Spontaneous Transition of Micelle–Vesicle–Micelle in a Mixture of

Article pubs.acs.org/Langmuir

Spontaneous Transition of Micelle−Vesicle−Micelle in a Mixture of Cationic Surfactant and Anionic Surfactant-like Ionic Liquid: A Pure Nonlipid Small Unilamellar Vesicular Template Used for Solvent and Rotational Relaxation Study Surajit Ghosh, Chiranjib Ghatak, Chiranjib Banerjee, Sarthak Mandal, Jagannath Kuchlyan, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India S Supporting Information *

ABSTRACT: The micelle−vesicle−micelle transition in aqueous mixtures of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and the anionic surfactant-like ionic liquid 1-butyl-3-methylimidazolium octyl sulfate, [C4mim][C8SO4] has been investigated by using dynamic light scattering (DLS), transmission electron microscopy (TEM), surface tension, conductivity, and fluorescence anisotropy at different volume fractions of surfactant. The surface tension value decreases sharply with increasing CTAB concentration up to ∼0.38 volume fraction and again increases up to ∼0.75 volume fraction of CTAB. Depending upon their relative amount, these surfactants either mixed together to form vesicles and/or micelles, or both of these structures were in equilibrium. Fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in this system at different composition of surfactant indicates the formation of micelle and vesicle structures. The apparent hydrodynamic diameter of these large multilamellar vesicles is about ∼200 nm−300 nm obtained by DLS measurement and finally confirmed by TEM micrographs. The large multilamellar vesicles are transformed into small unilamellar ones by sonication using a Lab-line instruments probe sonicator with a diameter of ∼90−125 nm. To investigate the heterogeneity, solvent, and rotational relaxation of coumarin-153 (C-153) have been investigated in these unilamellar vesicles by using picosecond time-resolved fluorescence spectroscopic technique. The solvation dynamics of C-153 in these vesicles is found to be biexponential with average time constant ∼580 ps. This indicates the slow relaxation of water molecules in the surfactant bilayer. In accordance with solvation dynamics, fluorescence anisotropy analysis of C-153 in unilamellar vesicles also indicates hindered rotation compared to bulk water.

1. INTRODUCTION Cationic and anionic (catanionic) surfactant assemblies in aqueous solution are the fascinating arena of extensive investigations,1 the outcome of which can provide a basic understanding of the crucial phenomena in surfactant sciences, especially for mixed surfactants in solution. There has been much interest in studying the microstructures and properties of spontaneous vesicles at thermodynamic equilibrium.2,3 By varying the mixing ratio between the ionic amphiphilic surfactants, the charge of the aggregates as well as their spontaneous curvature (or the effective surfactant packing parameter) can be tuned. This gives rise to a broad range of aggregation forms,2−4 the formation of which is also influenced by factors such as molecular structure, total surfactant concentration, temperature, and salt. Aggregates as diverse as mixed micelles (spheres, disks, ribbons, and elongated micelles), unilamellar or multilamellar vesicles, planar lamellar phases, and hollow regular icosahedra can be formed. Generally, at equimolar mixing the amphiphiles precipitate in a 1:1 neutral complex salt, also called a catanionic surfactant.5 For mixing ratios except 1, there is excess charge that © XXXX American Chemical Society

destabilizes the precipitate and induces vesicle formation. These vesicles have been proven to be not only spontaneously formed and stable in time with respect to size2,6 but also independent of the formation path.7 The reversible transitions between micelles and vesicles upon temperature or composition changes are also biological and colloidal processes of great interest, with direct relevance, for instance, in membrane protein reconstitution and drug release. Hence, a lot of work has been put into understanding these transitions, which can be induced in a number of different ways using common surfactants (cationic, anionic, and zwitterionic), lipids, bile salts, and so on.8−17 The formation and transition of self-assembled aggregates greatly depend on electrostatic, van der Waals, hydrophobic, and steric interactions and their delicate balance.18−20 However, systematic research is still necessary to obtain a better understanding of these weak interaction. Received: May 29, 2013 Revised: July 15, 2013

