Dynamic Percolation and Swollen Behavior of Nanodroplets in 1-Ethyl

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Dynamic Percolation and Swollen Behavior of Nanodroplets in 1-Ethyl-3-Methylimmidazolium Trifluoromethanesulphonate/Triton X-100/Cyclohexane Microemulsions Adhip Rahman, M. Muhibur Rahman, Mohammad Yousuf A. Mollah, and Md. Abu Bin Hasan Susan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b04763 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

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The Journal of Physical Chemistry

Dynamic Percolation and Swollen Behavior of Nanodroplets in 1-Ethyl-3methylimmidazolium Trifluoromethanesulphonate/Triton X-100/Cyclohexane Microemulsions Adhip Rahman,a M. Muhibur Rahman,b M. Yousuf A. Mollahb and Md. Abu Bin Hasan Susana* a

b

Department of Chemistry, University of Dhaka, Dhaka 1000, Bangladesh

University Grants Commission of Bangladesh, 29/1 Agargaon, Dhaka 1207, Bangladesh

CORRESPONDING AUTHOR INFORMATION: *E-mail: [email protected] (M.A.B.H. Susan) *Telephone/Fax: 880-29661920 Ext. 7162; Fax: +88 029667222

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Abstract Microemulsions

comprising

an

ionic

liquid

(IL)-

1-ethyl-3-methylimmidazolium

trifluoromethane sulphonate ([emim][OTf]) - as the polar component, Triton X-100 as a surfactant and cyclohexane as the non-polar medium were prepared and characterized. Conductivity and dynamic viscosity data were critically analyzed to confirm dynamic percolation among the droplets that are in continuous motion, aggregation and fission. The transition from oil-continuous phase to bi-continuous phase was observed at the conductance and viscosity percolation thresholds and sharp changes in the values of conductivity and dynamic viscosity could be identified. Dynamic light scattering measurements revealed swelling of the droplets, which varied within the hydrodynamic diameter range of 10-100 nm. Diffusivity of the droplets suggested less Brownian movement with increased amount of the IL. Moreover, changes in the droplet sizes and diffusivity- with increase in IL content- supported dynamic percolation within the systems.


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Introduction Microemulsions are thermodynamically stable and optically clear isotropic solutions of one liquid dispersed in another one- the dispersed component being surrounded by a surfactant film and/or co-surfactant molecules. One of the liquids is polar, water for example, and the other is non-polar, particularly an organic solvent. In contrast to emulsions, microemulsions form upon only mixing of components- with no additional mechanical forces being implemented on.1-5 In a microemlusion, two liquid components, a polar and a non-polar, are separated by surfactant and/or co-surfactant molecules. The surfactant and/or co-surfactant molecules constitute monolayers at the interface of the polar and non-polar components. The hydrophilic parts of surfactant molecules head towards the polar component, while hydrophobic tails direct towards the non-polar component. The shapes of these self-assembled structures in microemulsions vary from spherical and cylindrical forms to lamellar and bi-continuous structures.6,7 The size of the dispersed droplets in microemulsion is usually less than 100 nm; hence they are termed as nanodroplets.3 The outstanding properties of microemulsions, such as high capacity to solubilize both polar and non-polar liquids, large interfacial area, spontaneous formation, fine nanostructures and tunable microenvironment and physicochemical properties render them as excellent candidates for a plethora of applications. They have been applied to conduct chemical reactions, in industrial coating and textile finishing, preparation of nanoparticles and drug delivery systems.8-15 The preparation of microemulsions has not been confined in using water as the only polar medium; other polar solvents have also been used. For instance, water has been replaced by polar nonaqueous solvents such as ethyl laurate, formamide, glycerol, and dimethylformamide, which have relatively high dielectric constants and are immiscible in non-polar solvents.16-19 In recent



