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Microregion detection of ionic liquid microemulsions
Journal of Colloid and Interface Science 301 (2006) 612–616 www.elsevier.com/locate/jcis
Microregion detection of ionic liquid microemulsions Yanan Gao a , Suqing Wang b , Liqiang Zheng a,∗ , Shuaibing Han a , Xuan Zhang a , Deming Lu a , Li Yu a , Yongqiang Ji a , Gaoyong Zhang b a Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, China b Department of Chemistry, Weifang University, Weifang 261061, China
Received 2 February 2006; accepted 3 May 2006 Available online 12 June 2006
Abstract Nonaqueous ionic liquid (IL) microemulsion consisting of IL, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4 ), surfactant TX-100, and toluene was prepared and the phase behavior of the ternary system was investigated. Electrical conductivity measurement was used for investigating the microregions of the nonaqueous IL microemulsions. On the basis of the percolation theory, the bmimBF4 -in-toluene (IL/O), bicontinuous, and toluene-in-bmimBF4 (O/IL) microregions of the microemulsions were successfully identified using insulative toluene as the titration phase. However, this method was invalid when conductive bmimBF4 acted as the titration phase. The microregions obtained by conductivity measurements were further proved by electrochemical cyclic voltammetry experiments. The results indicated that the conductivity method was feasible for identifying microstructures of the nonaqueous IL microemulsions. © 2006 Elsevier Inc. All rights reserved. Keywords: Microemulsion; Ionic liquid; Microstructure; Conductivity; Cyclic voltammetry
1. Introduction Microemulsions are thermodynamically stable and macroscopically homogeneous; however, the structure is heterogeneous on a microscopic scale. The extensive interest in studying microemulsions is due to their microstructures and microenvironment. Ordered microstructures such as oil-in-water (O/W) or water-in-oil (W/O) microdroplets, similar to the structure of micelles, with the overall larger size of microemulsion droplets, can be formed at high water or oil content. Whereas, bicontinuous microstructures, as a network of water tubes in an oil matrix or a network of oil tubes in a water matrix, with hydrocarbon and water regions stretching over large distances have been identified in the intermediate regions [1,2]. Conductivity is the most frequently used simple technique to investigate microstructure and structural changes on the basis of the percolation theory, which has been generally accepted to de* Corresponding author. Fax: +86 531 88564750.
E-mail address: [email protected] (L. Zheng). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.05.010
termine the microstructures of microemulsions. The static percolation model has been proposed to describe the mechanism of percolation in traditional microemulsion which attributes percolation to the appearance of a bicontinuous water structure. It is assumed in this description that the open water channel is responsible for electrical conduction [2,3]. The sharp increase of the electrical conductivity in a W/O microemulsion can be explained by a connected water path in the system. In addition, a dynamic percolation model is also developed based on the attractive interactions between the water droplets or micelles [2]. In this viewpoint, the approach considered that the attractive interactions between water globules are responsible for the formation of percolation clusters and the charge transport is assured by hopping of ions on globule clusters which rearrange in time. Recently, ionic liquids (ILs) have received much attention as a class of neoteric solvents, because of their special properties, such as low volatility, nonflammability, and high thermal stability [4–8]. As ideal alternative solvents, they have been widely applied to chemical reactions, separations, electrochemical applications, biopolymers, molecular self-assembly, and in-
Y. Gao et al. / Journal of Colloid and Interface Science 301 (2006) 612–616
terfacial syntheses [4,9,10]. Moreover, ILs have been used to substitute for water or organic solvents to prepare novel IL microemulsions. For example, Han and his co-workers [6] found that hydrophilic IL, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4 ) can substitute for water and form a nonaqueous IL microemulsion with adequate surfactant. Freezefracture electron microscopy (FFEM) indicated that the microemulsion droplets were the same shape as “classic” droplets of water-in-oil microemulsions [6]. Eastoe et al. investigated the same microemulsion system by small-angle neutron scattering (SANS) which showed a regular increase in droplet volume as micelles were progressively swollen with added bmimBF4 , behavior consistent with classic W/O microemulsions [11]. Our recent studies have indicated that the hydrophobic IL, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6 ) may substitute for organic solvents and formed aqueous IL microemulsions with water in the presence of surfactant TX-100 or Tween 20 [12,13]. These aqueous IL microemulsions were found to have potential in the production of metal nanomaterials, used in biological extractions or as solvents for enzymatic reactions [13]. The structure of microemulsions is of much current interest [1,14,15]. For these novel IL microemulsions, it is more necessary to investigate the microstructure and structural transitions. Cyclic voltammetry has been successfully used to obtain information about the microstructure of micelles or microemulsions [16–30]. Changes in the microstructure were identified by using electrochemical probes such as ferrocene, ferricyanide, or methyl viologen [1]. These probes can identify different microenvironments in that the electrochemical reversibility of the probes is affected by the structures of the microemulsions, and thus may appear to reflect the ease of mobility by diffusion coefficient [1]. Gao et al. have successfully detected the three regions of aqueous IL microemulsions: water-in-bmimPF6 (W/IL), bicontinuous, and bmimPF6 in-water (IL/W) using this electrochemical method [12,13]. However, the microstructures of the aqueous IL microemulsions cannot be differentiated through the conductivity method because ILs are essentially molten salts. This means that no insulative media are present in these systems; thus the percolation theory is not applicable. However, if a hydrophilic IL such as bmimBF4 substitutes for water and builds an IL/oil nonaqueous microemulsion in which IL is conductive and oil acts as an insulative medium, then the systems are in accord with the percolation theory according to its theoretical description. In this paper, bmimBF4 was used as a substitute for water and the microstructures and structural transitions of bmimBF4 / TX-100/toluene three-component nonaqueous microemulsion were investigated. Three different microregions: IL-in-oil (IL/O), bicontinuous, and oil-in-IL (O/IL) were differentiated by electrical conductivity and cyclic voltammetry, respectively. The result showed that the simple electrical conductivity method was also valid for investigating microstructures of the nonaqueous IL microemulsions.