A

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vesicles by using a certain volume ratio of [C4mim][C8SO4]/ CTAB mixture by proper sonication. Solvation dynamics can provide useful information regarding the heterogeneity of various microenvironments.33−37 Dynamics of solvent molecules and their effects in chemical and biological processes have been interesting topics for recent research.38−43 Since, micelles, reverse micelles, and lipid bilayers can act as model biological systems, both experimental and theoretical methods have been used by various research groups to exploit the solvation dynamics in different organized assemblies.44−49 Solvation in bulk water is very fast (∼1 ps), but in organized assemblies, an additional slow component of solvation arises in the ∼100−1000 ps time scale.50 The coexistence of both slow and fast components of solvation in organized media is explained by the famous phenomenological model of Nandi and Bagchi.51,52 Several reports are available about solvation dynamics in lipid-containing vesicles,46,53 but such kinds of study are less in mixed-surfactant-containing vesicles. So, in this paper, we have used coumarin-153 (C-153) as a probe molecule to monitor solvation dynamics in unilamellar vesicles composed of [C4mim][C8SO4] and CTAB and to verify whether a distinct difference is present compared to liposomes.

In the present work, we focus on vesicles formed by combining the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the surfactant-like ionic liquid 1-butyl-3methylimidazolium octylsulfate [C4mim][C8SO4] as anionic surfactant. [C4mim][C8SO4] is a long-chain room-temperature ionic liquid (RTIL) that exhibits interesting and unusual physical properties, such as low vapor pressure, high thermal stability, wide liquidous temperature range, and wide electrochemical windows. Due to their unusual properties, RTILs have been used in many chemical reactions.21,22 The impressive solvation ability of ionic liquids facilitates their interaction with classical surfactants.23 It was shown that solvatophobic interactions are present between ionic liquid and the hydrocarbon portion of the surfactant, thus leading to the formation of surfactant micelles in ionic liquids and enhancing the solvation characteristics of the (ionic liquid + surfactant) system.24 Here, one can combine two reagents of completely different natures (hydrophilic and hydrophobic) into a macroscopically homogeneous solution. Among the 1-alkyl-3methylimidazolium family of cations, [Cnmim]+, with different linear alkyl chain lengths and different counterions, [Cnmim]Cl with n > 8 unambiguously form aggregates in solution, and the nature of this self-aggregation is discussed in terms of the electrostatic versus hydrophobic contributions of the isolated cation.24 By contrast, the short chain containing ionic liquid behaves, as anticipated, as simple salts.25 Several reports are available where ionic liquids have been used as the reaction medium for vesicle formation26 or used as host molecules to construct vesicles. When ionic liquids are mixed with oppositely charged surfactants, it will lead to the formation of advanced aggregates and surfactant phases, since under these conditions, ILs behave in a manner similar to ionic surfactants.27−29 Now, we have used the opposite combination of cation and anion counterparts regarding surfactant and ILs. Miskolczy et al.30 showed that [C4mim][C8SO4] can form micelles in aqueous solution. There are very few reports that cover detailed information about vesicle formation using long-chain ionic liquid when mixed with a normal surfactant.27,28 We want to explore the minute picture of the phase behavior of CTAB/ [C4mim][C8SO4] solution at various compositions ranging from 0 to 1.0 volume fraction of CTAB. Turbidity, electrical conductivity and dynamic light scattering (DLS) measurement, and also optical photographs are used to construct the phase behavior of these mixed surfactant solution. Anisotropy measurement and surface tension determination also help us to draw the borderlines between various phases observed in this present case. Absolute proof of existence of large multilamellar vesicle was confirmed by clear TEM photographs, and these vesicles are stable for long periods. Although the extensive use of lipids for the preparation of small unilamellar vesicles and use for drug delivery and interaction study, now-a-days there is a growing rate for discovering such templates in a nonlipidic manner. Liposomes are made of expensive lipids and quite unstable carriers, and, to improve these contexts, scientists are now substituting lipids by nontoxic surfactants having versatile tunable outcomes.31,32 It is not obvious but interesting to look at the heterogeneity of the microenvironment of such kinds of vesicular systems compared to liposomes, which gives us useful information about their inherent structural differences. Dense white multilamellar vesicular solution is not suitable for optical study, and in this context we have prepared small unilamellar

2. EXPERIMENTAL SECTION 2.1. Materials. [C4mim][C8SO4] and CTAB (>99%) were obtained from Sigma-Aldrich and used as received. Milli-Q water was used for the preparation of samples. C-153 was purchased from Exciton and also used as received. All experiments were carried out at 298(±1) K. The structures of [C4mim][C8SO4], CTAB, and C-153 are shown in Scheme 1.

