Combined phase behavior, dynamic light scattering, viscosity and spectroscopic investigations of a pyridinium-based ionic liquid-in-oil microemulsion

Abstract

Although several studies of imidazolium-based ionic liquid-in-oil microemulsions are available in the literature, studies on pyridinium-based ionic liquid microemulsions are uncommon. Pyridinium-based ionic liquids have superior properties, but their properties in the polar part of the microemulsion have yet to be explored. 1-Butyl-4-methyl pyridinium tetrafluoroborate ([b₄mpy][BF₄])–(Tween 20 + n-pentanol)–n-heptane ionic liquid-in-oil microemulsion system has been studied by combined phase behavior, dynamic light scattering, viscosity, and UV-visible absorption and emission spectroscopic techniques. As the ratio of Tween 20 to n-pentanol (S/CS) was decreased, the stability of the microemulsions also decreased, although it was not possible to achieve a stable microemulsion without the n-pentanol cosurfactant. Dynamic light scattering and viscosity studies revealed that the size of the microemulsion droplets increased with increasing volume fraction (φ_d) of ionic liquid. Viscosity also increased with φ_d. With an increasing amount of n-pentanol, the variation was less sensitive due to the reduced polarity of the medium induced by the alkanol. Increase in the size of microemulsion droplets was overshadowed by the increase in the fluidity of the medium, for which viscosity decreased with increasing temperature, as is common for Newtonian fluids. The state of the ionic liquid in the microemulsion was monitored by fluorescence and absorption spectroscopy with and without curcumin as the molecular probe, respectively. While a continuous increase in polarity of the IL part occurred with an increasing amount of IL, the fluorescence anisotropy results revealed that the rigidity of the part passed through maxima for all S/CS combinations. A spherical morphology of the microemulsion droplets was established by transmission electron microscopy measurements.

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1. Introduction

A microemulsion can be defined as an optically transparent, thermodynamically stable dispersion of one liquid in an otherwise immiscible second liquid, stabilized by a surfactant.¹ Sometimes, short-chain alkanols and amines can assist microemulsion formation.¹ It has also been reported that an ionic liquid (IL) can substitute for the polar component (water) in a microemulsion.² There has been a considerable increase in research involving ILs. ILs with melting points below 100 °C are considered as neoteric components of microemulsions because of its specific properties, viz., non-flammability, non-corrosiveness, high ionic conductivity and inertness towards different thermal and chemical environments.³ One of the outstanding properties of ILs lies in their use as alternatives to traditional organic solvents. ILs are also called “designer solvents” because their properties can be tuned by altering the substituents as well as the counter ions.⁴ In spite of the considerable research on ILs, what remains unknown is considered to be one of the barriers in utilizing them for practical applications. Thus more fundamental research on ILs is warranted.

Microemulsions containing ILs in the polar part have some unknown but novel properties owing to the unique and combined features of the ILs and microemulsions. Ionic liquid microemulsions find application in various fields, viz., preparation and characterization of polymeric nanoparticles,⁵ synthesis of inorganic nanoparticles,⁶ renewable lubricants,⁷ catalysis,⁸ etc. Additional uses of microemulsions include drug delivery, nanoparticle synthesis, media for organic reactions, biochemical reactions, separation, cosmetics,⁹ etc.

Most of the studies on ILs are associated with the imidazolium ion. However, there has been a recent trend to search for easily available and low-cost alternative ILs other than the imidazolium ion. According Domanska et al.,¹⁰ pyridinium-based ILs have specific properties, viz., broad temperature range in the liquid state, inertness to air, moisture and superior solubilization capacity. These unique features of pyridinium-based ILs have already been explored with special reference to antistatic thermoplastic resins,¹¹ adhesive films,¹² electrochemical probes,¹³ electron transfer processes,¹⁴ reaction acceleration,¹⁵ organocatalysis,¹⁶ extraction processes,¹⁷ etc. In spite of the high potential, there has been little research on microemulsions containing 1-butyl-4-methyl pyridinium tetrafluoroborate ([b₄mpy][BF₄]) although it is one of the most researched pyridinium-based ILs.¹⁸ In a very recent report by Takumi et al.,¹⁹ mutual miscibility of imidazolium and pyridinium ILs with [BF₄]⁻ as the common counter anion was explored. Another advantage of using the pyridinium-based IL is that the system itself can be investigated without any molecular probe (because of the presence of the pyridinium ring) in the UV-visible region.