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2. Experimental 2.1. Materials TX-100 was obtained from Alfa Aesar and used as received. IL bmimBF4 was prepared as reported earlier [31]. To avoid the moisture in air, bmimBF4 was made and used freshly. Toluene was provided by Beijing Chemical Reagents Company. Potassium ferricyanide K3 Fe(CN)6 , used as an electrochemical probe, was purchased from Fisher. Triple-distilled water was used throughout this study. 2.2. Apparatus and procedures A low-frequency conductivity meter (Model DDS-307, Shanghai Cany Precision Instrument Co., Ltd.) with an accuracy of ±1% was used to measure the solution conductivities along the dilution lines shown in Fig. 1 at 25.0 ±0.2 ◦ C. For cyclic voltammetry, an electrochemical analyzer, CHI832A (CH Instruments, Austin, TX), was used. The electrochemical measurements were conducted using a threeelectrode configuration, which consisted of a glassy carbon working electrode, an Ag/AgCl reference electrode, and a platinum flake counterelectrode. The spacing between adjacent electrodes was set at 2.0 cm. All potentials quoted are with respect to the Ag/AgCl reference. Before each measurement, the working electrode was polished using 0.05 µm aluminum oxide slurries and then washed carefully with distilled water. After polishing, the electrode was ultrasonicated in distilled water for about 5 min and the water on the surface of the electrode was wiped off by soft tissue paper immediately before use. The working electrode area was determined as earlier report (the electrode area was 4.15 × 10−6 m2 ) [12]. The potential was scanned between 0.5 and −0.5 V, and the sweep rate range was 20–100 mV s−1 . The experiments were carried out under a
Fig. 1. Phase diagram of the bmimBF4 /TX-100/toluene three-component system at 25.0 ◦ C. For lines a, b, c, d, and e, the initial TX-100 weight fractions are I = 0.93, 0.88, 0.75, 0.63, and 0.50, respectively. For the lines f and g, the initial TX-100 weight fractions are 0.40 and 0.50, respectively.
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nitrogen atmosphere to avoid the effect of oxygen. All experiments were carried out at 25 ± 0.5 ◦ C. In preparation of the IL microemulsion, each component was added by weight and the solution was mechanically stirred until clear and homogeneous. No phase separation was found in these microemulsions which had been stored at room temperature for at least 2 months. The phase diagram was constructed by titration with toluene as follows: the required masses of TX-100 and bmimBF4 were initially mixed. For each titration, the initial TX-100 weight fraction (I ) was fixed. The phase boundaries were determined by observing the transition from turbidity to transparency or from transparency to turbidity. In Fig. 1, the region marked “single phase” was transparent and the region marked “polyphasic” was turbid. By repeating the experiment for other I values, the phase diagram was established. Furthermore, for cyclic voltammetry, the bmimBF4 solution of probe was freshly made and used immediately. 3. Results and discussion 3.1. Phase behavior of the bmimBF4 /TX-100/toluene three-component system The phase behavior of the TX-100/bmimBF4 /toluene threecomponent microemulsion at 25.0 ◦ C is shown in Fig. 1. A continuous single-phase microemulsion region can always be observed over the bmimBF4 or toluene weight content range 0–100%. Generally, the W/O microemulsions can be formed at low water content and the oil is continuous phase; with increasing water content progressively, as long as there is no phase separation and the system remains isotropic, there should be some kind of gradual structural transition in traditional aqueous microemulsions. The transition should span the range of possible structures from microdroplets to bicontinuous structures. Similarly, for the current nonaqueous IL microemulsions, the single-phase channels are suitable for the study of the microstructure and structural transitions [12,13]. 3.2. Conductivity measurements and microregions of the IL microemulsions As the bmimBF4 /TX-100/toluene microemulsion system is in accord with the percolation theory from the viewpoint of the static percolation model as well as dynamic percolation model, the microstructure investigation can be carried out by electrical conductivity measurement. Clausse et al. [32] demonstrated that, for traditional aqueous microemulsions, with increasing water content, the microemulsion electrical conductivity (k) changed according to four successive stages: the initial nonlinear increase of k revealed the existence of a percolation phenomenon that could be attributed to inverse microdroplet aggregation. The next linear increase was due to the formation of aqueous microdomains which resulted from the partial fusion of clustered inverse microdroplets. The phenomenon suggested that a W/O microemulsion was formed in this low water content region. The third nonlinear curve increase indicated that the
Fig. 2. The electric conductivity k of the nonaqueous IL microemulsion as a function of toluene weight content with I = 0.88 (A) and 0.75 (B), respectively.