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years, water has frequently been replaced by ionic liquids (ILs)- which are defined as organic salts with a melting temperature below 100°C, consisting of organic cations and organic or inorganic anions. Owing to their unique physicochemical properties (low volatility, wider liquidus ranges compared to molecular solvents, large electrochemical window, nonflammability, and high thermal stability).20-22 and manifold applications in material and chemical science, a good deal of interest has been shown in IL-based microemulsions.23-27 The advantages of ILs in the formulation of microemulsions are their applicability as polar phases (substituent of water), non-polar phases (substituent of oils), as cosurfactants and/or amphiphilic components.28,29 In particular, IL-in-oil microemulsions have been investigated extensively as the solubility limitations for apolar solutes can be overcome in such non-aqueous moieties. This, in turn has provided hydrophilic nanodomains, which has expanded the potential of hydrophilic ILs as reaction or extraction media.30 For instance, Gao et al.31 replaced water by 1-butyl-3methylimidazolium tetrafluoroborate, [bmim][BF4], and used Triton X-100 (TX-100) as a surfactant to form IL-oil microemulsions. Oil-in-IL (O/IL) micro-regions at low volume fraction of cyclohexane, followed by bi-continuous and IL-in-oil (IL/O) structures with increasing IL content were identified via conductivity measurements. The results were consistent with apparent hydrodynamic radii extracted from dynamic light scattering (DLS) measurements. Freeze fracture transmission electron micrographs (FF-TEM) in IL/O micro-region indicated increase in the sizes of droplet structures with increasing IL-to-surfactant molar ratio, R.31 Eastoe et al. performed small angle neutron scattering (SANS) experiments on [bmim][BF4] based microemulsions, where the scattering curves at different R could well be described by a model of homogeneous diluted ellipsoidal droplets.30 They observed that the volume of the droplets increases as they are progressively swollen with the addition of [bmim][BF4].30 Rojas et al.


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identified

three

phases

in

microemulsions

composed

of

IL

surfactant

1-butyl-3-

methylimidazolium dodecylsulfate, 1-ethyl-3-methylimidazolium ethylsulfate ([emim][etSO4]) and toluene; in addition, they reported the formation of spherical micellar aggregates that could solubilize [emim][etSO4].32 Dynamic percolation is associated with clustering and fusion of the nano-domains, or droplets, formed in the microenvironments of microemulsions.33 The numbers and sizes of droplets are, in general, directly related to the amount or volume fraction of the continuous media in microemulsions. The increased number of droplets facilitates the movement of masses/ions through the channels, as the droplets are in continuous Brownian motion and experience collisions with one another. The collisions are relatively high near percolation threshold, i.e. the concentration or volume fraction of the polar component at which change of conductivity occurs abruptly. Droplets cluster or associate, which results in ‘fusion’ and the formation of channeled structures. Transfer of masses or ions takes place through the channels during the aggregation. Moreover, aggregated structures may experience fission and thus be separated to smaller droplets. These processes happen in a concerted manner- implying simultaneous aggregations and fissions of the droplet.34-37 Changes of conductivity in microemulsion due to dynamic percolation are associated with the changes of dynamic viscosity of the systems as well.38-43 The works related to percolation behavior, however, mostly involved the traditional water-in-oil or oil-in-water microemulsions. The current study aims at the study of the physicochemical properties of microemlusions containing 1-ethyl-3-methylimmidazolium trifluoromethanesulphonate (triflate) [emim][OTf] as the polar medium. Triflate ILs are thermally stable and less volatile; they have widely been used in organic separation, polymerization and as reaction media.44-47 However, there has been no 5 ACS Paragon Plus Environment


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report on preparation and investigation of [emim][OTf]-based microemulsions. We have for the first time attempted to explain the changes of conductivity and dynamic viscosity in [emim][OTf]/TX-100/cyclohexane microemulsion and explained change of the size and shapes of the droplets with the change of the volume fraction of the IL. We have for the first time attempted to explain the changes of the conductivity and the dynamic viscosity in [emim][OTf]/TX-100/cyclohexane microemulsions as well as changes of the size and shapes of the droplets with respect to the volume fraction of the IL. In brief, we have focused on the investigation of size, shapes and percolation of droplets as well as the nature of phase transitions within [emim][OTf]/TX-100/cyclohexane microemulsions. The ultimate goal is to gain an insight on the percolation phenomenon of the micromulsions for efficient and controlled use of the micro-environment as non-aqueous media for chemical reactions or for controlling the morphology of nanomaterials.