Curcumin is a natural polyphenolic compound isolated from the rhizome of turmeric (Curcuma longa). Apart from its biological activities, curcumin nowadays is being used as a molecular probe.^20,21 When used in IL-in-oil microemulsions, curcumin can only reside in the polar part of the microemulsion, as it is insoluble in n-heptane. Thus curcumin can conveniently be used as a molecular probe in investigating the microenvironment of the polar part of the ionic liquid-in-oil microemulsion.

The use of nonionic surfactants is always advantageous in microemulsion formulations as these surfactants are stable over a wide pH range and have a high solubilisation capability.^22–24 Use of nonionic surfactants in combination with short-chain alkanols is well documented in the literature.^25–38 Kundu et al.^30–33 and Bardhan et al.^25,26 have recently reported studies involving Brij 56, Brij 58, Brij 76, Triton X 100 and Tween 80 in the presence of ionic liquids. Bardhan et al.²⁶ have studied Brij 35–n-pentanol–isopropyl myristate water-in-oil microemulsion systems in combination with the anionic surfactant sodium dodecyl sulfate. Li et al.³⁹ have investigated the quaternary mixed system comprising water, Triton X 100, alkanol and n-heptane. Among the nonionic surfactants, polyoxyethylene sorbitan esters (Tweens) are the most studied surfactants because of their low price and easy availability.^35,40–43 According to Yaghmur et al.,⁴³ the presence of a bulky polyoxyethylene head group, attached to a sorbitan ring, results in the hydrophilicity of Tweens. In another report by Shevachman et al.,⁴¹ it was proposed that Tween-based surfactants can form a less compact structure at the oil–water interface. In spite of tremendous application potential, only a few studies have been carried out using Tween 20 in combination with IL.^44,45

Use of short-chain alcohols as a cosurfactant in preparing microemulsions is well documented in the literature.^{27,35,36,41,43,46,47} Short-chain alkanols can decrease the polar part–oil interfacial tension, which is otherwise unachievable by single-chain commercial surfactants, and thus modify the spontaneous curvature of the surfactant film at the interface. Cosurfactant molecules can also act as spacers between surfactant molecules at the interface,⁴⁰ and additionally can alter the polarity of the polar and apolar phases when used as a cosolvent.⁴¹ Digout et al.,²⁸ Li et al.,³⁹ Moulik et al.³⁶ and Abuin et al.⁴⁶ have carried out systematic investigations using n-alkanol to understand the effect of chain length on the stability of different microemulsions. Among the alkanols tested by Digout et al.,²⁸ n-pentanol is the smallest one that can achieve formation of microemulsion droplets with an enhanced interfacial rigidity. In our present study we therefore used n-pentanol in combination with Tween 20.

The current manuscript describes comprehensive studies on an IL-in-oil-type microemulsion comprising 1-butyl-4-methyl pyridinium tetrafluoroborate. The effect of Tween 20 (surfactant)–n-pentanol (cosurfactant) mass ratio, volume fraction of IL and temperature have been studied using a number of techniques, viz., phase manifestation, dynamic light scattering, viscosity, UV-visible absorption and emission spectroscopy. Detailed phase diagram studies helped in identifying the single-phase microemulsion and the two-phase turbid regions. Dynamic light scattering studies provided information about the size and distribution of the droplets at different temperatures; viscosity measurements were carried out and correlated with the DLS data. The ionic liquid in the polar part of the microemulsion was investigated by UV-visible absorption spectroscopy. Fluorescence spectroscopic studies using curcumin in the polar part helped in understanding the polarity and rigidity of the microenvironment.