medium underwent further structural transitions and a bicontinuous microstructure formed, which was ascribed to the progressive growth and interconnection of the aqueous microdomains. The final decrease of k with increase of water content corresponded to the appearance of water-continuous microemulsiontype media. That is, an O/W microemulsion formed at high water content. The decrease of k merely resulted from the fact that the concentration of the O/W microemulsion droplets was progressively diluted with water. However, due to the molten salts essential for ILs, if adding bmimBF4 progressively to the bmimBF4 /TX-100/toluene microemulsion, the electrical conductivity of the microemulsion will always increase, until approaching pure bmimBF4 conductivity. As a result, it is difficult to identify three different types of microstructures because no obvious breaks appear in the conductivity curve [33]. In this case, we have attempted to identify the structural transitions of bmimBF4 /TX-100/toluene microemulsion by using insulative toluene as the titration phase. Test samples are shown in Fig. 1 (lines a–e). As two typical examples, Fig. 2 shows the variation of electrical conductivity k as a function of toluene weight fraction. The initial increase of k, due to the successive increase of conductive O/IL microemulsion droplets, could indicate the formation of O/IL microemulsions. The next nonlinear decrease revealed that the
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medium underwent a structural transition and became bicontinuous, owing to the progressive growth and interconnection of the O/IL microdomains. The third section of curve, linear decrease of k, could be interpreted as the consequence of the formation of IL/O microdomains resulting from the partial fusion of clustered inverse microdroplets. The final nonlinear decrease of k with further increasing oil content corresponded to the existence of a percolation phenomenon that could be ascribed to inverse microdroplet aggregation. The conductivity curves in Fig. 2 evidently illustrate the presence of three different types of microstructure: O/IL, bicontinuous, and IL/O microemulsions. By repeating the experiment for other samples with different I values, three types of microregions can be determined. The subregions identified were also marked in Fig. 1. 3.3. Electrochemical behavior of the K3 Fe(CN)6 probe in the ionic liquid microemulsion In cyclic voltammetry, the peak current ip for a redox-active reversible system is given by the Randles–Sevcik equation [24], 0.447F 3/2 An3/2 D 1/2 Cv 1/2 , (1) R 1/2 T 1/2 where n is the number of electrons involved in oxidation or reduction, A is the area of the working electrode, D is the diffusion coefficient of the electroactive probe, C is the concentration of electroactive probe in the solution, v is the sweep rate, F is the Faraday constant, R is the gas constant, and T is the absolute temperature. It follows from Eq. (1) that ip will increase linearly with v 1/2 for a given electrode and a constant electroactive probe concentration. Diffusion coefficient values can be obtained from a linear regression of the slope of ip versus v 1/2 using the known surface area of the electrode. With microemulsion systems involving an electrochemical probe completely solubilized, the diffusion coefficient D in Eq. (1) corresponds to the microemulsion diffusion coefficient since the probe diffuses with the microemulsion droplets [16]. The cyclic voltammetry experiments were carried out at various scan rates in microemulsion environments in order to verify the diffusioncontrolled nature of the process. Electrochemical charge transfer must be diffusion-controlled in order to study the microstructure of microemulsions by cyclic voltammetry [24]. Fig. 3 shows typical plots of ip versus v 1/2 for three microemulsion systems, and the plots are straight lines passing through the origin. This result indicates that the electron transport for the [Fe(CN)6 ]3− /[Fe(CN)6 ]4− electrode reactions in the nonaqueous IL microemulsion medium is diffusioncontrolled [16,20,21]. Similar results were observed for all the other nonaqueous IL microemulsion systems we prepared. Thus, potassium ferricyanide K3 Fe(CN)6 is a suitable electrochemical probe for study of the microstructures and structural transitions of these microemulsions.
Fig. 3. Scan rate dependence of peak current in the nonaqueous IL microemulsions: (a) 26.7% TX-100 + 44.0% bmimBF4 + 29.3% toluene; (b) 26.1% TX-100 + 45.3% bmimBF4 + 28.6% toluene; (c) 25.5% TX-100 + 46.6% bmimBF4 + 27.9% toluene. 0.02 g K3 Fe(CN)6 added as electroactive probe for all systems.
ip =
3.4. Diffusion behavior of probe in microemulsion Test microemulsions were prepared along the dashed line shown in the phase diagram (Fig. 1, line g). This range of com-
Fig. 4. The diffusion coefficient of K3 Fe(CN)6 as a function of bmimBF4 weight content with initial TX-100 weight fraction, 0.50 (Fig. 1, line g).
