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Combined phase

Department of Chemistry, University of N

Bengal, India. E-mail: akpanda1@yahoo.co

† Electronic supplementary informa10.1039/c4ra01209g

Cite this: RSC Adv., 2014, 4, 32383

Received 11th February 2014Accepted 2nd July 2014

DOI: 10.1039/c4ra01209g

www.rsc.org/advances

This journal is © The Royal Society of C

behavior, dynamic lightscattering, viscosity and spectroscopicinvestigations of a pyridinium-based ionicliquid-in-oil microemulsion†

Sujoy Paul and Amiya Kumar Panda*

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 (4d) of ionic liquid. Viscosity also increased with 4d.

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.

1. Introduction

A microemulsion can be dened as an optically transparent,thermodynamically stable dispersion of one liquid in anotherwise immiscible second liquid, stabilized by a surfactant.1

Sometimes, short-chain alkanols and amines can assist micro-emulsion formation.1 It has also been reported that an ionicliquid (IL) can substitute for the polar component (water) in amicroemulsion.2 There has been a considerable increase inresearch involving ILs. ILs with melting points below 100 �C areconsidered as neoteric components of microemulsions becauseof its specic properties, viz., non-ammability, non-corrosive-ness, high ionic conductivity and inertness towards differentthermal and chemical environments.3 One of the outstandingproperties of ILs lies in their use as alternatives to traditional

orth Bengal, Darjeeling 734 013, West

m

tion (ESI) available. See DOI:

hemistry 2014

organic solvents. ILs are also called “designer solvents” becausetheir properties can be tuned by altering the substituents as wellas the counter ions.4 In spite of the considerable research onILs, what remains unknown is considered to be one of thebarriers in utilizing them for practical applications. Thus morefundamental research on ILs is warranted.

Microemulsions containing ILs in the polar part have someunknown but novel properties owing to the unique andcombined features of the ILs and microemulsions. Ionic liquidmicroemulsions nd application in various elds, viz., prepa-ration and characterization of polymeric nanoparticles,5

synthesis of inorganic nanoparticles,6 renewable lubricants,7

catalysis,8 etc. Additional uses of microemulsions include drugdelivery, nanoparticle synthesis, media for organic reactions,biochemical reactions, separation, cosmetics,9 etc.

Most of the studies on ILs are associated with the imidazo-lium ion. However, there has been a recent trend to search foreasily available and low-cost alternative ILs other than theimidazolium ion. According Domanska et al.,10 pyridinium-based ILs have specic properties, viz., broad temperature range

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in the liquid state, inertness to air, moisture and superiorsolubilization capacity. These unique features of pyridinium-based ILs have already been explored with special reference toantistatic thermoplastic resins,11 adhesive lms,12 electro-chemical probes,13 electron transfer processes,14 reactionacceleration,15 organocatalysis,16 extraction processes,17 etc. Inspite of the high potential, there has been little research onmicroemulsions containing 1-butyl-4-methyl pyridinium tetra-uoroborate ([b4mpy][BF4]) although it is one of the mostresearched pyridinium-based ILs.18 In a very recent report byTakumi et al.,19 mutual miscibility of imidazolium and pyr-idinium ILs with [BF4]

� as the common counter anion wasexplored. Another advantage of using the pyridinium-based ILis that the system itself can be investigated without anymolecular probe (because of the presence of the pyridiniumring) in the UV-visible region.

Curcumin is a natural polyphenolic compound isolated fromthe rhizome of turmeric (Curcuma longa). Apart from its bio-logical activities, curcumin nowadays is being used as amolecular probe.20,21 When used in IL-in-oil microemulsions,curcumin can only reside in the polar part of the micro-emulsion, as it is insoluble in n-heptane. Thus curcumin canconveniently be used as a molecular probe in investigating themicroenvironment of the polar part of the ionic liquid-in-oilmicroemulsion.