2. Experimental section

2.1. Materials

The IL 1-butyl-4-methyl pyridinium tetrafluoroborate, [b₄mpy][BF₄] was purchased from Sigma-Aldrich Chemicals Pvt. Ltd., USA. The nonionic surfactant polyoxyethylene sorbitan monolaurate (Tween 20) and the cosurfactant n-pentanol were obtained from Fluka, Switzerland and Lancaster, England respectively. They were stated to be more than 99.5% pure. HPLC-grade n-heptane was obtained from E. Merck, Germany. Curcumin, [1,7-bis(4-hydroxy-3-methoxy-phenyl)-1,6-heptadiene-3,5-dione] was obtained from Sigma-Aldrich Chemicals Pvt. Ltd., USA. All the chemicals, except Tween 20, were used as received. Tween 20, as stated by the supplier, comprised <3 wt% water. Water was removed following the protocol of Schubert et al.⁴⁸ Karl-Fischar titration revealed the presence of <0.5 wt% water in the purified sample.

2.2. Methods

2.2.1. Phase diagram construction. In this work, Tween 20 (S) and n-pentanol (CS) were used in three different mass ratios (1 : 0.5, 1 : 1 and 1 : 2) to explore the effect of cosurfactant on the microemulsion system. A pseudoternary phase diagram comprising [b₄mim][BF₄]–(Tween-20 + n-pentanol)–n-heptane was constructed using titration and visual inspection. Known amounts of Tween 20 + n-pentanol and n-heptane or IL were placed in a stoppered test tube. IL or n-heptane was then progressively added using a Hamilton (USA) microsyringe under constant stirring.^49,50 The whole process was carried out in a controlled temperature bath (298 ± 0.1 K). The phase boundary was detected by the appearance of turbidity. The same experiment was carried out for a number of compositions by varying the amount of oil or IL as well as in different S/CS ratios.

2.2.2. Dynamic light scattering (DLS) studies. DLS measurements were carried out using a Zetasizer Nano ZS90 (ZEN3690, Malvern Instruments Ltd, U.K.). A 0.2 M Tween 20 mixed with appropriate amount of n-pentanol dissolved in n-heptane was used for such studies. Tween 20–n-pentanol ratios (w/w) were the same as in the phase diagram construction studies. A He–Ne laser operating at a wavelength of 632.8 nm was used and the data were collected at a 90° angle. Temperature was controlled to within ±0.05 K using a built-in Peltier heating-cooling device. The instrument actually measures the diffusion coefficient (D) from which the diameter of the microemulsion droplet (d) was determined according to Stokes-Einstein's formalism:^49,50where k, T and η indicate the Boltzmann constant, temperature and viscosity of the solvent (herein n-heptane) respectively.

2.2.3. Viscosity measurement. Viscosities of microemulsion systems were measured with an LVDV-II+PCP cone and plate type roto-viscometer (Brookfield Eng. Lab, USA). The same set of solutions that was used in the DLS measurements was also employed for size analyses. Temperature was controlled to within ±0.1 K using a cryogenic circulatory water bath (DC-1006 M/S Hahntech Corporation, S. Korea). Shear rate (r) was varied in the range 20–60 sec⁻¹ with an increment of 5.0 sec⁻¹ in each step. Zero shear viscosity (η) was obtained using the relation η= τ/r,^49,50 where τ indicates the shear stress.

2.2.4. Spectral studies.
2.2.4.1. Absorption spectra. UV-visible absorption spectra of the microemulsion systems without any other molecular probes were recorded on a UVD-2950 spectrophotometer (Labomed Inc., USA) in the range 200–400 nm using a matched pair of cells of 1.0 cm path length. While recording the spectra of the systems containing IL, a corresponding surfactant solution without the IL was used as a reference.
2.2.4.2. Emission spectra. Steady-state fluorescence spectroscopic measurements were carried out using a benchtop spectrofluorimeter (Quantamaster-40, Photon Technology International Inc., NJ, USA). The steady-state emission spectra were recorded in the range 400–650 nm with an excitation of curcumin at 426 nm. While recording the fluorescence spectra, the overall concentration of curcumin was always kept constant at 10 μM. Initially, the required amount of curcumin in methanol–chloroform (1 : 3 v/v) was placed in a test tube. The solvent was evaporated under vacuum. A microemulsion of known composition was then added and homogenized by keeping the solution in an ultrasonic water bath. Note that since curcumin is insoluble in n-heptane, it could be assumed that these dye molecules reside in the polar part.

