Volumetric properties of ionic liquid 1,3-dimethylimidazolium methyl sulfate + molecular solvents at T = (298.15–328.15) K

Abstract

The experimental density, d, values as function of ionic liquid concentration, m, for binary mixtures, [MMIm]MSO₄ + methanol, + water, + DMSO, + DMF, and + MeCN at T = (298.15–328.15) K are reported. Apparent molar volumes V_ϕ have been determined from the experimental densities and fitted to a Redlich–Mayer equation for the calculation of limiting apparent molar volume values $V_{\varphi }^0$. The values of $V_{\varphi }^0$ have the following order in the investigated systems: MeCN < MeOH < DMF < DMSO < H₂O. The calculated $V_{\varphi }^0$ values indicate that ionic liquid + molecular solvent interactions in water and DMSO are stronger than the other solvents. The limiting apparent molar expansibilities $E_{\varphi }^0$ show that at each temperature $E_{\varphi }^0$ values for ionic liquid in H₂O and DMSO have positive values and decrease with rising temperature. The calculated values of ${({\partial }^2{V}_{\varphi }^0/\partial T^2)}_P$ for binary mixtures of containing [MMIm]MSO₄ have small and negative values for studied systems. Therefore, ionic liquid under study is predominantly a structure breaker.

Introduction

One of the most important types of organic salts is ionic liquids (ILs), which are liquids at or near room temperature in their pure state. They have been widely used in a number of fields in both academia and industry and exhibit many advantages such as a low melting point (<373 K), a wide liquid range, suitable viscosity, thermal stability, ability to dissolve a variety of materials and most important, negligible vapor pressure [1], [2], [3]. Ionic liquids have been suggested as green and benign replacements for traditional volatile organic solvents and a growing number of applications such as catalysis, chemical reactions, separations, electrochemistry, and nanoscience investigations [4], [5], [6]. These developments demand reliable reference data on the thermodynamic and thermophysical properties of their mixtures with other compounds. These properties of ionic liquids depend on structural factory, ion-association and ion–solvation effects, are receiving increasing interest and are being studied more systematically. It is very important to understand the interactions between the traditional solvents and ionic liquids, which can be accomplished by the measurements of the physical properties.

Binary mixtures of ionic liquids with molecular solvents have been used in many processes mainly for synthesis, design of chemical, extractive processes, and heat transfer mediate involving ionic liquids on an industrial scale. To date, a number of papers have measured some of the thermodynamic and thermophysical properties of binary mixtures systems containing an ionic liquid in molecular solvents such as density, viscosity, speed of sound, surface tension, thermal conductivity and phase equilibria [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. These papers mainly report these quantities for the whole composition range of ionic liquids in solvents, and only some other works have been performed for dilute region solution of ionic liquids. It is well known that ionic liquids can be viewed as kind of organic electrolyte. Therefore, at even higher dilutions, ionic liquid acts a conventional but rather strongly associated electrolyte [17]. Therefore, electrolyte solution models may treat their properties.

The most commonly studied ionic liquids are based on the imidazolium cation and fluorinated anions such as BF₄⁻, PF₆⁻, NTf⁻ and so on. However, the hydrolytical instability of such anions has become obvious, and decomposition of these fluorinated anions leads to the formation of highly toxic and corrosive HF, the use of these ionic liquids will be limited. Therefore, the synthesis and application of halogen free ionic liquids is necessary. Several types of these systems have been described, e.g. alkyl sulfunates, organoborates and alkyl sulfate anions. The most important properties of alkyl sulfate based ionic liquids are easily synthesis, using cheap alkylating agent, solvent free synthesis, high reaction rate, excellent purity, large scale synthesis, safe and nontoxic chemical, low viscosities and low melting points [18].

One of the most important ionic liquid based on methyl sulfate anion is 1,3-dimethylimidazolium methyl sulfate. Activity coefficients at infinite dilution of various solutes in the ionic liquid, [MMIm]MSO₄, were measured [19]. The surface tensions were measured in a series of alcoholic solutions of [MMIm]MSO₄ [20]. The osmotic coefficient of [MMIm]MSO₄ has also been measured with water by Macedo and co-workers [21], [22]. The liquid–liquid equilibria of several binary system containing [MMIm]MSO₄ and dialkyl carbonates, ketones or acetates and other solvents were studied and similar studies were made on alkyl sulfate containing ionic liquids [23], [24], [25], [26], [27], [28], [29], [30], [31].

In this work, the procedure to synthesize [MMIm]MSO₄ is described and volumetric properties of binary mixtures containing this ionic liquid and molecular solvents have been studied. Volumetric properties have been extensively studied in order to know the nature and extent of various intermolecular interactions existing between different species present in mixture [32]. In this work densities were measured for the binary mixtures containing an ionic liquid, 1,3-dimethylimidazolium methyl sulfate, [MMIm]MSO₄ with water, N,N-dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF), acetonitrile (MeCN), and methanol (MeOH) at T = (298.15–328.15) K and at atmospheric pressure from which the values for infinite dilution apparent molar volumes are calculated with the help of Redlich–Mayer [33].