The use of nonionic surfactants is always advantageous inmicroemulsion formulations as these surfactants are stableover a wide pH range and have a high solubilisation capa-bility.22–24 Use of nonionic surfactants in combination withshort-chain alkanols is well documented in the literature.25–38

Kundu et al.30–33 and Bardhan et al.25,26 have recently reportedstudies involving Brij 56, Brij 58, Brij 76, Triton X 100 and Tween80 in the presence of ionic liquids. Bardhan et al.26 have studiedBrij 35–n-pentanol–isopropyl myristate water-in-oil micro-emulsion systems in combination with the anionic surfactantsodium dodecyl sulfate. Li et al.39 have investigated thequaternary mixed system comprising water, Triton X 100,alkanol and n-heptane. Among the nonionic surfactants, poly-oxyethylene sorbitan esters (Tweens) are the most studiedsurfactants because of their low price and easy availability.35,40–43

According to Yaghmur et al.,43 the presence of a bulky polyoxy-ethylene head group, attached to a sorbitan ring, results in thehydrophilicity of Tweens. In another report by Shevachmanet al.,41 it was proposed that Tween-based surfactants can form aless compact structure at the oil–water interface. In spite oftremendous application potential, only a few studies have beencarried out using Tween 20 in combination with IL.44,45

Use of short-chain alcohols as a cosurfactant in preparingmicroemulsions is well documented in the litera-ture.27,35,36,41,43,46,47 Short-chain alkanols can decrease the polarpart–oil interfacial tension, which is otherwise unachievable bysingle-chain commercial surfactants, and thus modify thespontaneous curvature of the surfactant lm at the interface.Cosurfactant molecules can also act as spacers betweensurfactant molecules at the interface,40 and additionally canalter the polarity of the polar and apolar phases when used as acosolvent.41 Digout et al.,28 Li et al.,39 Moulik et al.36 and Abuin

32384 | RSC Adv., 2014, 4, 32383–32390

et al.46 have carried out systematic investigations using n-alka-nol to understand the effect of chain length on the stability ofdifferent microemulsions. Among the alkanols tested by Digoutet al.,28 n-pentanol is the smallest one that can achieve forma-tion of microemulsion droplets with an enhanced interfacialrigidity. In our present study we therefore used n-pentanol incombination with Tween 20.

The current manuscript describes comprehensive studies onan IL-in-oil-type microemulsion comprising 1-butyl-4-methylpyridinium tetrauoroborate. The effect of Tween 20 (surfac-tant)–n-pentanol (cosurfactant) mass ratio, volume fraction ofIL and temperature have been studied using a number oftechniques, 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 aboutthe size and distribution of the droplets at different tempera-tures; viscosity measurements were carried out and correlatedwith the DLS data. The ionic liquid in the polar part of themicroemulsion was investigated by UV-visible absorptionspectroscopy. Fluorescence spectroscopic studies using curcu-min in the polar part helped in understanding the polarity andrigidity of the microenvironment.

2. Experimental section2.1. Materials

The IL 1-butyl-4-methyl pyridinium tetrauoroborate,[b4mpy][BF4] was purchased from Sigma-Aldrich Chemicals Pvt.Ltd., USA. The nonionic surfactant polyoxyethylene sorbitanmonolaurate (Tween 20) and the cosurfactant n-pentanol wereobtained from Fluka, Switzerland and Lancaster, Englandrespectively. 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-hepta-diene-3,5-dione] was obtained from Sigma-Aldrich ChemicalsPvt. Ltd., USA. All the chemicals, except Tween 20, were used asreceived. Tween 20, as stated by the supplier, comprised <3 wt%water. Water was removed following the protocol of Schubertet al.48 Karl-Fischar titration revealed the presence of <0.5 wt%water in the puried 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 onthe microemulsion system. A pseudoternary phase diagramcomprising [b4mim][BF4]–(Tween-20 + n-pentanol)–n-heptanewas constructed using titration and visual inspection. Knownamounts of Tween 20 + n-pentanol and n-heptane or IL wereplaced in a stoppered test tube. IL or n-heptane was thenprogressively added using a Hamilton (USA) microsyringeunder constant stirring.49,50 The whole process was carried outin a controlled temperature bath (298 � 0.1 K). The phaseboundary was detected by the appearance of turbidity. The same

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experiment was carried out for a number of compositions byvarying the amount of oil or IL as well as in different S/CS ratios.