Scheme 1. Structures of [C4mim][C8SO4], CTAB, and C-153

2.2. Instrument. 2.2.1. Turbidity Measurement. The turbidity measurement was done by determining the transmittance of the solution using a Shimadzu (model no. UV-2405) spectrophotometer. 2.2.2. Size Distribution Measurement. DLS measurements were performed using a Malvern Nano ZS instrument equipped with a thermostatted sample chamber. All experiments were carried out using a 4 mW He−Ne laser (λ = 632.8 nm). In Zetasizer Nano ZS, the detector angle is fixed at 173°. We have used this instrument for DLS measurement. All the measurements were performed at 298 K. 2.2.3. Transmission Electron Microscopy (TEM). For TEM micrographs, 15 μL of solution were placed on a 300 mesh size carbon-coated copper grid (50 nm carbon film) allowed to adsorb for 2 min. Excess liquid was removed by use of a piece of filter paper, airdried, and then negatively stained with freshly prepared 0.1 wt % aqueous uranyl acetate. The specimens were kept in desiccators until before use. The specimens were examined under a transmission electron microscope (JEOL-JEM 2100, Japan) operating at an accelerating voltage of 200 kV at room temperature (298 K). B

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2.2.4. Surface Tension Measurement. The surface tension (γ) of the surfactant solutions were measured using the Du Nuöy ring detachment method with an automated surface tensiometer (3S, GBX, France) at ∼298 K. The instrument was calibrated through loading the proper weight (for 600 mg, the γ shows 49 mN m−1) and checked by measuring the surface tension of distilled water before each experiment. The solution was magnetically stirred for 30 s following each addition of aliquot and allowed to stand for about 3 min at room temperature (∼298 K) to achieve the equilibrium before surface tension was measured. For each concentration, three measurements for γ were performed, and their mean was taken as the value of the equilibrium surface tension. 2.2.5. Electrical Conductivity Measurements. A low-frequency conductivity analyzer manufactured by Integrated Electrolife System (India), model 213R, having an EP-type connector was used to measure the electrical conductivities of the solutions at 298 K. The electrical conductivities of the two-phase solutions were measured with stirring. 2.2.6. Determination of Zeta Potential (ζ). We used a Malvern Nano ZS instrument to determine zeta potential (ζ) of aqueous 100 mM [C4mim][C8SO4], CTAB, and [C4mim][C8SO4]−CTAB mixture. 2.2.7. Viscosity (η) Measurements. Viscosities of the 100 mM [C4mim][C8SO4], CTAB and mixture of [C4mim][C8SO4]−CTAB at a volume fraction of CTAB were measured using a Brookfield DV-II+ Pro viscometer at 298 K. Rheology measurements were performed on a Bohlin RS D-100 (Malvern, U.K.) rheometer, and parallel-plate geometry was used for the measurement. The gap between the parallel plates was maintained at 100 μm, and a stress-amplitude sweep experiment was performed at a constant oscillation frequency of 1.0 Hz at 300 K. 2.2.8. Anisotropy Measurement. For anisotropy measurement, we used a Perkin-Elmer LS-55 luminescence spectrometer equipped with a filter polarizer and a thermostatted cell holder. The anisotropy is given by r0 =

(VCTAB + VIL)] using appropriate volumes of respective stock solution (40 mM, 60 mM, and100 mM). The surfactant mixtures were stirred, and bubbles in the solution were removed by ultrasonication. The sample solutions were thermostatted at 298 K and for at least 15 days in order to reach an equilibrium state. The phase behavior of the solutions was examined by visual inspection, and the boundaries of the phase regions were determined by conductivity measurements.

3. RESULTS AND DISCUSSION 3.1. Phase Behavior. It is already known that [C4mim][C8SO4] can form tiny micelles (∼3 nm) in aqueous solution, acting as a surfactant molecule.30 Aqueous solutions with concentrations of 40 mM, 60 mM, and 100 mM are lowviscosity clear solutions (micellar solutions). When various amounts of CTAB (40 mM, 60 mM, and 100 mM) were added into [C4mim][C8SO4] (40 mM, 60 mM, and 100 mM) aqueous solution, a series of phase changes were observed in all the sets as the volume fraction (χCTAB) of CTAB increased from 0 to 1. Photographs of typical samples are shown in Figure 1.

Figure 1. Phase behavior (optical micrographs) of the aqueous [C4mim][C8SO4]−CTAB with increasing volume fraction of CTAB.