Experimental 1-Ethyl-3-methylimmidazolium trifluoromethanesulphonate, [emim][OTf] (purity ≥ 99.0%) was obtained from Sigma- Aldrich and used without further purification. TX-100 and cyclohexane were also obtained from Sigma-Aldrich. Determination of water content in pure [emim][OTf] was carried out by Karl-Fischer autotitration (Metrohm-Titrando 890) and was found to be less than 0.5% (w/w) during the preparation of samples. Prior to preparing microemulsions, TX-100 was heated in an oven at 80°C for 4-5 hours in order to remove moisture. Microemulsions were prepared gravimetrically, using an analytical balance (Unilab UB-110) with a precision of ±0.0001 g. A 0.92 M solution of TX-100 in cyclohexane was prepared in a 100 mL volumetric flask and used as the stock solution. The IL, [emim][OTf] was added gradually to obtain


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isotropic solutions at different IL-to-surfactant molar ratio, R (R= amount of IL in molar unit/ amount of surfactant in molar unit). Transparent solutions did not form at ambient condition when R exceeded 1.05. Meanwhile, oil-to-surfactant molar ratio (α) (α= amount of oil in molar unit/ amount of surfactant in molar unit) was kept constant to 4.88 throughout the preparation process. The samples were sonicated in a digital ultrasonic bath (Labnics LU-2) for 30-40 min at 25°C to ensure homogeneous mixing. Autotitration of the microemulsions estimated the water content be up to maximum 0.81% of the mass of each of the samples. A Jenway 4510 conductivity meter with platinum electrodes and a calibration constant of 1.05 cm-1 was used to observe the change in conductivity of the micromeulsions with the change of IL content. Conductivity of the system was measured at 25°C. Change of dynamic viscosity as a function of R was estimated using an Anton-Paar Lovis-2000 falling ball automated viscometer (Ø 1.8 steel). Hydrodynamic diameters (dh) of the droplets and diffusion co-efficient of the microenvironments were measured using a Zetasizer Nano ZS90 (Malvern Instruments Ltd, UK), with the He-Ne Laser beam being set at a wavelength of 632.8 nm. The measurements for 1 mL of each of the samples in a universal glass dip-cell were performed with a scattering angle being fixed at 90° and temperature at 25°C. Density values of the microemulsons were measured with an Anton-Paar DMA-4500 vibrating tube density meter. The temperature of the apparatus was controlled by a built-in Peltier device. Refractive indices were directly measured using an Anton-Paar Abbemat-350 automated refractometer with high resolution optical sensor.



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Results and Discussion Figure 1 depicts the change of conductivity of the microemulsions with the increase in [emim][OTf] mass content. Numerous previous works on various IL-in-oil microemulsions have demonstrated that gradual increase of the polar component to a percolation threshold increases conductivity.48-51 We observed that conductivity of [emim][OTf]/TX-100/cyclohexane microemulsions increases with the gradual increase of R. When R is near 0.65, that is- IL content is about 14 wt%, a sharp increase in conductivity is observed- suggesting a percolation threshold volume located near that IL mass fraction. The sharp increase ceases when R reaches near 1.00equivalent to [emim][OTf] content of about 20 wt%. Subsequently, a less-sharp increase of the conductivity occurs again.

Figure 1: Variation of conductivity of [emim][OTf]/TX-100/cyclohexane microemulsions with [emim][OTf]-to-TX-100 molar ratio.

Figure 2 shows the change of dynamic viscosity of [emim][OTf]/TX-100/cyclohexane microemulsions with the change in R at three different temperatures. Increase in temperature results in an overall decrease of dynamic viscosity for all the samples. Additionally, an increase 8 ACS Paragon Plus Environment

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of the IL content brings about an increase in dynamic viscosity of the microemulsions. This is analogous to the change in conductivity. Dynamic viscosity drastically increases when R reaches approximately 0.60. The shift is tangible until the R value has reached about 0.90- implying significant alteration at the microenvironment within the system

Figure 2: Change of dynamic viscosity with [emim][OTf]-to-TX-100 molar ratio at 25°C (dots), 30°C (triangles) and 35°C (crosses).

The [emim][OTf] is mostly responsible for the transport of charge in [emim][OTf]/TX100/cyclohexane microemulsions, as it contains ions- both positively and negatively chargedwhereas cyclohexane and TX-100 are non-polar. When the continuous medium is non-polar cyclohexane, ions of [emim][OTf] are to be dispersed and encapsulated in nano-structures or droplets surrounded by TX-100 molecules. Qualitatively, smaller R values indicate lesser and larger R values point out greater numbers of droplets. This has been confirmed for the present compositions by the size distribution of the droplets (vide infra). Lesser amount of IL present in



