother correlations in the region around 1–1.2 ppm; these representcross peaks between the x -CH3 peak of the cationic surfactantsand the C-11 protons of the SDS, as well as cross peaks betweenthe methyl protons and the C-3 protons of the cationic amphi-philes. These cross peaks may be due to the folding of the carbonchains of SDS and [HexPy]+ cations in the micellar interior. Thisis consistent with cationic surfactant being located in the aggre-gate with its headgroup in intimate contact with the SDS headgroups, and its hydrocarbon chain pointed inwards towards thecentre of the aggregates.

In order to account for the behaviour of these systems in thecationic rich region, i.e. a non-monotonic variation of vesicle sizeupon addition of SDS and the packing of the two aggregates to-gether as deduced by the NOESY spectra, we propose a model forthe mixing of cationic amphiphiles and SDS that will account forthe complex interplay between the geometric packing and electro-

Fig. 3. (a) TEM photographs of aggregates obtained by the negative-staining method forvesicular nature of the aggregates. (b) TEM photographs of [hexylpyridinium][BF4] syst[hexylpyridinium][BF4] system showing the presence of small vesicles of a size close[hexylpyridinium][BF4] system showing the presence of linked vesicles. The lamellar na

static interactions. Initially, with no SDS present, (i.e. pure cationicamphiphile) we do not observe any aggregation, which is to be ex-pected from the small size of the alkyl chain of the cationic surfac-tants. Addition of SDS leads to the formation of ion-pairs betweenthe anion from SDS and the cation from the cationic amphiphiles,due to strong electrostatic interactions. These ion-pairs then willquickly aggregate to form vesicles. Non existence of micelles orany other form of aggregation can be attributed to the geometryof the ion-pair.

3.2.1. TEM measurementsTEM photographs, generated by the negative staining tech-

nique, are presented in Fig. 3a–d for a solution of 0.145 M of thecationic surfactants mixed with 0.010 M SDS. In all the figures,we can clearly see the formation of vesicle like structure due tothe induced self-assembly of the cationic surfactant with added an-ionic amphiphile (SDS). In Fig. 3a, we can clearly see the presence

the system 0.145 M [hexylpyridinium][BF4] mixed with 0.010 M SDS showing theem showing the presence of additional large vesicles. (c) TEM photographs of the

to the size reported from the DLS measurements. (d) TEM photographs of theture of the vesicles is also visible in this photograph.

110 K. Singh et al. / Journal of Colloid and Interface Science 335 (2009) 105–111

of some large, fused, multilamellar vesicles of the system [Hex-Py][BF4], with a diameter of about 200 nm; clearly visible inFig. 3a is the onion like nature of the vesicle strongly suggestingthat these soft aggregates are of the multi-lamellar type. InFig. 3b and c, the presence of additional large vesicles (in the rangeof 150–200 nm) are clearly visible, as well as a significant numberof ‘‘networked” structures, superimposed on a large number ofsmall structures with a size in the range of about 40 nm, whichis consistent with the size measurements reported above fromPCS. In Fig. 3d, the appearance of large networked (linked) struc-tures and the significant number of small vesicles supports themechanism where the smaller vesicles (in the range of 20–40 nm, consistent with the results from the DLS measurements)can be surrounded by additional bilayers. These large structuresindicate that substantial fusion is possible with the vesicles formedby the ionic liquid type cationic amphiphile when mixed with SDS.In the case of the [HexPy][Br]/SDS catanionic system (Fig. 4), weobserved the presence of large structures (�80–120 nm, again inagreement with the DLS results). However, we did not see anylarge fused vesicle systems (in the range of 200 nm) similar tothose we had observed in the case of the [HexPy][BF4]/SDScatanionic system. The TEM measurements for the [HexPy][Br]/SDS systems confirmed the presence of larger aggregates, onaverage, for that system versus the smaller averaged sized aggre-gates for the [HexPy][BF4]/SDS system, in excellent agrrement withthe results we obtained above from the DLS measurements.The fact that we did not observe any fused structures in the[HexPy][Br]/SDS system must indicate a substantial contributionto the self-assembly process from the IL.