2.2.2. Dynamic light scattering (DLS) studies. DLSmeasurements were carried out using a Zetasizer Nano ZS90(ZEN3690, Malvern Instruments Ltd, U.K.). A 0.2 M Tween 20mixed 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 constructionstudies. A He–Ne laser operating at a wavelength of 632.8 nmwas used and the data were collected at a 90� angle. Tempera-ture was controlled to within �0.05 K using a built-in Peltierheating-cooling device. The instrument actually measures thediffusion coefficient (D) from which the diameter of themicroemulsion droplet (d) was determined according to Stokes-Einstein's formalism:49,50

D ¼ kT

3phd(1)

where k, T and h indicate the Boltzmann constant, temperatureand viscosity of the solvent (herein n-heptane) respectively.

2.2.3. Viscosity measurement. Viscosities of micro-emulsion systems were measured with an LVDV-II+PCP coneand plate type roto-viscometer (Brookeld Eng. Lab, USA). Thesame set of solutions that was used in the DLS measurementswas also employed for size analyses. Temperature wascontrolled to within �0.1 K using a cryogenic circulatory waterbath (DC-1006 M/S Hahntech Corporation, S. Korea). Shear rate(r) was varied in the range 20–60 sec�1 with an increment of 5.0sec�1 in each step. Zero shear viscosity (h) was obtained usingthe relation h¼ s/r,49,50 where s indicates the shear stress.

2.2.4. Spectral studies2.2.4.1. Absorption spectra. UV-visible absorption spectra of

the microemulsion systems without any other molecular probeswere recorded on a UVD-2950 spectrophotometer (LabomedInc., USA) in the range 200–400 nm using amatched pair of cellsof 1.0 cm path length. While recording the spectra of thesystems containing IL, a corresponding surfactant solutionwithout the IL was used as a reference.

2.2.4.2. Emission spectra. Steady-state uorescence spectro-scopic measurements were carried out using a benchtop spec-trouorimeter (Quantamaster-40, Photon TechnologyInternational Inc., NJ, USA). The steady-state emission spectrawere recorded in the range 400–650 nm with an excitation ofcurcumin at 426 nm. While recording the uorescence spectra,the overall concentration of curcumin was always kept constantat 10 mM. Initially, the required amount of curcumin in meth-anol–chloroform (1 : 3 v/v) was placed in a test tube. The solventwas evaporated under vacuum. A microemulsion of knowncomposition was then added and homogenized by keeping thesolution in an ultrasonic water bath. Note that since curcuminis insoluble in n-heptane, it could be assumed that these dyemolecules reside in the polar part.

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

r ¼ (IVV � GIVH)/(IVV + 2GIVH) (2)

This journal is © The Royal Society of Chemistry 2014

and,

G ¼ IHV/IHH (3)

where IVV, IVH are the intensities obtained with the excitationpolarizer oriented vertically and the emission polarizer orientedvertically and horizontally respectively; IHV and IHH refer to thesimilar parameters as above for the horizontal positions of theexcitation polarizer. In case of anisotropy measurements, theuorescence data were collected at an emission wavelength(lem) of 550 nm. Further details are available in the literature.51

Both the absorption and uorescence spectra were recorded atambient but controlled temperature.

2.2.5. Morphological studies: transmission electronmicroscopy (TEM). In order to further understand themorphology of the microemulsion droplets, transmission elec-tron microscopic (TEM) studies were carried out. A micro-emulsion dispersion was diluted 10-fold in n-heptane. 2 mL ofthe diluted solution was then drop-casted onto a TEM grid(Formavar/Carbon, 300 mesh Cu, Agar Scientic, UK). Thesolution was allowed to dry in air at room temperature over-night and vacuum dried before imaging. TEM was performedon a Technai F30 UHR version electron microscope, using aeld emission gun (FEG) operating at an accelerating voltage of200 kV. As this method may lead to the disruption of themicroemulsion, freeze-fracture TEM (FF-TEM) analyses werealso carried out. However, due to the very low surface tension ofthe continuous medium (n-heptane) the freeze replica could notbe generated. Hence only the TEM images obtained by theconventional technique have been reported.