To understand the microviscosity of the solvent surrounding the probe molecules, steady-state anisotropy (r) values were determined using the following expressions:⁵¹

r = (I_VV − GI_VH)/(I_VV + 2GI_VH) (2)

and,

G = I_HV/I_HH (3)

where I_VV, I_VH are the intensities obtained with the excitation polarizer oriented vertically and the emission polarizer oriented vertically and horizontally respectively; I_HV and I_HH refer to the similar parameters as above for the horizontal positions of the excitation polarizer. In case of anisotropy measurements, the fluorescence data were collected at an emission wavelength (λ_em) of 550 nm. Further details are available in the literature.⁵¹ Both the absorption and fluorescence spectra were recorded at ambient but controlled temperature.

2.2.5. Morphological studies: transmission electron microscopy (TEM). In order to further understand the morphology of the microemulsion droplets, transmission electron microscopic (TEM) studies were carried out. A microemulsion dispersion was diluted 10-fold in n-heptane. 2 μL of the diluted solution was then drop-casted onto a TEM grid (Formavar/Carbon, 300 mesh Cu, Agar Scientific, UK). The solution was allowed to dry in air at room temperature overnight and vacuum dried before imaging. TEM was performed on a Technai F30 UHR version electron microscope, using a field emission gun (FEG) operating at an accelerating voltage of 200 kV. As this method may lead to the disruption of the microemulsion, freeze-fracture TEM (FF-TEM) analyses were also carried out. However, due to the very low surface tension of the continuous medium (n-heptane) the freeze replica could not be generated. Hence only the TEM images obtained by the conventional technique have been reported.

3. Results and discussions

3.1. Phase diagram construction

From the application point of view, construction of the phase diagram is a primary task towards a microemulsion formulation. Fig. 1 describes the pseudoternary phase diagram of [b₄mpy][BF₄]–(Tween-20 + n-pentanol)–n-heptane systems at different surfactant–cosurfactant ratios (w/w). The shaded areas under the curves represent the two-phase turbid region, where the stable microemulsions were not formed. The unshaded portions correspond to the microemulsion zone. In the currently studied systems, oil-rich (ionic liquid-in-oil microemulsion), ionic liquid-rich (oil-in-ionic liquid microemulsion) as well as bicontinuous states were observed. However, for simplicity, all these three different regions were represented as a single phase (microemulsion zone; the unshaded portion in the pseudoternary phase diagram). All further experiments were carried out using the oil-rich (i.e., ionic liquid-in-oil) microemulsion systems. The areas under the microemulsion region (unshaded portion) and the turbid regions were calculated by simply weighing the individual areas as previously described.^49,50 With increasing amount of cosurfactant (with respect to surfactant), stability of the microemulsion decreased as indicated by the decrease in the area of the unshaded portions of the phase diagrams. Results have been graphically presented in Fig. S1 (ESI†). Percentage area of the microemulsion zone was plotted against the wt% of the cosurfactant (w_cs%) and it was observed that such a profile followed a second-order polynomial relation whereby the maximum microemulsion zone would have appeared in the absence of cosurfactant:

%Area of microemulsion zone = 20.34–8.46 × w_CS% + 1.61 × (w_CS%)² (4)

Fig. 1 Pseudoternary phase diagram of [b₄mpy][BF₄]–(Tween-20 + n-pentanol)–n-heptane system at different Tween 20 (S)–n-pentanol (CS) ratio (w/w): (A) 1 : 0.5; (B) 1 : 1 and (C) 1 : 2. n-Heptane was used as oil (Oil) and [b₄mpy][BF₄] was used as the ionic liquid (IL). Temp. 298 K.