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the domains is reflected by comparatively lower conductivity and dynamic viscosity of the systems. We assume that the droplets are isolated from one another and cannot form adequate channels in between. The increase in R ensures the presence of more ions capable of transporting charges and greater number of droplets. Moreover, increase of the number of droplets escalates the possibility of increased collisions. As the droplets collide more frequently, it is more feasible to form channels within or aggregates. The boundary of the channels is formed by TX-100 molecules and the core is filled up with [emim]+ and [OTf]- ions. Thus, passage of more ions through the channels or the boundary surface of the aggregated droplets may occur with the increase of R, which is reflected by the increase of the conductivity in Figure 1. Such trait is, nevertheless, a commonplace in IL based microemulsions where the dispersed phase is the IL. We predict that at the R ~0.60-0.65, transition from IL-in-oil to bi-continuous phase may have occurred. In a bi-continuous phase, where the number of droplets is very high, almost all the droplets are either aggregated or connected via channels; the dispersed and the continuous phase cannot be differentiated. As the IL content is increased gradually up to R~1.00, formation of droplets, droplet-clusters and channels increases. The bi-continuous phase again, possibly goes through a transition at an R value near 1.00. Transition from bi-continuous to the oil-in-IL region could not be confirmed since clear and transparent solutions only formed up to R~1.05 at ambient condition. As suggested by Figure 2, dynamic viscosity for each composition decreases with the increase in temperature. If the droplets are considered to be present in different layers in the systems, possibility of faster flow of the layers comprising the droplets submerged in cyclohexane pool increases at high temperatures. Thus droplets in individual layers experience less collision and the systems go through an increase in dynamic viscosity. Moreover, percolation process involves 10 ACS Paragon Plus Environment

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channeling and clustering among droplets in three dimensions- resulting in the increase of sheer stress, at a given temperature and resistance to the frequent motion of the droplet layers. Eventually, this leads to an increase in dynamic viscosity. In the present systems, increase in R possibly involves formation of more droplets and more 3-D channeling among them, which is related to the increase in dynamic viscosity. The hump akin to R~0.60 has been related to the phase transition from oil continuous to bi-continuous region involving broad passages connecting the droplets. Dynamic viscosity of the microemulsions goes through relatively higher values when bi-continuous regions persist. The observations are in agreement with that of Paul et al., Borkovec et al. and Łuczak et al. - that an increase in conductivity with compositions of microemulsions involves increase of dynamic viscosity provided the structural transitions are from oil continuous to polar continuous phases.41,42,52 The plots in Figure 3 depict the percolation threshold of the microemulsions with respect to the [emim][OTf] volume fraction. Percolation threshold values are important parameters to interpret both the conductance and viscosity changes of not only the water-based but IL-based microemulsions as well, provided the droplets (micelles or reverse micelles) are in frequent interactions. The conductivity of microemulsions in the percolation range is generally explained by the following scaling equations = Ac − -s for c > or ln κ = ln A − lnc − .........(1) = B − ct for c < or ln κ = ln B + ln − c.........(2) Here wc is the percolation threshold volume fraction of [emim][OTf], A and B are prefactors and s and t are the scaling exponents below and above percolation threshold respectively. The wc has



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been calculated using the sigmoidal Boltzmann fitting (SBF) equation- which Hait et al. described to be precise-

log = log f +

log i – log f … … . . 3 1 + exp − c/d

where κi and κf are the initial and final conductivities at a specific range of compositions.53 The value of wc has been found to be 0.1354. This value may be important to predict the shapes of the nano-environment within the microemulsions. For instance, reports indicated that lower values of the wc, preferrebly near 0.15, imply overlapping and strong attractive interactions among the droplets; in the process, the droplets may alter in their shapes to ellipsiodal from spherical.43,54 In this work, the calculated value of the wc suggests the phenomenon in [emim][OTf]/TX100/cyclohexane microemulsions.

Figure 3: Scaling behavior of (a) conductivity and (b) dynamic viscosity below (circles) and above (triangles) conductivity and viscosity percolation threshold volume fractions respectively.