3. Results and discussions3.1. Phase diagram construction

From the application point of view, construction of the phasediagram is a primary task towards a microemulsion formula-tion. Fig. 1 describes the pseudoternary phase diagram of[b4mpy][BF4]–(Tween-20 + n-pentanol)–n-heptane systems atdifferent surfactant–cosurfactant ratios (w/w). The shaded areasunder the curves represent the two-phase turbid region, wherethe stable microemulsions were not formed. The unshadedportions correspond to the microemulsion zone. In thecurrently studied systems, oil-rich (ionic liquid-in-oil micro-emulsion), ionic liquid-rich (oil-in-ionic liquid microemulsion)as well as bicontinuous states were observed. However, forsimplicity, all these three different regions were represented asa single phase (microemulsion zone; the unshaded portion inthe pseudoternary phase diagram). All further experiments werecarried out using the oil-rich (i.e., ionic liquid-in-oil) micro-emulsion systems. The areas under the microemulsion region(unshaded portion) and the turbid regions were calculated bysimply weighing the individual areas as previouslydescribed.49,50 With increasing amount of cosurfactant (withrespect to surfactant), stability of the microemulsion decreasedas indicated by the decrease in the area of the unshadedportions of the phase diagrams. Results have been graphicallypresented in Fig. S1 (ESI†). Percentage area of the

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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 asoil (Oil) and [b4mpy][BF4] was used as the ionic liquid (IL). Temp. 298 K.

Fig. 2 Variation in diameter (d) and viscosity (h) of [b4mpy][BF4]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil microemulsion systemwith the volume fraction (4d) of [b4mpy][BF4]. Temp. 308 K. Tween 20–n-pentanol ratio (w/w): O, 1 : 0.5; -, 1 : 1 and D, 1 : 2.

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microemulsion zone was plotted against the wt% of the cosur-factant (wcs%) and it was observed that such a prole followed asecond-order polynomial relation whereby the maximummicroemulsion zone would have appeared in the absence ofcosurfactant:

%Area of microemulsion zone ¼ 20.34–8.46 � wCS%

+ 1.61 � (wCS%)2 (4)

This observation implies that in the absence of n-pentanolthe area of microemulsion zone would have been 20% whenthat for the turbid region is 80%. Interestingly a stable micro-emulsion without n-pentanol was unachievable. Hence, unlikethe other systems,2,52 use of cosurfactant was mandatory inorder to achieve a stable microemulsion. Use of cosurfactant forsingle-tailed surfactants is not uncommon in literature.49,50 Thepresent set of results is also comparable with the similar set ofcomponents where water49 and another ionic liquid, 1-butyl-3-methyl imidazolium methane sulphonate [bmim][MS],50 wereused. Compared to the other systems, the % area of themicroemulsion zone in the current work is less, which could bedue 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 signicant role inreducing the stability of the microemulsion; thus more two-phase turbid regions resulted.

As already mentioned in the Introduction, n-pentanol servesa dual purpose for nonionic microemulsions. Firstly, it reducesthe interfacial tension at the oil–polar part interface through themodication of spontaneous curvature. This phenomenonhelps in the formation of a stable microemulsion. However thesecond contribution of the cosurfactant, reduction of interfacialpolarity in the present case, resulted in destabilization of themicroemulsion. Here, the polar part comprises ionic liquid;

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hence, it is not unexpected that addition of n-pentanol woulddestabilise the microemulsion by reducing the interfacialpolarity.

3.2. Dynamic light scattering (DLS) and viscosity studies

DLS studies on microemulsions can provide information on itsdroplet size, its distribution, and hence the polydispersityindex. Variation in the diameter of [b4mpy][BF4]–(Tween 20+n-pentanol)–n-heptane IL-in-oil microemulsion with the volumefraction of the IL (4d) at 308 K is graphically presented in Fig. 2at different surfactant to cosurfactant ratios (panel A). Dropletswere fairly monodispersed as indicated by the size distributionshown in Fig. S2 (ESI†). If the other conditions are kept thesame, there is a correlation between the size of droplets andviscosity, hence the viscosity (h)–4d prole for the same systemsare also presented in panel B of the same gure (Fig. 2).Experiments were carried out in the temperature range 293–323K and the representative results at 308 K (intermediatetemperature) are shown. Results for the other systems are pre-sented in the ESI.† The size of the microemulsion dropletsincreased with increasing volume fraction (4d) of IL. Increase insize with increasing volume fraction of the dispersed phase isnot an uncommon phenomenon and has been documented byothers.2,44,50,52,53