positions corresponds to a wide range of IL content and the system is always isotropical. It is suggested that the compositions of the microemulsions changed in a systematic fashion to span the range of possible structures from microdroplets to a bicontinuous structure. The electroactive probe concentration had little effect on the diffusion coefficient D values since the shape and size of microemulsions were not disturbed by the presence of the electroactive probe over a large concentration range [25]. For the investigated nonaqueous IL microemulsion systems, this has been confirmed by the fact that the electron transport of [Fe(CN)6 ]3− /[Fe(CN)6 ]4− electrode reactions was diffusion-controlled. Therefore, our experiments were carried out with a fixed initial K3 Fe(CN)6 quantity and the concentration of the probe changed with the addition of IL. The change in the diffusion coefficient of the probe, calculated from the Randles–Sevcik equation as a function of added bmimBF4 in microemulsions, is illustrated in Fig. 4. It can be seen that the diffusion coefficients of K3 Fe(CN)6 increase with increasing bmimBF4 weight content in the whole single-phase microemulsion region. When the bmimBF4 weight content was less than about 16%, the diffusion coefficients increased grad-
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ually, because at low bmimBF4 content, a bmimBF4 -in-toluene microemulsion formed. K3 Fe(CN)6 is expected to probe the bmimBF4 environment preferentially due to its limited solubility in toluene. The diffusion coefficient of K3 Fe(CN)6 , therefore, should correspond to the diffusion coefficient of bmimBF4 -in-toluene microemulsion droplets. Thus the value of the diffusion coefficient of the probe is relatively small. Guering and Lindman [34] also discovered that molecules confined in a closed domain will give rise to low self-diffusion coefficients. The diffusion coefficients increase with increasing droplet size, but as long as the diffusion of probe is confined in microemulsion droplets, the curve increases slowly. Therefore, the gradual change indicates that the microenvironment of the microemulsions is unchanged. A similar trend has also been observed for this microemulsion system when the bmimBF4 weight content was above 41%. In the latter case, the toluene microdroplets were dispersed in a continuous bmimBF4 medium. The diffusion coefficient of the probe may correspond to the continuous bmimBF4 medium and show high mobility, thus a high value. The gradual increase of the diffusion coefficients derives from the fact that the IL microenvironment is not sensitive to bmimBF4 weight content in continuous bmimBF4 phase microemulsions. However, a relatively dramatic change in probe diffusion coefficients is observed when the bmimBF4 weight content is in the range of 16–41%, which indicates that the microenvironment of the microemulsions is different from that of IL/O or O/IL microemulsions. According to the principle of distinguishing subregions in microemulsions using the apparent diffusion coefficient [16,17,19], the result suggests that a bicontinuous microstructure, in which bmimBF4 and immiscible toluene are both local continuous phases, is formed. Guering and Lindman [34] stated that the molecules occurring in the continuous medium will be characterized by rapid diffusion. Thus a continuous structure is expected to give a higher self-diffusion coefficient than a droplet structure [22]. The curve of the diffusion coefficient of the probe in Fig. 4 illustrates the occurrence of the three microregions: IL/O (<16% bmimBF4 ), bicontinuous (16–41% bmimBF4 ), and O/IL (>41% bmimBF4 ) microemulsions. Thus, the results obtained by electrical conductivity and cyclic voltammetry measurements are in agreement, indicating that the electrical conductivity method is also feasible for identifying microstructures of the nonaqueous IL microemulsions. 4. Conclusion In summary, microemulsions consisting of bmimBF4 , surfactant TX-100, and toluene were prepared and the phase behavior of the ternary system was investigated. The bmimBF4 in-toluene (IL/O), bicontinuous, and toluene-in-bmimBF4 (O/IL) microregions of the nonaqueous IL microemulsions were initially identified by traditional electrical conductivity measurements on the basis of percolation theory. The microstructures of the microemulsion were further investigated by electrochemical cyclic voltammetry. The data obtained by electrical conductivity are the same as those by cyclic voltammetry, indicating that the simple electrical conductivity method
