Self-aggregation of ionic liquids in aqueous media: A thermodynamic study

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Abstract

Aggregation process of the ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4], 3-methyl-1-octylimidazolium tetrafluoroborate [C8mim][BF4], 3-methyl-1-octylimidazolium chloride [C8mim][Cl], and N-butyl-3-methylpyridinium chloride [C4mpy][Cl], in the aqueous solutions has been characterized by electrical conductivity, density, and speed of sound measurements at 298.15 K. The critical aggregation concentration (CAC), the standard Gibbs energy of aggregation ΔGagg., isentropic compressibility κs and changes in the isentropic compressibility upon aggregation Δκs,agg for the ionic liquids were derived from the experimental data. The dependence of the CAC, ΔGagg., and Δκs,agg on the nature of the cations and anions has been examined. The aggregation number n for various ILs was determined by applying the mass action theory to the concentration dependence of electrical conductivity and compressibility. Aggregation properties showed fairly good agreement with our previously reported results of 1H NMR and fluorescence spectroscopy.

Introduction

In recent years, room temperature ionic liquids (ILs) have aroused great interest among scientific community due to their unique physical and chemical properties. ILs have an edge over conventional organic solvents in terms of very low vapor pressure, large liquid range, high-thermal stability, nonflammability under ambient conditions, wide electrochemical window and strong ability to dissolve many organic or inorganic solutes. They have been finding place in a variety of chemical applications such as organic synthesis, liquid–liquid extraction, electrochemical processes, and separation science [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Until now, the focus of the studies on room temperature ionic liquids have been mainly concerned to the temperature or pressure dependent physical–chemical characterization based on variation of cationic or anionic species. Considerable efforts have been also made towards the understanding of effect of water on the thermodynamic properties of pure ILs [11], [12], [13], [14] or their binary mixtures in the aqueous solutions [15], [16], [17], [18], [19], [20], [21].

The studies performed in aqueous solutions of ILs are mainly devoted to the understanding of nature of solute–solvent (hydrogen bonding or ion solvent) interactions. This is only recently when focus has been turned towards the composition region where ILs are shown to form organized assemblies due to the inherent amphiphilic nature of their cations [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. In a previous study, we investigated the aggregation behavior of some short hydrocarbon chain imidazolium or pyridinium based ILs in their aqueous solutions and defined the role of aromatic ring, alkyl chain, and counter ions on the aggregation properties using 1H NMR and fluorescence spectroscopy [33]. As an extension of previous work, in the present work we have thermodynamically characterized the aggregation process of various ILs. The aggregate formation was followed by electrical conductivity, density and speed of sound measurements at 298.15 K. From the experimental measurements, different thermodynamic functions such as free energy of aggregation, isentropic compressibility and the changes in isentropic compressibility upon aggregate formation were deduced. Further, the dependence of various thermodynamic functions upon nature of cations and anions has been examined. For estimation of aggregation numbers and degree of ionization of aggregates of ILs, the electrical conductivity and speed of sound data were analyzed according to the mass action model of micellar solution. Aggregation properties determined by various thermodynamic methods have been compared with those of obtained from our previously reported 1H NMR and steady-state fluorescence measurements.

Section snippets

Materials and methods

1-Butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4], 3-methyl-1-octylimidazolium tetrafluoroborate [C8mim][BF4], 3-methyl-1-octylimidazolium chloride [C8mim][Cl], N-butyl-3-methylpyridinium chloride [C4mpy][Cl] with stated purities higher than 99% mass fraction, were purchased from Solvent Innovation, GmbH, Germany. All of the ILs, were dried under vacuum at 343 K for few days to remove trace amount of water. Karl-Fisher analysis of the samples indicated that the water content was reduced

Conductivity

Experimental values of the specific conductivity σ as a function of IL concentration are shown in Fig. 1(a–d). Precise values of critical aggregation concentration (CAC) for the various ILs were obtained by analyzing conductivity data using the Philips [35] definition of the critical aggregation concentration as the point corresponding to the maximum change in the gradient of a physical property of solution against concentration. Distinct break points in the specific conductivity versus

Conclusions

Thermodynamic studies have been performed for investigating the aggregation behavior of some imidazolium or pyridinium based ionic liquids. Mass action model of micellar solution has been applied to the conductivity and compressibility data of the aqueous solutions of ionic liquids and found fairly adequate for the estimation of aggregation number. Thermodynamic results suggest that the aggregation in longer chain ionic liquids is driven by the alkyl chain-ion inductive force and the

Acknowledgements

We are thankful to Dr. P.K. Ghosh, Director of the Institute, for his interest in this work. TS is thankful for financial assistance by DST, Government of India (Project no. SR/S1/PC-36/2005).

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