When we examine the results for the self-assembly of thecatanionic systems investigated here, in terms of nature of the cat-ionic surfactant, the environment around the N atom in the [Hex-Py]+ cation is obviously very sensitive to both the charge densityand possibly the ‘‘hardness” of the counterion. A number of papersrecently have discussed the specific interactions that exist betweenspecies containing charged headgroups (i.e., micelles, vesicles,

Fig. 4. TEM photographs of the 0.145 M [hexylpyridinium][Br] mixed with 0.010 MSDS systems showing the presence of vesicles in the size range reported from DLSmeasurements.

polymer/surfactant complexes, and macromolecules) in terms ofa Hofmeister type ordering [16,26–29]. In the present set of exper-iments, the formation of stable vesicles between the [HexPy]+ cat-ion and the dodecylsulfate anion is demonstrated; however, it isclear that both the size of the vesicles thus formed as well as thepresence of large, fused vesicles is dependent on the nature ofthe counterions of both components of the catanionic system.The effect of various ions on the formation of vesicles will dependon the manner in which these ions alter the area per molecule atthe interface. According to Israelachivili [5], the formation of vesi-cles is favoured when the packing parameter is close to one. This ismainly due to the decreased surface area occupied by the surfac-tant headgroups. As these ions and co-ions occupy the surface re-gions of the aggregates, they can effectively compete with theheadgroups for water and that consequently the headgroups areless and less hydrated with increasing concentration of ions. Wenote that the effects of the nature and type of ion depend on thecosmotropic or chaotropic nature of the ions. Due to the differ-ences in surface charge density and hence, the hydration of chao-tropes and cosmotropes, chaotropes will form direct ion pairswith other chaotropes, and cosmotropes will interact more favour-ably with other cosmotropes [27,28]. Hence, the effect of variousco-ions in increasing the size of surfactant aggregates of the twosurfactant headgroups can be explained following Collin’s conceptof matching water affinities; chaotropes do not come into closecontact with cosmotropes. In this case of the vesicles formed be-tween the dodecylsulfate anion and the hexylpyridinium ion, theextremely soft nature of the hexylpyridinium ion will not allowthe formation of close ion pairs with the dodecylsulfate anion.Hence, due to the closer matching of the affinities of the pairingof the BF4

� and the [HexPy]+ cation, these ion pairs or dipoles willbe much less hydrated than separate ions and headgroups. Thissmaller hydration is reflected in smaller effective headgroup areasa leading to higher packing parameters and the presence of largerstructures in these systems, compared to the system with Br� asthe counterion of the hexylpyridinium salt.

The presence of the counterion’s salt obviously promotes theformation of the bilayer type structures; if the counterions are ofthe hard anion type, the electrical bilayer of both molecules inthe complex are compressed and they are allowed to more closelyapproach to form the fused structures. With the change in the sur-face charges, the two polar headgroups of the two surfactant mol-ecules can approach more closely. Additional molecules enter thebilayer of the vesicle, which leads to the decrease of the curvatureand the increase of the diameter. Note the more the bilayer is com-pressed, the closer the molecules are and the more difficult it is forthe outer molecules to enter the inner core of the vesicle. Thediameter of the vesicle will reach a maximum size, and additionalbilayers will lead to a bridging of two vesicles to eventually pro-duce fused vesicles.