IVV − GIVH IVV + 2GIVH

For solutions with χCTAB between 0 and 0.25, a transparent solution with a low-viscous micellar phase is observed. This region is comprised of pure micelle and mixed micelle formed by [C4mim][C8SO4] alone and with CTAB. Increase in volume fraction of CTAB from 0.25 to 0.60 causes the solution to become turbid, and a bluish appearance phase is observed. This change is actually composed of two close transitions: At χCTAB = 0.25, a light bluish color appeared, which became more turbid in nature and dense in color. This transition cannot be distinguished without the help of surface tension and conductivity measurement. Above χCTAB = 0.60, white deep precipitate appears, confirming the spontaneous aggregation of normal size vesicles followed by thermodynamically more stable larger multilamellar vesicles. At higher molar ratios (χCTAB = 0.7−0.8), an ambiguous phase evolution is observed, which is clear but highly viscous in nature. The existence of such a viscous solution demands further minute study to characterize it, which is discussed in a forthcoming section. At volume ratio greater than ∼0.8, a clear micellar phase is expected and observed. The above naked eye detection of phase separation was then examined spectrophotometrically by measuring the change in turbidity (100% T) of the solutions at different volume ratios. The appearance of cloudiness during the change in volume fraction after 0.25 was easily detected by the sharp increment in turbidity of the solution as shown in Figure 2. Such steep variation results only when a phase separation occurs having a greater difference in their properties. A plateau up to χCTAB = 0.60 indicates the existence of quite similar types of species, and it is difficult to differentiate between them by this technique. Such higher value of turbidity supports the

where G is the correction factor, IVV and IVH are the fluorescence intensity polarized parallel and perpendicular to the polarization of the excitation light, respectively. 2.2.7. Steady-State and Time-Resolved Measurements. The absorption and fluorescence spectra of C-153 were collected using a Shimadzu (model no. UV-2450) spectrophotometer and a Hitachi (model no. F-7000) spectrofluorimeter, respectively. For steady-state measurements, all the samples were excited at 410 nm to collect the emission spectra. For time-resolved measurements, we used a timecorrelated single-photon-counting (TCSPC) instrument from IBH (U.K.), and details of the time-resolved fluorescence setup were described in our earlier publication.54 Briefly, the samples were excited at 410 nm using a picosecond laser diode (IBH, Nanoled), and the signals were collected at the magic angle (54.7°) using a Hamamatsu microchannel plate photomultiplier tube (3809U). The same setup was used for anisotropy measurements. The instrument response function of our setup is ∼90 ps. We used a motorized polarizer on the emission side. The emission intensities at parallel I∥(t) and perpendicular I⊥(t) polarizations were collected alternatively until a certain peak difference between parallel I∥(t) and perpendicular I⊥(t) decays was reached. The analysis of the data was done using IBH DAS, version 6, decay analysis software. 2.3. Solution Preparation. For the preparation of aqueous solution of [C4mim][C8SO4] and CTAB solution, we used Milli-Q water. Initially, water is filtered through syringe filter (0.2 μm) to remove dust particles. The required amount of CTAB and [C4mim][C8SO4] was added separately in two different volumetric flask. It was stirred and kept overnight. Then, aqueous [C4mim][C8SO4] and CTAB solution were mixed together to prepare the desired volume fraction [χCTAB = volume fraction of CTAB = VCTAB/ C

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[C8SO4], the intensity of the neat [C4mim][C8SO4] main peak gradually decreases while the intensity of the additional peak becomes pronounced. At χCTAB = 0.10−0.40, two distinct peaks in the size distribution plot are observed, indicating the presence of free micelles, mixed micelles, and vesicles. However, as the volume fraction of CTAB increases up to ∼0.6, the distinct peak around ∼400 nm is observed, indicating the presence of multilamellar vesicles. DLS measurements cannot distinguish the shapes of these aggregates. In the region χCTAB = 0.7−0.8, the solution viscosity is very high; these may be due to the formation of elongated micelles. This difference between the existence of micelle-vesicle phase and sole vesicle phase can also be followed by observing the TEM image and conductivity data. In Figure 4, we have shown the variation of size of the aggregates with variation of volume fraction of CTAB. 3.3. Transmission Electron Microscopy (TEM) Measurement. Direct evidence of formation of vesicles and elongated micelles was obtained from TEM measurements. The micrographs of aqueous solutions of [C4mim][C8SO4]/ CTAB mixture at χCTAB = 0.60 and χCTAB = 0.70 have been depicted in Figure 5. To visualize the morphology of aggregates, freshly prepared 0.1 wt.% aqueous uranyl acetate is used as the staining agent. The TEM images clearly indicate the formation spherical vesicles (χCTAB = 0.60). The average size of the vesicles is ∼200−400 nm. However, at a higher χCTAB value (χCTAB = 0.70), elongated micelles are also observed. These results are consistent with our DLS measurement studies. Figure 5A−D shows the TEM images of multilamellar vesicles with size distribution ranges from ∼200−400 nm. The presence of bilayer, which is indicated