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Using the value of the wc, the scaling behavior for the case of conductance percolation of the present systems has been evaluated, as shown in the ln (wc-w) vs. ln κ and ln (w-wc) vs. ln κ plots in Figure 3(a). By fitting the data on equations (1) and (2), s and t have been estimated to be 1.03 and 0.12 respectively. However, the plot of ln (w-wc) vs. ln κ fits better on the fourth degree polynomial equationy = y0 + tx + bx2 + cx3 + dx4 .......(4) where y = ln κ, y0= ln B, x = ln (w-wc) and b, c and d are constants. Using equation (4), we obtain the value of t=1.67. To calculate the exponents in the case of viscosity percolation for the current systems- as shown in Figure 3(b)- we have used the following equations ! = Av − -s for v > , or ln η = ln A − lnv − ..........(5) ! = B − vt for v < , or ln η = ln B + ln − v ..........(6) Where η is the dynamic viscosity of a particular composition and A and B are prefactors.39 Using the SBF procedure, the viscosity percolation threshold volume fraction of [emim][OTf], wv, is calculated to be 0.1330. This value is close to the conductance percolation threshold fraction we calculated earlier. For better fitting, we have utilized third and fourth degree polynomial equations to calculate s and t for the viscosity percolation.55 The values of s and t, according to third degree polynomial fitting, have been 1.08 and 0.49, but the value for t optimized from fourth degree polynomial fitting has been 0.92. The pre-percolation exponent s is suggested to be near 1.00 to1.20 and post-percolation exponent t be near 1.50 to 2.00- provided dynamic percolation dominates over static percolation in microemulsions.40,43,52,56,57



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We observed that the values of s- calculated for both the conductance and viscosity percolationfit well within that range. The case of t is somewhat complicated nevertheless; on one hand, the value calculated through fitting on equation (2) is anomalous to the suggested value ~1.50-2.00. Nevertheless, some recent researches unveiled that the value of t may deviate significantly and be within 0.10-0.30

40,52

. For example, Łuczak et al. investigated the scaling parameters for

H2O/TX-100/[bmim][Tf2N] and H2O/ TX- 100/[bmim][PF6] microemulsions and observed the critical parameter t to be significantly low, within the range 0.11 to 0.17, and attributed the fact to dynamic percolation in the systems.52 On the other hand, fitting of the data in equation (4) gives the value of t, although for the conductance percolation only, over 1.50. The value of t therefore depends on the choice of scaling equations. DLS in one of the most common techniques applied to interpret the sizes, shapes and characteristics of the droplets in IL-based microemulsions. Gao et al. and several other groups for example, applied DLS measurements to observe the hydrodynamic diameter- the total diameter of a droplet and the molecules of the continuous phase adjacent to the droplet- shapes and diffusion behavior of droplets in IL-based microemulsions. The technique provides useful information on the microenvironment in microemulsions.48,50,51,58-60 The size distribution of the droplets at different compositions of [emim][OTf]/TX-100/cyclohexane microemulsions is shown in Figure 4. Eight specific compositions at R from 0.10 to 0.80 for these experiments were selected and subjected for three to four runs. In each case the polydispersity indices were in between 0.1-0.6, which confirms that solutions subjected to DLS measurements were isotropic. At R=0.10 single sinusoidal peaks are observed for individual runs. The maxima, with the


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Figure 4: Size distribution of the droplets and/or aggregates formed in [emim][OTf]/TX-100/ cyclohexane microemulsions at different R values.



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highest probability of the particles to be found, are near 10 nm. The narrow width of the peaks indicates that with less [emim][OTf] content, the droplets consist a small range of hydrodynamic diameters. For this system therefore, the distribution of sizes of the droplets is not quite random. Similar peaks for the microemulsions with R=0.20-0.50 are observed in Figures 4 (B) to 4 (E). In each case, amount of IL [emim][OTf] has been increased gradually. As the volume fraction of cyclohexane is too high compared to that of [emim][OTf], it is obvious that IL molecules reside in the droplets, surrounded by self-organized TX-100 molecules. There is a possibility that [emim]+ ions are attracted to the lone pairs of oxygen atoms of the ethylene oxide part in TX-100 molecules; thus some cations may be distributed in between those ethylene oxide parts as well. With more IL ions being encapsulated in the cores, the ions increase the three-dimensional hydrodynamic volume of the droplets. The droplets are said to be swollen with the dispersed phase.61 For the R values from 0.20 to 0.50, gradual increases in both hydrodynamic diameter and statistical distribution of the particles in between a given range of sizes, as indicated by the increase of peak areas, confirm the usual swollen behavior in the present systems. For instance, the average peak-maxima for the systems studied here increased from about 20 nm to about 40 nm for R being increased from 0.20 to 0.40 respectively. The droplets aggregate and form clusters- which are dynamic processes and do not cease. Therefore laser generally detects not only discrete droplets, but also the clusters (in three dimensions, the single droplets and clusters may give rod-like, ellipsoidal, cone-shaped and other structures). Different behavior is observed in the microemulsions with R=0.60 and 0.70, when multiple sinusoidal peaks occur. Earlier we observed that the conductivity and the dynamic viscosity for the current systems have predicted transition from oil-continuous phase to bi-continuous phase at R ~0.60-0.65. Thus, DLS studies show a good correlation with conductivity and viscosity values involving the transition near