is also feasible for identifying microstructures of the IL microemulsions. Acknowledgments The authors are grateful to the Natural Science Foundation of China and Shandong Province (50472069, Z2004B02) and Ministry of Science and Technology for financial support (2003CCA02900). References [1] R.A. Mackay, S.A. Myers, L.A. Bodalbhai, Anal. Chem. 62 (1990) 1084– 1090. [2] P.G. Gennes, C. Taupin, J. Phys. Chem. 86 (1982) 2294–2304. [3] A. Ponton, T.K. Bose, J. Chem. Phys. 94 (1991) 6879–6886. [4] T. Welton, Chem. Rev. 99 (1999) 2071–2084. [5] S.G. Kazarian, B.J. Briscoe, T. Welton, Chem. Commun. (2000) 2047– 2048. [6] H.X. Gao, J.C. Li, B.X. Han, W.N. Chen, J.L. Zhang, R. Zhang, D.D. Yan, Phys. Chem. Chem. Phys. 6 (2004) 2914–2916. [7] J.L. Anderson, J. Ding, T. Welton, D.W. Armstrong, J. Am. Chem. Soc. 124 (2002) 14247–14254. [8] J.L. Anderson, V. Pino, E.C. Hagberg, V.V. Sheares, D.W. Armstrong, Chem. Commun. (2003) 2444–2445. [9] B.P. Binks, A.K.F. Dyab, P.D.I. Fletcher, Chem. Commun. (2003) 2540– 2541. [10] T. Nakashima, N. Kimizukua, J. Am. Chem. Soc. 125 (2003) 6386–6387. [11] J. Eastoe, S. Gold, S.E. Rogers, A. Paul, T. Welton, R.K. Heenan, I. Grillo, J. Am. Chem. Soc. 127 (2005) 7302–7303. [12] Y. Gao, S. Han, B. Han, G. Li, D. Shen, Z. Li, J. Du, W. Hou, G. Zhang, Langmuir 21 (2005) 5681–5684. [13] Y.A. Gao, N. Li, L.Q. Zheng, X.Y. Zhao, S.H. Zhang, B.X. Han, W.G. Hou, G.Z. Li, Green Chem. 8 (2006) 43–49. [14] P.G. de Gennes, C. Taupin, J. Phys. Chem. 86 (1982) 2294–2304. [15] E.W. Kaler, K.E. Bennett, H.T. Davies, L.E. Scriven, J. Chem. Phys. 79 (1983) 5673–5684. [16] C.S. Mo, Langmuir 18 (2002) 4047–4053. [17] C.S. Mo, M.H. Zhong, Q. Zhong, J. Electroanal. Chem. 493 (2000) 100– 107. [18] J.F. Rusling, C.N. Shi, T.F. Kumosinski, Anal. Chem. 60 (1988) 1260– 1267. [19] B. Geetha, A.B. Mandal, Langmuir 11 (1995) 1464–1467. [20] A.B. Mandal, Langmuir 9 (1993) 1932–1933. [21] A.B. Mandal, B.U. Nair, J. Phys. Chem. 95 (1991) 9008–9013. [22] S.A. Myers, R.A. Mackay, A. Brajter-Toth, Anal. Chem. 65 (1993) 3447– 3453. [23] A.B. Mandal, B.U. Nair, D. Ramaswamy, Langmuir 4 (1988) 736–739. [24] K. Chokshi, S. Qutubuddin, A. Hussam, J. Colloid Interface Sci. 129 (1989) 315–326. [25] D.M. Shah, K.M. Davies, A. Hussam, Langmuir 13 (1997) 4729–4736. [26] X.L. Zu, J.F. Rusling, Langmuir 13 (1997) 3693–3699. [27] J. Santhanalakshmi, G. Vijayalakshmi, Langmuir 13 (1997) 3915–3920. [28] J.F. Rusling, G.N. Kamau, J. Electroanal. Chem. 187 (1985) 355–359. [29] D.M. Shah, K.M. Davies, A. Hussam, Langmuir 13 (1997) 4729–4736. [30] G. Gounili, J.M. Bobbitt, J.F. Rusling, Langmuir 11 (1996) 2800–2805. [31] J. Dupont, C.S. Consorti, P.A.Z. Suarez, R.F. Souza, Org. Synth. 79 (1999) 236–241. [32] M. Clausse, A. Zradba, L. Nicolas-Morgantini, in: H.L. Rosano, M. Clausse (Eds.), Microemulsion Systems, Dekker, New York, 1987, pp. 387–425. [33] We have investigated the bmimBF4 /TX-100/p-xylene three-component system along the line f using bmimBF4 as the titration phase and found that the curve was almost linear after the percolation phenomenon happened. [34] P. Guering, B. Lindman, Langmuir 1 (1985) 464–468.
Microregion detection of ionic liquid microemulsions Yanan Gao a , Suqing Wang b , Liqiang Zheng a,∗ , Shuaibing Han a , Xuan Zhang a , Deming Lu a , Li Yu a , Yongqiang Ji a , Gaoyong Zhang b a Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, China b Department of Chemistry, Weifang University, Weifang 261061, China
Received 2 February 2006; accepted 3 May 2006 Available online 12 June 2006
Abstract Nonaqueous ionic liquid (IL) microemulsion consisting of IL, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4 ), surfactant TX-100, and toluene was prepared and the phase behavior of the ternary system was investigated. Electrical conductivity measurement was used for investigating the microregions of the nonaqueous IL microemulsions. On the basis of the percolation theory, the bmimBF4 -in-toluene (IL/O), bicontinuous, and toluene-in-bmimBF4 (O/IL) microregions of the microemulsions were successfully identified using insulative toluene as the titration phase. However, this method was invalid when conductive bmimBF4 acted as the titration phase. The microregions obtained by conductivity measurements were further proved by electrochemical cyclic voltammetry experiments. The results indicated that the conductivity method was feasible for identifying microstructures of the nonaqueous IL microemulsions. © 2006 Elsevier Inc. All rights reserved. Keywords: Microemulsion; Ionic liquid; Microstructure; Conductivity; Cyclic voltammetry
1. Introduction Microemulsions are thermodynamically stable and macroscopically homogeneous; however, the structure is heterogeneous on a microscopic scale. The extensive interest in studying microemulsions is due to their microstructures and microenvironment. Ordered microstructures such as oil-in-water (O/W) or water-in-oil (W/O) microdroplets, similar to the structure of micelles, with the overall larger size of microemulsion droplets, can be formed at high water or oil content. Whereas, bicontinuous microstructures, as a network of water tubes in an oil matrix or a network of oil tubes in a water matrix, with hydrocarbon and water regions stretching over large distances have been identified in the intermediate regions [1,2]. Conductivity is the most frequently used simple technique to investigate microstructure and structural changes on the basis of the percolation theory, which has been generally accepted to de* Corresponding author. Fax: +86 531 88564750.