Figure 2. Variation of turbidity of aqueous [C4mim][C8SO4]−CTAB system.

formation of bigger particles having size in the micrometer range in that volume ratio range (0.25−0.65). Again a sharp decrement in turbidity at molar ratio χCTAB = 0.70 indicates the transition from bigger particle to smaller particle in solution. 3.2. Dynamic Light Scattering (DLS) Study. The spontaneous microstructural transformation of [C4mim][C8SO4]/CTAB mixture by balancing the charges and alignment of trimethyl ammonium−sulfate group in aqueous [C4mim][C8SO4] with increasing CTAB concentration has been confirmed by DLS measurements. The average hydrodynamic diameter (dh) of the [C4mim][C8SO4] micelle is ∼3.0 nm. The micelle-to-vesicle transition is supported by DLS measurement, and the size distribution plots at various concentration of mixture are shown in Figure 3. It has been observed that upon addition of CTAB to aqueous [C4mim]-

Figure 3. Size distribution (diameter) plot of [C4mim][C8SO4]/CTAB solution at different volume fraction values. D

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Figure 4. Variation of size (diameter) of IL/CTAB solution at different volume fraction values.

Figure 6. Surface tension measurement of IL/CTAB solution at different volume fraction values.

Figure 5. TEM images of vesicles and elongated micelles composed of [C4mim][C8SO4] and CTAB. Figure 7. Variation of conductivity of [C4mim][C8SO4]/CTAB solution at different volume fraction values.

by a red arrow, is clearly observed in high-resolution TEM images. At higher χCTAB values (χCTAB = 0.70−0.80), the formation of elongated micelles is also confirmed by TEM micrographs (Figure 5E,F). 3.4. Surface Tension and Conductivity Measurements. We have measured surface tension (γ) of [C4mim][C8SO4]/ CTAB at different compositions, and the variation of surface tension with composition is shown in Figure 6. This plot showed that surface tension values of surfactant mixture are much lower than those of individual surfactants. This result indicates that surface mixtures behave as better surface active agents than CTAB or [C4mim][C8SO4]. Strong electrostatic interaction between ammonium cation and sulfate anion and hydrophobic interaction between hydrocarbon chains of the pseudo-double-chain surfactant results in lowering of the surface tension value. To distinguish different phase boundaries, electrical conductivity measurements are carried out. The variation of electrical conductivity of [C4mim][C8SO4]/CTAB at different compositions is shown in Figure 7. Initially, with the addition of CTAB in aqueous [C4mim][C8SO4], electrical conductivity increases due to the increasing amount of counterions as charge carriers. At χCTAB = 0.25−0.65, electrical conductivity decreases sharply, indicating the formation of vesicles. Once vesicles are formed, the counterions are absorbed, which decreases the number of conductive ions. In the region χCTAB = 0.70−1.00,

due to formation of elongated micelles and free micelles, the conductivity value again increases. 3.5. Zeta Potential (ζ) Measurement. We have measured the zeta potential (ζ) of aqueous 100 mM [C4mim][C8SO4] solution and in the presence of different amounts of CTAB. Zeta potential (ζ) indicates the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed micellar aggregates. It also gives an indication about the stability of systems. The variation in zeta potential of aqueous [C4mim][C8SO4] solution with the addition of CTAB is shown in Figure 8. Initially, with addition of CTAB, the slight increase in ζ value clearly indicates the interaction of CTA cation with the surface charge of our anionic [C8SO4] micellar aggregates. However, after χCTAB = 0.30, sudden decrease in ζ value followed by a regular increase is observed up to χCTAB = 0.70. At χCTAB = 0.70−0.80, due to the formation of elongated micelles, a sudden change in ζ value is also observed. In general, the [C4mim][C8SO4] micelle carries a negative charge, so the ζ value is about −24.2 mV. Upon addition of CTAB, the negative charge decreases, and at χCTAB = 0.50, the ζ value becomes almost zero. In the region χCTAB = 0.80−1.0, it becomes positive. In conclusion, measurement of zeta potential of 100 mM [C4mim][C8SO4], CTAB, and their mixture can provide E