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R=0.60. Around this composition, very small droplets (with hydrodynamic diameter below 10 nm), moderate-sized droplets (40-60 nm) and droplets of dh about 100-120 nm reveal their existence. The abrupt change and presence of droplets with multiple hydrodynamic diameters indicate a transition to the micro-structure of the microemulsions from oil-continuous to bicontinuous. As there is frequent networking in bi-continuous region, the droplets are no longer categorized as spherical. A laser beam, at a given time, therefore may identify a small interconnected region and simultaneously large interconnected regions as well. Comparing the conductivity and

viscosity measurements

with

sizes of droplets,

[emim][OTf]/TX-

100/cyclohexane microemulsions at R=0.60 and 0.70 may be considered as bi-continuous systems. At R=0.80, again single sinusoidal peaks are observed, but with comparatively larger width-ranges than the other systems explained earlier. The peak maxima in this composition are near 60 nm- though presence of droplets with sizes from 10-100 nm is evident here. The system is perhaps a well-settled bi-continuous microemulsion with less motion of the interconnected regions. Observing comparatively much wider peaks within R= 0.60-0.80, it can be stated that the bi-continuous regions for the current microemulsions are more swollen than the oilcontinuous ones. Figure S1 in Supporting Information shows the correlograms- logarithm-based plots produced by the particle size analyzer and signifies the stability of microemulsions. The correlation function predicts whether the microemulsions have been stable or not, even if they show optical transparency and distribution curves in DLS measurements. In our work, we observed the correlation functions in between a time-range of between 0-100 µs - indicating the stability of the dispersions.



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For microemulsions, the more the increase of the hydrodynamic diameter/sizes of the droplets, the less is the diffusion of the droplets into the continuous medium. With the addition of dispersed component, droplets become heavy; moreover, clustering and channeling of droplets hinder the movement of discrete ones- hindering the diffusion of droplets in the process. According to Stoke-Einstein equation, hydrodynamic diameter is related to diffusivity as-

d h=

$B%

(7)

&πη'

where kB is the Boltzmann constant, η is dynamic viscosity and D is diffusion coefficient. Figure 5 shows the variation of the diffusion coefficient as a function of R in [emim][OTf]/TX100/cyclohexane microemulsions. As the droplets become more swollen with increase in R, their Brownian motion decreases. This is facilitated by clustering and channeling in three-dimensions. As the hindrance to the movement of droplets occurs, the probability of the droplets to be diffused decreases. The trend of decreasing diffusion co-efficient is observed up to R=0.80.

Figure 5: Variation of diffusion coefficient at different R for [emim][OTf]/TX 100/cyclohexane microemulsions.


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At the composition with R=1.00, diffusion coefficient again increases- indicating some change in the microstructures of the microemulsion; however, it cannot be ascertained at the moment whether this is due to the transition from bi-continuous to IL continuous phase.

Conclusion The conductivity and dynamic viscosity of [emim][OTf]/TX-100/cyclohexane microemulsions show an increase with increasing R. The trend of changes can be explained in the light of dynamic percolation for R being low to moderate, which is supported by the values of the scaling exponents. The percolation threshold for IL is ca. 0.14 mass content of the IL. The systems go through a transition from oil-continuous to bi-continuous phases at R~0.60 at room temperature. DLS measurements have showed regular swollen behavior of the nano-droplets in the systems. At R< 0.60, the size of the droplets lies in between about 10-60 nm. The appearance of multiple peaks at R~0.60-0.70 is attributed to complex three-dimensional structures near the percolation threshold. The increase of [emim][OTf] in the microemulsions brings about an increase both in the sizes of the droplets and their statistical distribution at a wider size-range. Diffusion coefficient descends with the increase of IL, which suggests less Brownian movement- thereby enhances inter-droplet networking. These would help establishing a fundamental knowledgebase on triflate IL-based microemulsions and underpin further development for other microemulsions comprising ILs.



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ACKNOWLEDGEMENTS The authors acknowledge the use of laboratory equipment for the present work procured under subproject CP 231 of the Higher Education Quality Enhancement Project of the Ministry of Education, Government of Bangladesh. The authors also gratefully acknowledge financial support for a research project on Ionic Liquid-Based Microemulsions (2013-14) from the Ministry of Science and Technology, Bangladesh.

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Dynamic Percolation and Swollen Behavior of Nanodroplets in 1-Ethyl

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