E-mail address: [email protected] (L. Zheng). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.05.010
termine the microstructures of microemulsions. The static percolation model has been proposed to describe the mechanism of percolation in traditional microemulsion which attributes percolation to the appearance of a bicontinuous water structure. It is assumed in this description that the open water channel is responsible for electrical conduction [2,3]. The sharp increase of the electrical conductivity in a W/O microemulsion can be explained by a connected water path in the system. In addition, a dynamic percolation model is also developed based on the attractive interactions between the water droplets or micelles [2]. In this viewpoint, the approach considered that the attractive interactions between water globules are responsible for the formation of percolation clusters and the charge transport is assured by hopping of ions on globule clusters which rearrange in time. Recently, ionic liquids (ILs) have received much attention as a class of neoteric solvents, because of their special properties, such as low volatility, nonflammability, and high thermal stability [4–8]. As ideal alternative solvents, they have been widely applied to chemical reactions, separations, electrochemical applications, biopolymers, molecular self-assembly, and in-
Y. Gao et al. / Journal of Colloid and Interface Science 301 (2006) 612–616
terfacial syntheses [4,9,10]. Moreover, ILs have been used to substitute for water or organic solvents to prepare novel IL microemulsions. For example, Han and his co-workers [6] found that hydrophilic IL, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4 ) can substitute for water and form a nonaqueous IL microemulsion with adequate surfactant. Freezefracture electron microscopy (FFEM) indicated that the microemulsion droplets were the same shape as “classic” droplets of water-in-oil microemulsions [6]. Eastoe et al. investigated the same microemulsion system by small-angle neutron scattering (SANS) which showed a regular increase in droplet volume as micelles were progressively swollen with added bmimBF4 , behavior consistent with classic W/O microemulsions [11]. Our recent studies have indicated that the hydrophobic IL, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6 ) may substitute for organic solvents and formed aqueous IL microemulsions with water in the presence of surfactant TX-100 or Tween 20 [12,13]. These aqueous IL microemulsions were found to have potential in the production of metal nanomaterials, used in biological extractions or as solvents for enzymatic reactions [13]. The structure of microemulsions is of much current interest [1,14,15]. For these novel IL microemulsions, it is more necessary to investigate the microstructure and structural transitions. Cyclic voltammetry has been successfully used to obtain information about the microstructure of micelles or microemulsions [16–30]. Changes in the microstructure were identified by using electrochemical probes such as ferrocene, ferricyanide, or methyl viologen [1]. These probes can identify different microenvironments in that the electrochemical reversibility of the probes is affected by the structures of the microemulsions, and thus may appear to reflect the ease of mobility by diffusion coefficient [1]. Gao et al. have successfully detected the three regions of aqueous IL microemulsions: water-in-bmimPF6 (W/IL), bicontinuous, and bmimPF6 in-water (IL/W) using this electrochemical method [12,13]. However, the microstructures of the aqueous IL microemulsions cannot be differentiated through the conductivity method because ILs are essentially molten salts. This means that no insulative media are present in these systems; thus the percolation theory is not applicable. However, if a hydrophilic IL such as bmimBF4 substitutes for water and builds an IL/oil nonaqueous microemulsion in which IL is conductive and oil acts as an insulative medium, then the systems are in accord with the percolation theory according to its theoretical description. In this paper, bmimBF4 was used as a substitute for water and the microstructures and structural transitions of bmimBF4 / TX-100/toluene three-component nonaqueous microemulsion were investigated. Three different microregions: IL-in-oil (IL/O), bicontinuous, and oil-in-IL (O/IL) were differentiated by electrical conductivity and cyclic voltammetry, respectively. The result showed that the simple electrical conductivity method was also valid for investigating microstructures of the nonaqueous IL microemulsions.
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2. Experimental 2.1. Materials TX-100 was obtained from Alfa Aesar and used as received. IL bmimBF4 was prepared as reported earlier [31]. To avoid the moisture in air, bmimBF4 was made and used freshly. Toluene was provided by Beijing Chemical Reagents Company. Potassium ferricyanide K3 Fe(CN)6 , used as an electrochemical probe, was purchased from Fisher. Triple-distilled water was used throughout this study. 2.2. Apparatus and procedures A low-frequency conductivity meter (Model DDS-307, Shanghai Cany Precision Instrument Co., Ltd.) with an accuracy of ±1% was used to measure the solution conductivities along the dilution lines shown in Fig. 1 at 25.0 ±0.2 ◦ C. For cyclic voltammetry, an electrochemical analyzer, CHI832A (CH Instruments, Austin, TX), was used. The electrochemical measurements were conducted using a threeelectrode configuration, which consisted of a glassy carbon working electrode, an Ag/AgCl reference electrode, and a platinum flake counterelectrode. The spacing between adjacent electrodes was set at 2.0 cm. All potentials quoted are with respect to the Ag/AgCl reference. Before each measurement, the working electrode was polished using 0.05 µm aluminum oxide slurries and then washed carefully with distilled water. After polishing, the electrode was ultrasonicated in distilled water for about 5 min and the water on the surface of the electrode was wiped off by soft tissue paper immediately before use. The working electrode area was determined as earlier report (the electrode area was 4.15 × 10−6 m2 ) [12]. The potential was scanned between 0.5 and −0.5 V, and the sweep rate range was 20–100 mV s−1 . The experiments were carried out under a
Fig. 1. Phase diagram of the bmimBF4 /TX-100/toluene three-component system at 25.0 ◦ C. For lines a, b, c, d, and e, the initial TX-100 weight fractions are I = 0.93, 0.88, 0.75, 0.63, and 0.50, respectively. For the lines f and g, the initial TX-100 weight fractions are 0.40 and 0.50, respectively.