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of DPH allows it to be incorporated into the bilayer moiety. Hence, its rotational motion is hindered in aggregates compared to bulk solvent. As a result, a high steady-state anisotropy value is associated with a low degree of rotation of fluorescence probe, DPH. Therefore the variations of anisotropy are indicative of changes in the fluidity of the probe environment. The variation of steady state anisotropy (r) in different compositions of [C4mim][C8SO4]−CTAB is shown in Figure 10. It has been observed that in bilayer

Figure 8. Variation of zeta potential (ζ) of [C4mim][C8SO4]/CTAB solution at different volume fraction values.

an idea about the morphological change of surfactant aggregates. 3.6. Viscosity (η) Measurement. To get an idea about the change of viscosity of the [C4mim][C8SO4]−CTAB mixture at different volume fractions, we measured the bulk viscosity using a Brookfield DV-II+ Pro viscometer at 298 K. The viscosity of aqueous [C4mim][C8SO4] solution is about 1.05 cP, and it almost remains constant up to χCTAB = 0.25. A slight increment of viscosity is observed with further addition of CTAB. However, the η value reaches a maxima at the region χCTAB = ∼0.70−0.80 (Figure 9). Interestingly, this region is located after

Figure 10. Steady-state anisotropy measurement of [C4mim][C8SO4]/CTAB solution at different volume fraction values.

aggregates, the fluorescence anisotropy of DPH is very high and in spherical or rod like micelles, the anisotropy value is lower. In bilayer aggregates, the hydrocarbon chains are tightly packed, and therefore,the microenvironments are more rigid than normal micelles. In our system, at χCTAB = 0−0.20, the anisotropy value is low, and then it increases up to χCTAB = 0.65. However, at higher χCTAB, the anisotropy again decreases. These observations indicate that for χCTAB = 0.40−0.70, the aggregates are dominated by vesicles, while at lower and higher χCTAB values, the aggregates are mainly micelles, mixed micelles, or elongated micelles. The micelle-to-vesicle transition in aqueous [C4mim][C8SO4] solution with addition of CTAB is represented in Scheme 2. 3.7. Preparation and Characterization of Small Unilamellar Vesicles. Generally, Mixing of [C4mim][C8SO4] and CTAB at a volume ratio of ∼0.6 forms large multilamellar vesicles, and it is not suitable for spectroscopic study. For the preparation of desired small unilamellar vesicles, the mixture is subjected to freeze−thaw cycles followed by sonication at intervals of 30 s with 1 min between the intervals for 20 min using a Lab-line instruments probe sonicator. After all these exhaustive steps, SUVs were then prepared in the solution and again it was centrifuged at 10 000 rpm at 277 K for 15 min using a 5404R centrifuge for the removal of titanium particles introduced during sonication. To obtain direct evidence for size of unilamellar vesicles, we performed DLS and TEM measurements. The TEM images were taken using 0.1 wt.% uranyl acetate as the staining agent. Figure 11 shows the size distribution of small unilamellar vesicles obtained from DLS measurement with diameter ranges from ∼90 to 125 nm. The TEM images of the unilamellar vesicles are shown in Figure 12. The presence of a bilayer is clearly observed from the high-resolution TEM image of a single vesicle shown in Figure 10A. 3.8. Solvent and Rotational Relaxation Measurements. 3.8.1. Steady-State Spectra. For the investigation of

Figure 9. Variation of bulk viscosity of [C4mim][C8SO4]/CTAB solution at different volume fraction values.

the high turbidity region. It is already reported that mixed surfactant solution with high viscosity also suggests the formation of elongated micelles.55 The TEM images (Figure 5E,F) indicate that these micelles are entangled with each other, forming a network that results in high viscosity in this region. We also measured the shear viscosity at two volume fractions of CTAB (χCTAB = 0.75 and 0.80), which is depicted in Figure S1 (Supporting Information). As the other surfactant mixture is not viscous enough, this experiment is not possible due to the sensitivity of the rheological equipment. These viscosity properties further indicate the formation of elongated micelles in the region χCTAB = 0.75−0.80. 3.6. Anisotropy Measurement. Morphological change of surfactant aggregates can also be predicted by following the change in steady-state fluorescence anisotropy (r).56,57 We have used 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe molecule to predict the type of aggregates formed in aqueous solution of surfactant molecules. DPH was added to different compositions of [C4mim][C8SO4]−CTAB mixture. The hydrophobic nature F