This observation implies that in the absence of n-pentanol the area of microemulsion zone would have been 20% when that for the turbid region is 80%. Interestingly a stable microemulsion without n-pentanol was unachievable. Hence, unlike the other systems,^2,52 use of cosurfactant was mandatory in order to achieve a stable microemulsion. Use of cosurfactant for single-tailed surfactants is not uncommon in literature.^49,50 The present set of results is also comparable with the similar set of components where water⁴⁹ and another ionic liquid, 1-butyl-3-methyl imidazolium methane sulphonate [bmim][MS],⁵⁰ were used. Compared to the other systems, the % area of the microemulsion zone in the current work is less, which could be due to the greater ionicity of the components in the polar part, compared to water as well as [bmim][MS]. Although the cation [b₄mpy]⁺ was less polar, the BF₄⁻ ion played a significant role in reducing the stability of the microemulsion; thus more two-phase turbid regions resulted.

As already mentioned in the Introduction, n-pentanol serves a dual purpose for nonionic microemulsions. Firstly, it reduces the interfacial tension at the oil–polar part interface through the modification of spontaneous curvature. This phenomenon helps in the formation of a stable microemulsion. However the second contribution of the cosurfactant, reduction of interfacial polarity in the present case, resulted in destabilization of the microemulsion. Here, the polar part comprises ionic liquid; hence, it is not unexpected that addition of n-pentanol would destabilise the microemulsion by reducing the interfacial polarity.

3.2. Dynamic light scattering (DLS) and viscosity studies

DLS studies on microemulsions can provide information on its droplet size, its distribution, and hence the polydispersity index. Variation in the diameter of [b₄mpy][BF₄]–(Tween 20+n-pentanol)–n-heptane IL-in-oil microemulsion with the volume fraction of the IL (φ_d) at 308 K is graphically presented in Fig. 2 at different surfactant to cosurfactant ratios (panel A). Droplets were fairly monodispersed as indicated by the size distribution shown in Fig. S2 (ESI†). If the other conditions are kept the same, there is a correlation between the size of droplets and viscosity, hence the viscosity (η)–φ_d profile for the same systems are also presented in panel B of the same figure (Fig. 2). Experiments were carried out in the temperature range 293–323 K and the representative results at 308 K (intermediate temperature) are shown. Results for the other systems are presented in the ESI.† The size of the microemulsion droplets increased with increasing volume fraction (φ_d) of IL. Increase in size with increasing volume fraction of the dispersed phase is not an uncommon phenomenon and has been documented by others.^{2,44,50,52,53}

Fig. 2 Variation in diameter (d) and viscosity (η) of [b₄mpy][BF₄]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil microemulsion system with the volume fraction (φ_d) of [b₄mpy][BF₄]. Temp. 308 K. Tween 20–n-pentanol ratio (w/w): O, 1 : 0.5; ■, 1 : 1 and Δ, 1 : 2.