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nitrogen atmosphere to avoid the effect of oxygen. All experiments were carried out at 25 ± 0.5 ◦ C. In preparation of the IL microemulsion, each component was added by weight and the solution was mechanically stirred until clear and homogeneous. No phase separation was found in these microemulsions which had been stored at room temperature for at least 2 months. The phase diagram was constructed by titration with toluene as follows: the required masses of TX-100 and bmimBF4 were initially mixed. For each titration, the initial TX-100 weight fraction (I ) was fixed. The phase boundaries were determined by observing the transition from turbidity to transparency or from transparency to turbidity. In Fig. 1, the region marked “single phase” was transparent and the region marked “polyphasic” was turbid. By repeating the experiment for other I values, the phase diagram was established. Furthermore, for cyclic voltammetry, the bmimBF4 solution of probe was freshly made and used immediately. 3. Results and discussion 3.1. Phase behavior of the bmimBF4 /TX-100/toluene three-component system The phase behavior of the TX-100/bmimBF4 /toluene threecomponent microemulsion at 25.0 ◦ C is shown in Fig. 1. A continuous single-phase microemulsion region can always be observed over the bmimBF4 or toluene weight content range 0–100%. Generally, the W/O microemulsions can be formed at low water content and the oil is continuous phase; with increasing water content progressively, as long as there is no phase separation and the system remains isotropic, there should be some kind of gradual structural transition in traditional aqueous microemulsions. The transition should span the range of possible structures from microdroplets to bicontinuous structures. Similarly, for the current nonaqueous IL microemulsions, the single-phase channels are suitable for the study of the microstructure and structural transitions [12,13]. 3.2. Conductivity measurements and microregions of the IL microemulsions As the bmimBF4 /TX-100/toluene microemulsion system is in accord with the percolation theory from the viewpoint of the static percolation model as well as dynamic percolation model, the microstructure investigation can be carried out by electrical conductivity measurement. Clausse et al. [32] demonstrated that, for traditional aqueous microemulsions, with increasing water content, the microemulsion electrical conductivity (k) changed according to four successive stages: the initial nonlinear increase of k revealed the existence of a percolation phenomenon that could be attributed to inverse microdroplet aggregation. The next linear increase was due to the formation of aqueous microdomains which resulted from the partial fusion of clustered inverse microdroplets. The phenomenon suggested that a W/O microemulsion was formed in this low water content region. The third nonlinear curve increase indicated that the
Fig. 2. The electric conductivity k of the nonaqueous IL microemulsion as a function of toluene weight content with I = 0.88 (A) and 0.75 (B), respectively.
medium underwent further structural transitions and a bicontinuous microstructure formed, which was ascribed to the progressive growth and interconnection of the aqueous microdomains. The final decrease of k with increase of water content corresponded to the appearance of water-continuous microemulsiontype media. That is, an O/W microemulsion formed at high water content. The decrease of k merely resulted from the fact that the concentration of the O/W microemulsion droplets was progressively diluted with water. However, due to the molten salts essential for ILs, if adding bmimBF4 progressively to the bmimBF4 /TX-100/toluene microemulsion, the electrical conductivity of the microemulsion will always increase, until approaching pure bmimBF4 conductivity. As a result, it is difficult to identify three different types of microstructures because no obvious breaks appear in the conductivity curve [33]. In this case, we have attempted to identify the structural transitions of bmimBF4 /TX-100/toluene microemulsion by using insulative toluene as the titration phase. Test samples are shown in Fig. 1 (lines a–e). As two typical examples, Fig. 2 shows the variation of electrical conductivity k as a function of toluene weight fraction. The initial increase of k, due to the successive increase of conductive O/IL microemulsion droplets, could indicate the formation of O/IL microemulsions. The next nonlinear decrease revealed that the
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medium underwent a structural transition and became bicontinuous, owing to the progressive growth and interconnection of the O/IL microdomains. The third section of curve, linear decrease of k, could be interpreted as the consequence of the formation of IL/O microdomains resulting from the partial fusion of clustered inverse microdroplets. The final nonlinear decrease of k with further increasing oil content corresponded to the existence of a percolation phenomenon that could be ascribed to inverse microdroplet aggregation. The conductivity curves in Fig. 2 evidently illustrate the presence of three different types of microstructure: O/IL, bicontinuous, and IL/O microemulsions. By repeating the experiment for other samples with different I values, three types of microregions can be determined. The subregions identified were also marked in Fig. 1. 3.3. Electrochemical behavior of the K3 Fe(CN)6 probe in the ionic liquid microemulsion In cyclic voltammetry, the peak current ip for a redox-active reversible system is given by the Randles–Sevcik equation [24], 0.447F 3/2 An3/2 D 1/2 Cv 1/2 , (1) R 1/2 T 1/2 where n is the number of electrons involved in oxidation or reduction, A is the area of the working electrode, D is the diffusion coefficient of the electroactive probe, C is the concentration of electroactive probe in the solution, v is the sweep rate, F is the Faraday constant, R is the gas constant, and T is the absolute temperature. It follows from Eq. (1) that ip will increase linearly with v 1/2 for a given electrode and a constant electroactive probe concentration. Diffusion coefficient values can be obtained from a linear regression of the slope of ip versus v 1/2 using the known surface area of the electrode. With microemulsion systems involving an electrochemical probe completely solubilized, the diffusion coefficient D in Eq. (1) corresponds to the microemulsion diffusion coefficient since the probe diffuses with the microemulsion droplets [16]. The cyclic voltammetry experiments were carried out at various scan rates in microemulsion environments in order to verify the diffusioncontrolled nature of the process. Electrochemical charge transfer must be diffusion-controlled in order to study the microstructure of microemulsions by cyclic voltammetry [24]. Fig. 3 shows typical plots of ip versus v 1/2 for three microemulsion systems, and the plots are straight lines passing through the origin. This result indicates that the electron transport for the [Fe(CN)6 ]3− /[Fe(CN)6 ]4− electrode reactions in the nonaqueous IL microemulsion medium is diffusioncontrolled [16,20,21]. Similar results were observed for all the other nonaqueous IL microemulsion systems we prepared. Thus, potassium ferricyanide K3 Fe(CN)6 is a suitable electrochemical probe for study of the microstructures and structural transitions of these microemulsions.