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Scheme 2. Pictorial Representation of the Oscillation between Micelle−Vesicle−Micelle in Aqueous [C4mim][C8SO4] Solution with the Addition of Different Volume Fractions of CTAB

Table 1. Steady-State Absorption and Emission Maxima of C-153 in 40 mM [C4mim][C8SO4], 40 mM CTAB, and [C4mim][C8SO4]−CTAB Vesicle system

λmax abs (nm)

λmax em (nm)

[C4mim] [C8SO4] (40 mM) 40 mM CTAB (40 mM) vesicle

431 435 428

542 538 532

3.8.2. Time-Resolved Studies. 3.8.2.1. Time-Resolved Anisotropy Studies. Time-resolved anisotropy measurement provides information about the location of probe molecule in organized assembly. The time-resolved fluorescence anisotropy, r(t), is calculated using the following equation:57

Figure 11. Size distribution plot (DLS measurement) of a unilamellar vesicle.

r (t ) =

I − GI⊥ I + 2GI⊥

(9)

where G is the instrument correction factor of the detector sensitivity to the polarization direction of the emission, which is 0.6 for our instrumental set up. I∥(t) and I⊥(t) are fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively. The representative anisotropy decay profiles of C-153 in water and [C4mim][C8SO4]−CTAB vesicle are shown in Figure 14. In water, the anisotropy decay of C-153 is single

Figure 12. TEM images of unilamellar vesicles.

solvent and rotational relaxation studies in the vesicles, we used C-153 as the molecular probe. The absorption spectra of C-153 in micellar solution (40 mM [C4mim][C8SO4] and CTAB) and vesicle are shown in Figure S2(Supporting Information). C-153 exhibits an intense emission with an emission maximum ∼542 nm in 40 mm [C4mim][C8SO4] solution and ∼538 nm in 40 mM CTAB solution (Figure 13). In vesicles, the emission maxima of C−-153 is observed to be ∼532 nm, which is blueshifted compared to aqueous CTAB or [C4mim][C8SO4] solution. These results are summarized in Table 1.

Figure 14. Anisotropy decays of C-153 in water and [C4mim][C8SO4]−CTAB vesicle.

exponential with a time constant of ∼100 ps. However, in vesicle, the fluorescence anisotropy decay is found to be biexponential with much longer time constant ∼460 ps (49%) and ∼2250 ps (51%). Hence, the average rotational time of C153 in vesicle becomes ∼1370 ps. It suggested that the rotation of C-153 is more hindered in vesicle. The observed biexponential nature of rotational relaxation is due to the

Figure 13. Emission spectra of C-153 in a [C4mim][C8SO4]−CTAB vesicle, 40 mM CTAB, and 40 mM [C4mim][C8SO4]. G

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different types of rotational motion of probe molecules inside the vesicle bilayer. The anisotropy data support the wobblingin-cone model.58 The slow component arises due to the overall rotational motion of the cone containing the probe molecules, and the fast component is due to the wobbling-in-cone dynamics of the probe itself. Such a type of motion has already been reported in various biomimicking organized assemblies.58−60 3.8.2.2. Solvation Dynamics. A fast decay at the blue end and a rise preceeding the decay at the red end in time-resolved emission decays over the emission spectra of C-153 in unilamellar vesicle indicate that solvation is occurring in this system. The wavelength dependent decays of C-153 in this vesicle are shown in Figure S2 (Supporting Information). We have constructed time-resolved emission spectra (TRES) using time-resolved decays and steady state emission intensity by using the procedure of Fleming and Maroncelli.33 The TRES at a given time t, S(λ;t), is obtained by the fitted decays, D(t; λ), by relative normalization to the steady-state spectrum S0(λ), as follows: S(λ ; t ) = D(λ ; t )

Figure 15. Decay of solvent correlation function, C(t) of C-153 in a unilamellar vesicle.