Size–φ_d profiles were found to be dependent on S/CS ratio. The size of the microemulsion droplet comprising Tween 20 and n-pentanol for the systems with a S/CS mass ratio of 1 : 0.5 was found to be larger than that of 1 : 1, which was even larger than that of 1 : 2. These results clearly indicate that the cosurfactant caused size constriction. A large number of cosurfactant molecules at the IL-oil interface results in the formation of a compact structure. The n-pentanol cosurfactant enhances the compactness and hence rigidity of the interface, as also reported by Digout et al.²⁸ In a previous report we showed that increasing the amount of surfactant containing the polyoxyethylene head group (Tween 20) could lead to the formation of a larger number of droplets.^49,50 For a fixed amount of surfactant, increasing the number of droplets could result only at the expense of the size of the droplets. Such an observation further supports the decrease in the area of the microemulsion region with increasing amount of cosurfactant, as in the phase diagram construction studies. A close inspection of panel A of Fig. 2 shows that for an S/CS ratio of 1 : 0.5, the size of the microemulsion droplet increased linearly up to φ_d = 0.01, after which the slope of the increment profile changed. Initially when IL molecules are added they strongly coordinate with the polyoxyethylene head group of Tween 20. With the further addition of IL they become free and behave like bulk material.⁵⁰ Almost twice the volume of IL was required to attain the breakpoint in the microemulsion with S/CS ratio 1 : 1 (φ_d = 0.02). However, for such systems the slope after the threshold was higher. Such an ambiguity cannot be explained with the present level of knowledge. Diameter (d) vs. φ_d profile for the systems with a surfactant to cosurfactant ratio of 1 : 2 was almost linear. The d–φ_d profile for all the systems at different temperatures are graphically presented in Fig. S3 (ESI†). It could be concluded from the results that with the increasing amount of cosurfactant (n-pentanol), size increment becomes less sensitive to the volume fraction. The presence of a larger number of n-pentanol molecules at the interface and subsequent enhancement of the interfacial rigidity was the causative factor for such an observation. The viscosity profile for the similar systems followed the same trend line as in the variation of droplet size with φ_d. Thus, it could be concluded that variations in the viscosity with φ_d was a consequence of the size variation in the microemulsion droplets.

In order to understand the effect of temperature, DLS studies were carried out at different temperatures (293, 298, 303, 308, 313, 318 and 323 K). As shown in Fig. 3 panel A, it was observed that the dependence of diameter on temperature was almost linear for all the systems. With increasing temperature, the size of the droplets was observed to increase, as has also been reported by others.⁵⁴ An increase in the volume of the dispersed phase as well as the “softening” of the interfacial region with increasing temperature contribute to the increase in droplet size. Additionally, with the increase in temperature, the probability of interdroplet contact increases. Spherical geometries are usually assumed in dynamic light scattering studies. However according to the proposition of Nazário et al.,⁵⁵ Tweens in combination with alkanols can experience reasonable shape fluctuation which eventually impart increased droplet diameter. In the present set of studies, the microemulsion droplets were bigger than the conventional water-in-oil microemulsion droplets or swollen reverse micelles. It is not unexpected that such entities may have shapes/geometry other than spheres. To address such issues further studies, such as cryo-TEM and small angle neutron scattering, are warranted. The parallel nature of the size-temperature profiles imply that the effect of temperature was the same for all the systems (independent of S/CS ratio).

Fig. 3 Dependence of the size (A) and viscosity (B) on temperature for [b₄mpy][BF₄]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil microemulsion at different Tween 20–n-pentanol ratio (S/CS, w/w): O, 1 : 0.5; ■, 1 : 1 and Δ, 1 : 2.0.2 M Tween 20 was used in each case where the volume fraction (φ_d) of IL was kept constant at 0.02.

Although size increased with increasing temperature, viscosity decreased with increasing temperature for all the systems. Results for viscosity–temperature profile for the three combinations at φ_d = 0.02 are shown in Fig. 3 (panel B). A decrease in viscosity with an increase in temperature is a common phenomenon for Newtonian fluids. In the present set of studies, increase in droplet size was overshadowed by the increase in the fluidity of the medium. For all three systems, viscosity decreased almost linearly with increasing temperature, although the slopes were different for the different systems. Differences in the slopes were caused by the differences in the rigidity of the microemulsion droplets. Systems with a larger proportion of cosurfactant are expected to form more rigid structures. The dependence of viscosity on volume fraction and temperature is also graphically presented in Fig. S4 (ESI†) for all the systems.