Size–4d proles were found to be dependent on S/CS ratio.The size of the microemulsion droplet comprising Tween 20and n-pentanol for the systems with a S/CS mass ratio of 1 : 0.5was found to be larger than that of 1 : 1, which was even largerthan that of 1 : 2. These results clearly indicate that the cosur-factant caused size constriction. A large number of cosurfactantmolecules at the IL-oil interface results in the formation of acompact structure. The n-pentanol cosurfactant enhances thecompactness and hence rigidity of the interface, as also repor-ted by Digout et al.28 In a previous report we showed thatincreasing the amount of surfactant containing the polyoxy-ethylene head group (Tween 20) could lead to the formation of alarger number of droplets.49,50 For a xed amount of surfactant,increasing the number of droplets could result only at theexpense of the size of the droplets. Such an observation further

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supports the decrease in the area of the microemulsion regionwith increasing amount of cosurfactant, as in the phasediagram construction studies. A close inspection of panel A ofFig. 2 shows that for an S/CS ratio of 1 : 0.5, the size of themicroemulsion droplet increased linearly up to 4d ¼ 0.01, aerwhich the slope of the increment prole changed. Initially whenIL molecules are added they strongly coordinate with the poly-oxyethylene head group of Tween 20. With the further additionof IL they become free and behave like bulk material.50 Almosttwice the volume of IL was required to attain the breakpoint inthe microemulsion with S/CS ratio 1 : 1 (4d ¼ 0.02). However,for such systems the slope aer the threshold was higher. Suchan ambiguity cannot be explained with the present level ofknowledge. Diameter (d) vs. 4d prole for the systems with asurfactant to cosurfactant ratio of 1 : 2 was almost linear. The d–4d prole for all the systems at different temperatures aregraphically presented in Fig. S3 (ESI†). It could be concludedfrom the results that with the increasing amount of cosurfac-tant (n-pentanol), size increment becomes less sensitive to thevolume fraction. The presence of a larger number of n-pentanolmolecules at the interface and subsequent enhancement of theinterfacial rigidity was the causative factor for such an obser-vation. The viscosity prole for the similar systems followed thesame trend line as in the variation of droplet size with 4d. Thus,it could be concluded that variations in the viscosity with 4d wasa consequence of the size variation in the microemulsiondroplets.

In order to understand the effect of temperature, DLS studieswere carried out at different temperatures (293, 298, 303, 308,313, 318 and 323 K). As shown in Fig. 3 panel A, it was observedthat the dependence of diameter on temperature was almostlinear for all the systems. With increasing temperature, the sizeof the droplets was observed to increase, as has also beenreported by others.54 An increase in the volume of the dispersedphase as well as the “soening” of the interfacial region withincreasing temperature contribute to the increase in dropletsize. Additionally, with the increase in temperature, the

Fig. 3 Dependence of the size (A) and viscosity (B) on temperature for[b4mpy][BF4]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil micro-emulsion at different Tween 20–n-pentanol ratio (S/CS, w/w): O,1 : 0.5; -, 1 : 1 and D, 1 : 2.0.2 M Tween 20 was used in each casewhere the volume fraction (4d) of IL was kept constant at 0.02.

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probability of interdroplet contact increases. Spherical geome-tries are usually assumed in dynamic light scattering studies.However according to the proposition of Nazario et al.,55 Tweensin combination with alkanols can experience reasonable shapeuctuation which eventually impart increased droplet diameter.In the present set of studies, the microemulsion droplets werebigger than the conventional water-in-oil microemulsion drop-lets or swollen reverse micelles. It is not unexpected that suchentities may have shapes/geometry other than spheres. Toaddress such issues further studies, such as cryo-TEM and smallangle neutron scattering, are warranted. The parallel nature ofthe size-temperature proles imply that the effect of tempera-ture was the same for all the systems (independent of S/CSratio).