The complete mechanism for the formation of lamellar vesiclesis complex due to the interplay of different types of interactionsbetween the solute (the ions from both the inorganic salt and theionic liquid) and the solvent. Hydration theory and related waterstructuring effects can explain it in a simple and semi-quantitativeway. Water diluted ionic liquids are supposed to have their ion-pairs surrounded by water molecules, and upon further dilution,the ionic liquid does not dissociate into ions completely. The ion-pairing tendency of the ionic liquid leads to more dehydration ofheadgroups which consequently, causes a decrease in the valueof a (effective area per molecule at interface) or an increase inpacking parameter P, to satisfy the condition for the formation ofvesicles. Further more, the presence of the counterion’s salt alsopromotes the formation of the bilayer type structures; if the coun-terions are of the hard anion type, the electrical bilayer of bothmolecules in the complex are compressed and they are allowed

K. Singh et al. / Journal of Colloid and Interface Science 335 (2009) 105–111 111

to more closely approach to form the fused structures. With thechange in the surface charges, the two polar headgroups of thetwo surfactant molecules can approach more closely. Additionalmolecules enter the bilayer of the vesicle, which leads to the de-crease of the curvature and the increase of the diameter. We notethe more the bilayer is compressed, the more tightly packed the bi-layer molecules become, and the more difficult it is for the outermolecules to enter the inner core of the vesicle. The diameter ofthe vesicle will reach a maximum size, and additional bilayers willlead to fusion of vesicles.

4. Conclusions

Spontaneous vesicle formation in the binary mixture of an ionicliquid amphiphile and an anionic surfactant (SDS) has been con-firmed by PCS, 2D NMR and TEM images. The sizes of the aggre-gates formed in these mixtures are comparable to similarlyformed aggregates in mixtures of a typical short chain cationicamphiphile with the same anionic surfactant. The manner in whichboth systems self-assemble into vesicles has been confirmed beidentical via 2D NMR. Strong electrostatic interactions betweenbulky organic ions are the main cause of the formation of vesicleaggregates. Initial studies on the stability of the vesicles formedwith the ionic liquid amphiphile indicate that their size is very sta-ble. Based on the NMR investigations, a model of the aggregate for-mation between the IL and the anionic surfactant has beenproposed. TEM measurements reveal that in the case of the systemwhere the ionic liquid amphiphile is the cationic surfactant compo-nent, fused vesicles are visible, whereas in the other system, vesiclefusion is not observed. We propose that additional bilayer electro-static effects and water structuring is provided by the ionic liquidamphiphile over the more conventional cationic amphiphile, andthat this is a significant contributing factor to the fusion of the ves-icles in the ½HexPy�½BF�4 �=SDS system.

Acknowledgments

The authors thank NSERC (Discovery grants, D.G.M and R.D.S;Research Capacity Development Grant, StFX and SMU; USRA,J.G.Q.), the Atlantic Innovation Fund, the StFX University Councilfor Research and the Faculty of Graduate Studies and Research,Saint Mary’s University for financial support of this research.

References

[1] R.O. Brito, E.F. Marques, P. Gomes, S. Falcao, O. Soderman, J. Phys. Chem. B. 110(2006) 18158. http://dx.doi.org/10.1021%2Fjp061946j.

[2] B.A. Coldren, H. Warriner, R. van Zanten, J.A. Zasadzinski, Langmuir 22 (2006)2465.

[3] M. Rosa, M.R. Infante, M.D. Miguel, B. Lindman, Langmuir 22 (2006) 5588.[4] T. Bramer, G. Karlsson, K. Edwards, K. Edsman, J. Drug Deliv. Sci. Technol. 17

(2007) 285.[5] J.N. Israelachvili, J. Mitchell, B.W. Ninham, J. Chem. Soc., Faraday Trans. 2 72

(1976) 1525. http://dx.doi.org/10.1039/F29767201525.[6] J.B.F.N. Engberts, J. Kevelam, Curr. Opin. Colloid Interface Sci. 1 (1996) 779.[7] J. Bowers, C.P. Butts, P.J. Martin, M.C. Vergara-Gutierrez, R.K. Heenan, Langmuir

20 (2004) 2191. http://dx.doi.org/10.1021/la035940m.[8] T. Inoue, H. Ebina, B. Dong, L. Zheng, J. Colloid Interface Sci. 314 (2007) 236.

http://dx.doi.org/10.1016/j.jcis.2007.05.052.[9] A. Tesfai, B. El-Zahab, D.K. Bwambok, G.A. Baker, S.O. Fakayode, M. Lowry, I.M.