3.3. Spectral studies

Spectroscopic investigation on microemulsions using a suitable probe can provide useful information on the microenvironment, viz., polarity, fluidity, and state of the aggregates of the ionic liquid in the polar part of the microemulsion. Much research has been reported on these topics.^2,44,50,53 For the present system, since the ionic liquid itself exhibits a UV-visible absorption band, due to the presence of the pyridinium ring, the microemulsion was studied without any probe. Fig. 4 shows the absorption spectra of [b₄mpy][BF₄] confined in the polar part at different volume fractions. Two distinct peaks, one at 280 nm and the other at 335 nm, were recorded for the ionic liquid. The peak at 4.6. 280 nm was significantly more intense than the other. Intensity of both the bands increased with increasing volume fraction (φ_d) of the ionic liquid. Absorbance vs. φ_d profiles for both the peaks are graphically shown in Fig. 5. The increases in the absorbance values were mostly linear except for the system with Tween 20–n-pentanol in a ratio of 1 : 0.5 (w/w). With the initial addition of IL to reverse micelles, formation of strongly coordinated species between the IL cation and oxyethylene groups of Tween 20 are initiated, resulting in the formation of swollen reverse micelles. After many IL molecules are added, they becomes free and behave like bulk liquid; thus the microemulsion is formed. These results were the same as those from the phase behaviour, DLS and viscosity studies.

Fig. 4 Absorption spectra of 1-butyl-4-methyl pyridinium tetrafluoroborate [b₄mpy][BF₄] confined in the polar part at different volume fractions (φ_d): (1), 0.007; (2), 0.014; (3), 0.022 and (4), 0.03. System without the IL was used as reference.

Fig. 5 Dependence of absorbance on volume fraction (φ_d) for 1-butyl-4-methyl pyridinium tetrafluoroborate [b₄mpy][BF₄], which is confined to the polar part. Absorbance values were recorded at wavelengths of 283 nm (A) and 335 nm (B). Tween 20–n-pentanol ratio (w/w): O, 1 : 0.5; ■, 1 : 1 and Δ, 1 : 2.

The microenvironments of the polar part in the reverse micelles are supposed to be different at different ratios of surfactant and cosurfactant, as the polarity of the interface (oil/IL) are dependent on the relative abundance of cosurfactant at the interface. In order to understand the microheterogeneity of the reverse micelles (herein IL in oil microemulsion) curcumin was used as the fluorescent probe. For such systems the fluorescent probe can have different locations ranging from the IL/oil interface to the core of the polar part.⁵⁶ Fluorescence spectra of curcumin confined in the polar part of the IL-in-oil microemulsion are shown in Fig. 6. When excited at 426 nm, curcumin shows an emission maximum at ∼500 nm. Results were found to be comparable with the previous report.⁵⁷ A red shift in the emission maxima along with a decrease in the fluorescence intensity with increasing volume fraction of the ionic liquid were observed, as shown in Fig. 7. The decrease in fluorescence intensity (panel A) was caused by the localized dilution of the probe in the IL pool.^2,44,50,53 The progressive red shift (panel B, Fig. 7) in the emission maxima is due to the increased polarity of the part with increasing volume fraction of the ionic liquid.⁵⁰ As already mentioned previously, with the initial progressive addition of the IL to the reverse micelles, strongly coordinated species start forming between the IL cation and oxyethylene head group of Tween 20, resulting in the formation of swollen reverse micelles. Further addition of IL led to the occurrence of free IL (bulk), as in the case of ionic liquid-in-oil microemulsions. A transition from the swollen reverse micelles to microemulsions was confirmed through the appearance of breaks/halts in the red shift of the fluorescence maxima of curcumin with increasing volume of IL.⁵⁶ Such breakpoints were dependent on the Tween 20–n-pentanol mass ratio. Breakpoints appeared at higher volume fraction of the IL with increasing proportion of n-pentanol.

Fig. 6 Emission spectra of 10 μM curcumin in [b₄mpy][BF₄]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil microemulsions at different volume fractions (φ_d) of IL. φ_d values: (1), 0.00; (2), 0.007; (3), 0.014; (4), 0.022; (5), 0.03 and (6), 0.035. Tween 20–n-pentanol ratio (w/w): 1 : 1. Inset: variation in the fluorescence intensity (panel A) and λ_em (panel B) with φ_d for different Tween 20–n-pentanol ratio (w/w): O, 1 : 0.5; □, 1 : 1 and Δ, 1 : 2.