Although size increased with increasing temperature,viscosity decreased with increasing temperature for all thesystems. Results for viscosity–temperature prole for the threecombinations at 4d ¼ 0.02 are shown in Fig. 3 (panel B). Adecrease in viscosity with an increase in temperature is acommon phenomenon for Newtonian uids. In the present setof studies, increase in droplet size was overshadowed by theincrease in the uidity 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 therigidity of the microemulsion droplets. Systems with a largerproportion of cosurfactant are expected to form more rigidstructures. The dependence of viscosity on volume fraction andtemperature is also graphically presented in Fig. S4 (ESI†) for allthe systems.

3.3. Spectral studies

Spectroscopic investigation on microemulsions using a suitableprobe can provide useful information on the microenviron-ment, viz., polarity, uidity, and state of the aggregates of theionic liquid in the polar part of the microemulsion. Muchresearch has been reported on these topics.2,44,50,53 For thepresent system, since the ionic liquid itself exhibits a UV-visibleabsorption band, due to the presence of the pyridinium ring,the microemulsion was studied without any probe. Fig. 4 showsthe absorption spectra of [b4mpy][BF4] conned in the polarpart at different volume fractions. Two distinct peaks, one at280 nm and the other at 335 nm, were recorded for the ionicliquid. The peak at 4.6. 280 nm was signicantly more intensethan the other. Intensity of both the bands increased withincreasing volume fraction (4d) of the ionic liquid. Absorbancevs. 4d proles for both the peaks are graphically shown in Fig. 5.The increases in the absorbance values were mostly linearexcept for the system with Tween 20–n-pentanol in a ratio of1 : 0.5 (w/w). With the initial addition of IL to reverse micelles,formation of strongly coordinated species between the IL cationand oxyethylene groups of Tween 20 are initiated, resulting inthe formation of swollen reverse micelles. Aer many IL mole-cules are added, they becomes free and behave like bulk liquid;thus the microemulsion is formed. These results were the sameas those from the phase behaviour, DLS and viscosity studies.

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Fig. 4 Absorption spectra of 1-butyl-4-methyl pyridinium tetra-fluoroborate [b4mpy][BF4] confined in the polar part at differentvolume fractions (4d): (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 (4d) for 1-butyl-4-methyl pyridinium tetrafluoroborate [b4mpy][BF4], which isconfined to the polar part. Absorbance values were recorded atwavelengths of 283 nm (A) and 335 nm (B). Tween 20–n-pentanolratio (w/w): O, 1 : 0.5; -, 1 : 1 and D, 1 : 2.

Fig. 6 Emission spectra of 10 mM curcumin in [b4mpy][BF4]–(Tween-20 + n-pentanol)–n-heptane IL-in-oil microemulsions at differentvolume fractions (4d) of IL. 4d 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 lem (panel B)with 4d for different Tween 20–n-pentanol ratio (w/w): O, 1 : 0.5; ,,1 : 1 and D, 1 : 2.

Fig. 7 Variation in the fluorescence intensity (panel A) and lem (panelB) of 10 mM curcumin with 4d confined in [b4mpy][BF4]–(Tween-20 +n-pentanol)–n-heptane IL-in-oil microemulsion. Tween 20–n-pen-tanol ratio (w/w): O, 1 : 0.5; -, 1 : 1 and D, 1 : 2.

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The microenvironments of the polar part in the reversemicelles are supposed to be different at different ratios ofsurfactant and cosurfactant, as the polarity of the interface (oil/IL) are dependent on the relative abundance of cosurfactant atthe interface. In order to understand the microheterogeneity ofthe reverse micelles (herein IL in oil microemulsion) curcuminwas used as the uorescent probe. For such systems the uo-rescent probe can have different locations ranging from the IL/oil interface to the core of the polar part.56 Fluorescence spectraof curcumin conned in the polar part of the IL-in-oil micro-emulsion are shown in Fig. 6. When excited at 426 nm, curcu-min shows an emission maximum at �500 nm. Results werefound to be comparable with the previous report.57 A red shi inthe emission maxima along with a decrease in the uorescenceintensity with increasing volume fraction of the ionic liquidwere observed, as shown in Fig. 7. The decrease in uorescenceintensity (panel A) was caused by the localized dilution of theprobe in the IL pool.2,44,50,53 The progressive red shi (panel B,