Fig. 3. Scan rate dependence of peak current in the nonaqueous IL microemulsions: (a) 26.7% TX-100 + 44.0% bmimBF4 + 29.3% toluene; (b) 26.1% TX-100 + 45.3% bmimBF4 + 28.6% toluene; (c) 25.5% TX-100 + 46.6% bmimBF4 + 27.9% toluene. 0.02 g K3 Fe(CN)6 added as electroactive probe for all systems.
ip =
3.4. Diffusion behavior of probe in microemulsion Test microemulsions were prepared along the dashed line shown in the phase diagram (Fig. 1, line g). This range of com-
Fig. 4. The diffusion coefficient of K3 Fe(CN)6 as a function of bmimBF4 weight content with initial TX-100 weight fraction, 0.50 (Fig. 1, line g).
positions corresponds to a wide range of IL content and the system is always isotropical. It is suggested that the compositions of the microemulsions changed in a systematic fashion to span the range of possible structures from microdroplets to a bicontinuous structure. The electroactive probe concentration had little effect on the diffusion coefficient D values since the shape and size of microemulsions were not disturbed by the presence of the electroactive probe over a large concentration range [25]. For the investigated nonaqueous IL microemulsion systems, this has been confirmed by the fact that the electron transport of [Fe(CN)6 ]3− /[Fe(CN)6 ]4− electrode reactions was diffusion-controlled. Therefore, our experiments were carried out with a fixed initial K3 Fe(CN)6 quantity and the concentration of the probe changed with the addition of IL. The change in the diffusion coefficient of the probe, calculated from the Randles–Sevcik equation as a function of added bmimBF4 in microemulsions, is illustrated in Fig. 4. It can be seen that the diffusion coefficients of K3 Fe(CN)6 increase with increasing bmimBF4 weight content in the whole single-phase microemulsion region. When the bmimBF4 weight content was less than about 16%, the diffusion coefficients increased grad-
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ually, because at low bmimBF4 content, a bmimBF4 -in-toluene microemulsion formed. K3 Fe(CN)6 is expected to probe the bmimBF4 environment preferentially due to its limited solubility in toluene. The diffusion coefficient of K3 Fe(CN)6 , therefore, should correspond to the diffusion coefficient of bmimBF4 -in-toluene microemulsion droplets. Thus the value of the diffusion coefficient of the probe is relatively small. Guering and Lindman [34] also discovered that molecules confined in a closed domain will give rise to low self-diffusion coefficients. The diffusion coefficients increase with increasing droplet size, but as long as the diffusion of probe is confined in microemulsion droplets, the curve increases slowly. Therefore, the gradual change indicates that the microenvironment of the microemulsions is unchanged. A similar trend has also been observed for this microemulsion system when the bmimBF4 weight content was above 41%. In the latter case, the toluene microdroplets were dispersed in a continuous bmimBF4 medium. The diffusion coefficient of the probe may correspond to the continuous bmimBF4 medium and show high mobility, thus a high value. The gradual increase of the diffusion coefficients derives from the fact that the IL microenvironment is not sensitive to bmimBF4 weight content in continuous bmimBF4 phase microemulsions. However, a relatively dramatic change in probe diffusion coefficients is observed when the bmimBF4 weight content is in the range of 16–41%, which indicates that the microenvironment of the microemulsions is different from that of IL/O or O/IL microemulsions. According to the principle of distinguishing subregions in microemulsions using the apparent diffusion coefficient [16,17,19], the result suggests that a bicontinuous microstructure, in which bmimBF4 and immiscible toluene are both local continuous phases, is formed. Guering and Lindman [34] stated that the molecules occurring in the continuous medium will be characterized by rapid diffusion. Thus a continuous structure is expected to give a higher self-diffusion coefficient than a droplet structure [22]. The curve of the diffusion coefficient of the probe in Fig. 4 illustrates the occurrence of the three microregions: IL/O (<16% bmimBF4 ), bicontinuous (16–41% bmimBF4 ), and O/IL (>41% bmimBF4 ) microemulsions. Thus, the results obtained by electrical conductivity and cyclic voltammetry measurements are in agreement, indicating that the electrical conductivity method is also feasible for identifying microstructures of the nonaqueous IL microemulsions. 4. Conclusion In summary, microemulsions consisting of bmimBF4 , surfactant TX-100, and toluene were prepared and the phase behavior of the ternary system was investigated. The bmimBF4 in-toluene (IL/O), bicontinuous, and toluene-in-bmimBF4 (O/IL) microregions of the nonaqueous IL microemulsions were initially identified by traditional electrical conductivity measurements on the basis of percolation theory. The microstructures of the microemulsion were further investigated by electrochemical cyclic voltammetry. The data obtained by electrical conductivity are the same as those by cyclic voltammetry, indicating that the simple electrical conductivity method
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