Warner, Nano Lett. (2008), doi:10.1021/nl073184p.[10] A. Triolo, A. Mandanici, O. Russina, V. Rodriguez-Mora, M. Cutroni, C. Hardacre,

M. Nieuwenhuyzen, H.J. Bleif, L. Keller, M.A. Ramos, J. Phys. Chem. B 110(2006) 21357.

[11] A. Triolo, O. Russina, H.J. Bleif, E. Di Cola, J. Phys. Chem. B 111 (2007) 4641.[12] A. Triolo, O. Russina, C. Hardacre, M. Nieuwenhuyzen, M.A. Gonzalez, H.

Grimm, J. Phys. Chem. B 109 (2005) 22061.[13] M.C. Law, K.-Y. Wong, T.H. Chan, Green Chem. (2002) 4 328. http://dx.doi.org/

10.1039/b203122a.[14] A.K. Panda, F. Possmayer, N.O. Petersen, K. Nag, S.P. Moulik, Colloids Surf.

Physicochem. Eng. Aspects 264 (2005) 106, doi:10.1016/j.colsurfa. 2005.02.011.[15] B.F.B. Silva, E.F. Marques, J Colloid Interface Sci. 290 (2005) 267, doi:10.1016/

j.jcis.2005.04.012.[16] N. Vlachy, M. Drechsler, J. Verbavatz, D. Touraud, W. Kunz, J. Colloid Interface

Sci. 319 (2008) 542.[17] N. Zhao, Y. Wei, N.J. Sun, Q.J. Chen, J.W. Bai, L.P. Zhou, Y. Qin, M.X. Li, L.M. Qi,

Langmuir 24 (2008) 991. http://dx.doi.org/10.1021%2Fla702848x.[18] R. Bordes, K. Rbii, A. Gonzalez-Perez, S. Franceschi-Messant, E. Perez, I. Rico-

Lattes, Langmuir 23 (2007) 7526.[19] M.M. Teng, A.X. Song, L.P. Liu, J.C. Hao, J. Phys. Chem. B 112 (2008) 1671.[20] E.F. Marques, O. Regev, A. Khan, M. da GracaMiguel, B. Lindman, J. Phys. Chem.

B 103 (1999) 8353. http://dx.doi.org/10.1021/jp990852p.[21] E.F. Marques, O. Regev, A. Khan, M. da GracaMiguel, B. Lindman, J. Phys. Chem.

B 102 (1998) 6746. http://dx.doi.org/10.1021/jp980355t.[22] E.F. Marques, O. Regev, A. Khan, B. Lindman, Adv. Colloid Interface Sci. 100–

102 (2003) 83. http://dx.doi.org/10.1016/S0001-868(02)00068-4.[23] S.Z. Mao, Y.R. Du, Acta Phys-Chim. Sin. 19 (2003) 675.[24] H.Z. Yuan, L. Luo, L. Zhang, S. Zhao, S.Z. Mao, J.Y. Yu, L.F. Shen, Y.R. Du, Colloid

Polym Sci. 280 (2002) 479.[25] B.E. Hawrylak, D.G. Marangoni, Can. J. Chem. 77 (1999) 1241.[26] J. Lyklema, Chem. Phys. Lett. 467 (2009) 217. http://dx.doi.org/10.1016/

j.cplett.2008.11.013.[27] N. Vlachy, B. Jagoda-Cwiklik, R. Vácha, D. Touraud, P. Jungwirth, W. Kunz, Adv.

Colloid Interface Sci. 146 (2009) 42. http://dx.doi.org/10.1016/j.cis.2008.09.010.

[28] K.D. Collins, G.W. Neilson, J.E. Enderby, Biophys. Chem. 128 (2007) 95. http://dx.doi.org/10.1016/j.bpc.2007.03.009.

[29] Y. Zhang, P.S. Cremer, Curr. Opinion Chem. Biol. 10 (2006) 658. http://dx.doi.org/10.1016/j.cbpa.2006.09.020.