Table 2. Anisotropy Decay r(t) Parameters of C-153 in a [C4mim][C8SO4]−CTAB Unilamellar Vesicle

S0(λ) ∞

∫0 D(λ ; t ) dt

a

(4)

Each TRES was fitted by “log-normal line shape function”, which is defined as ⎡ ⎛ ln[1 + 2b(υ − υp)/Δ] ⎞2 ⎤ ⎟⎟ ⎥ g (υ) = g0 exp⎢( −ln 2)⎜⎜ ⎢ b ⎝ ⎠ ⎥⎦ ⎣

υ(t ) − υ(∞) υ(0) − υ(∞)

(5) a

τslow (ps)

afast

τfast (ps)

⟨τarot⟩ (ps)

bulk water vesicle

0.51

2250

1.00 0.49

100 460

100 1376

Experimental error ∼5%.

system

a1

τ1 (ps)

a2

τ2 (ps)

⟨τaav⟩ (ps)

missing component (%)

vesicle

0.40

250

0.60

800

580

34

Experimental error ∼5%.

of several concentric bilayers. Generally, unilamellar vesicles are produced by breaking of multilayer arrangement by ultrasonic irradiation. So, in such a system, two different kinds of water molecules are present. In vesicles, the location of probe molecules is expected to be in three different regions: bulk water, surfactant bilayer, or inner water pool. In vesicle, the emission maximum is blue-shifted to ∼532 nm relative to that of a micellar solution of CTAB or [C4mim][C8SO4]. In addition to steady state measurement, time-resolved anisotropy decay studies indicate that C-153 is mainly located in the headgroup region of surfactant bilayer. The observed solvation dynamics suggests that the water molecules in the bilayer relaxed at a much slower time scale than bulk water due to the hinder motion of water molecules in bilayer. In organized assemblies, the slow and fast solvation arise from the response of water molecules close to the probe and the collective response of a large number of water molecules that are away from the probe.51,52 The average solvation time of C-153 in a vesicle composed of [C4mim][C8SO4] and CTAB (χCTAB = 0.60) is found to be ∼580 ps (Table 3). Previously, it is reported that in [C4mim][C8SO4] micelle, the average solvation time is ∼230 ps, and it remains almost constant with increasing surfactant concentration.61 The solvation dynamics of C−153 is slower compared to [C4mim][C8SO4] micellar solution. This finding suggests that the water molecules in the vesicles are much more constrained compared to the micelles. Slow solvation dynamics in vesicles can be explained by the dynamic exchange model, and, according to this model, the slow component of the solvation dynamics originates from the interconversion of the bound and free water molecules.46,51−53

(6)

where υ(0), υ(t), and υ(∞) are the peak frequency at time zero, t, and infinity. The decays of C(t) are fitted by a biexponential function C(t ) = a1 exp−t / τ1 + a 2 exp−t / τ2

aslow

Table 3. Decay Parameters of C(t) of C-153 in a [C4mim][C8SO4]−CTAB Unilamellar Vesicle

where g0, b, νp, and Δ are the peak height, asymmetric parameter, peak frequency, and width parameter, respectively. We have calculated the peak frequency from the log-normal fitting of TRES. This peak frequency was used to construct the decay of the solvent correlation function C(t), which is defined as C(t ) =

system

(7)

where τ1 and τ2 are the two solvation times with amplitudes of a1 and a2, respectively. The C(t) versus time plot for this vesicle is given in Figure 15. The average lifetime (⟨τav⟩) is calculated using the following equation: τav = a1τ1 + a 2τ2 (8) The average solvation time of C-153 in vesicle is found to be ∼580 ps (Table 3), consisting of time constants of the fast component as ∼250 ps (with relative contribution of 40%) and the slow component as ∼800 ps (with relative contribution of 60%). To discuss the slow dynamics in vesicles, we have to consider the structural features of our system. In vesicle, the aqueous volume is enclosed by a surfactant bilayer or lipid membrane, and these aggregates are also dispersed in bulk water. Mixing of cationic and anionic surfactant or lipid molecules results in the formation of onion-like multilamellar arrangements consisting H

dx.doi.org/10.1021/la402053a | Langmuir XXXX, XXX, XXX−XXX

Langmuir

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In bilayer assemblies, slow solvation dynamics having a component near or greater than 10 ns have already been discussed by considering the slow movement of geometrically strained and fully hydrated polyoxyethylene chains.36,37,62 Our bilayer system contains long-chain surfactant molecules and a broad distribution of probe molecules inside the bilayer headgroup region cannot be negligible. However, in our system, we have obtained the slow component of ∼800 ps. Hence, the contribution of self-diffusion of dyes (long lifetime) along the radial direction of the vesicle vesicles is much less. Due to limited resolution of the TCSPC setup, we are missing the fast component of the solvation dynamics (