32388 | RSC Adv., 2014, 4, 32383–32390

Fig. 7) in the emission maxima is due to the increased polarityof the part with increasing volume fraction of the ionic liquid.50

As already mentioned previously, with the initial progressiveaddition of the IL to the reverse micelles, strongly coordinatedspecies start forming between the IL cation and oxyethylenehead group of Tween 20, resulting in the formation of swollenreverse micelles. Further addition of IL led to the occurrence offree IL (bulk), as in the case of ionic liquid-in-oil micro-emulsions. A transition from the swollen reverse micelles tomicroemulsions was conrmed through the appearance ofbreaks/halts in the red shi of the uorescence maxima ofcurcumin with increasing volume of IL.56 Such breakpoints weredependent on the Tween 20–n-pentanol mass ratio. Breakpointsappeared at higher volume fraction of the IL with increasingproportion of n-pentanol.

To know the exact state of the solvent in the pool, uores-cence anisotropy studies on curcumin were carried out. The

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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 (4d) of IL: 0.015. Scale bar: 200 nm.

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dependence of uorescence anisotropy (r) on volume fraction(4d) of the ionic liquid is graphically shown in Fig. 8.

Anisotropy values passed through maxima with respect tothe volume fraction of IL. Initially ILs are used up in formingcoordinated species with the oxyethylene head groups of Tween20 for which structured entities are formed (like swollen reversemicelles). At that stage the dye molecules did not have freedomof movement. Aer the coordination of surfactant head groupsby IL cations was completed, excess ILs became free and couldbehave as bulk IL, as in the case of ionic liquid-in-oil micro-emulsions. Under this situation, the mobility of the dye mole-cules increased. This eventually led to the decrease in theuorescence anisotropy values. A linear increase in the anisot-ropy value for the system comprising Tween 20–n-pentanol in a1 : 2 w/w ratio was due to the continued solvation of the cationiccomponent of the ionic liquid, which was assisted by thepresence of a larger number of alkanol molecules.

3.4. Morphological study: TEM analyses

Morphological studies on the microemulsion droplets ofdifferent compositions were investigated by transmission elec-tron microscopy. A representative TEM image for a micro-emulsion system with Tween 20–n-pentanol at a 1 : 1 mass ratiois shown in Fig. 9.

The particles studied by transmission electron microscopywere found to be spherical. However, particle surfaces were notsmooth, which was possibly due to evaporation of the solvents.Sizes of the microemulsion droplets as derived from dynamiclight scattering were found to be smaller than those from TEM.Such a difference was due to the variation in the sample prep-aration and the subsequent analyses. However, sphericalmorphologies, as obtained from the TEM measurements,further support the assumption as made in the dynamic lightscattering studies. The relatively large average size of the

Fig. 8 Values of fluorescence anisotropy (r) for 10 mM curcumin atdifferent volume fractions (4d) 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 D, 1 : 2. Temp, 298 K.

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droplets, as indicated by the TEM measurements, was possiblydue to the aggregation of smaller precursor droplets.

4. Conclusions

Comprehensive studies of a 1-butyl-4-methyl pyridinium tetra-uoroborate ([b4mpy][BF4])–(Tween 20 + n-pentanol)–n-heptanemicroemulsion system were carried out using a number ofdifferent physicochemical techniques. Although high concen-trations of n-pentanol destabilized the microemulsion, some ofthis cosurfactant was nevertheless essential for the formation ofa stable microemulsion. Cosurfactant controlled the curvatureof the microemulsion droplets, imparted better stability bysolvating the cationic component of the ionic liquid, and aidedthe formation of a larger number of droplets. Temperaturesensitivity decreased as the proportion of cosurfactant withrespect to Tween 20 increased, as revealed through thecombined dynamic light scattering and viscosity measure-ments. Oxyethylene groups of Tween 20 formed coordinatelinkages with the IL cation, which enhanced the rigidity of thepolar part. IL, when in excess of the amount required forcoordinating the surfactant head groups, behaved like a bulkcomponent according to the occurrence of red shi in theuorescence maxima and appearance of maxima in uores-cence anisotropy values. Electron microscopic studies estab-lished the spherical morphology of the droplets.

Acknowledgements

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

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