Fig. 7 Variation in the fluorescence intensity (panel A) and λ_em (panel B) of 10 μM curcumin with φ_d confined in [b₄mpy][BF₄]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil microemulsion. Tween 20–n-pentanol ratio (w/w): O, 1 : 0.5; ■, 1 : 1 and Δ, 1 : 2.

To know the exact state of the solvent in the pool, fluorescence anisotropy studies on curcumin were carried out. The dependence of fluorescence anisotropy (r) on volume fraction (φ_d) of the ionic liquid is graphically shown in Fig. 8.

Fig. 8 Values of fluorescence anisotropy (r) for 10 μM curcumin at different volume fractions (φ_d) of IL for the [b₄mpy][BF₄]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil microemulsion system. Tween 20–n-pentanol ratio (w/w): O, 1 : 0.5; □, 1 : 1 and Δ, 1 : 2. Temp, 298 K.

Anisotropy values passed through maxima with respect to the volume fraction of IL. Initially ILs are used up in forming coordinated species with the oxyethylene head groups of Tween 20 for which structured entities are formed (like swollen reverse micelles). At that stage the dye molecules did not have freedom of movement. After the coordination of surfactant head groups by IL cations was completed, excess ILs became free and could behave as bulk IL, as in the case of ionic liquid-in-oil microemulsions. Under this situation, the mobility of the dye molecules increased. This eventually led to the decrease in the fluorescence anisotropy values. A linear increase in the anisotropy value for the system comprising Tween 20–n-pentanol in a 1 : 2 w/w ratio was due to the continued solvation of the cationic component of the ionic liquid, which was assisted by the presence of a larger number of alkanol molecules.

3.4. Morphological study: TEM analyses

Morphological studies on the microemulsion droplets of different compositions were investigated by transmission electron microscopy. A representative TEM image for a microemulsion system with Tween 20–n-pentanol at a 1 : 1 mass ratio is shown in Fig. 9.

Fig. 9 Representative TEM image of [b₄mpy][BF₄]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil microemulsion. Tween 20–n-pentanol (w/w): 1 : 1. Volume fraction (φ_d) of IL: 0.015. Scale bar: 200 nm.

The particles studied by transmission electron microscopy were found to be spherical. However, particle surfaces were not smooth, which was possibly due to evaporation of the solvents. Sizes of the microemulsion droplets as derived from dynamic light scattering were found to be smaller than those from TEM. Such a difference was due to the variation in the sample preparation and the subsequent analyses. However, spherical morphologies, as obtained from the TEM measurements, further support the assumption as made in the dynamic light scattering studies. The relatively large average size of the droplets, as indicated by the TEM measurements, was possibly due to the aggregation of smaller precursor droplets.

4. Conclusions

Comprehensive studies of a 1-butyl-4-methyl pyridinium tetrafluoroborate ([b₄mpy][BF₄])–(Tween 20 + n-pentanol)–n-heptane microemulsion system were carried out using a number of different physicochemical techniques. Although high concentrations of n-pentanol destabilized the microemulsion, some of this cosurfactant was nevertheless essential for the formation of a stable microemulsion. Cosurfactant controlled the curvature of the microemulsion droplets, imparted better stability by solvating the cationic component of the ionic liquid, and aided the formation of a larger number of droplets. Temperature sensitivity decreased as the proportion of cosurfactant with respect to Tween 20 increased, as revealed through the combined dynamic light scattering and viscosity measurements. Oxyethylene groups of Tween 20 formed coordinate linkages with the IL cation, which enhanced the rigidity of the polar part. IL, when in excess of the amount required for coordinating the surfactant head groups, behaved like a bulk component according to the occurrence of red shift in the fluorescence maxima and appearance of maxima in fluorescence anisotropy values. Electron microscopic studies established the spherical morphology of the droplets.

Acknowledgements

Financial support in the form of a research grant from the Department of Science and Technology, Govt. of India (grant number: SR/S1/PC/24/2008) is sincerely acknowledged. The manuscript is dedicated to Prof. Ajit Kumar Chakraborty, one of the mentors of AKP, on his 77^th birthday.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01209g

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