Physico-chemical Properties of Imidazolium-Based Ionic Liquids in Pure and Mixed Polar Solvents

Abstract

Ionic liquids comprise entirely ions and are fluid at temperatures around 100 °C or below. The physico-chemical properties of IL change dramatically upon addition of one or more molecular solvent/s and therefore, binary and ternary mixtures of ILs with molecular solvents have received significant attention in recent years. They provide the means of accumulation of knowledge regarding the types of interaction and may exhibit the appearance of a new phenomenon, which is absent in the pure state of ILs. With a view to a proper understanding molecular level interactions of IL based systems, in this work, binary liquid mixtures of N, N-dimethylformamide (DMF) with acetonitrile (ACN), 1-ethyl 3-methylimidazolium bis(triflouromethanesulfonylimide)([emim][TFSI]) with ACN and DMF in the whole composition range and 1-ethyl 3-methylimidazolium hexaflourophosphate([emim][PF6]) with ACN and DMF have been prepared. The different interactions in these binary mixtures have been investigated by measuring physico-chemical properties with varying the compositions of different components. Effects of temperature and composition on some fundamental physico-chemical properties- density, viscosity, refractive index and conductivity of these binary mixtures also have been investigated. The excess molar volume (VE), excess viscosity (∆η) and excess refractive index (∆n) were calculated and the results were fitted to Redlich-Kister polynomials. The values of VE and ∆η were negative and ∆n were positive over the whole composition range for these binary mixtures. The minima in VE and ∆η were observed at mole fraction of IL, XIL ~ 0.2 and 0.5 respectively for [emim][TFSI] and DMF mixtures, for [emim][TFSI]-ACN the minima in VE and ∆η were observed at mole fraction of IL, XIL ~ 0.2 and 0.6, respectively . With increasing XIL, the refractive index of these binary mixtures gradually increased. The conductivity of the binary mixtures have been measured by the impedance spectroscopy method and the data have been used to infer about the ionic characteristics of the binary systems. Finally, from the understanding of these three binary mixtures, ternary mixtures of [emim][TFSI] , DMF and ACN have been prepared and physico-chemical properties of these ternary mixtures have been studied in the temperature range from 20 to 50 oC with change in the composition of different components. The changes in density, viscosity and refractive index as a function of compositions have been correlated with the variation of these properties with temperature. The excess molar volume, excess viscosity and excess refractive index of these ternary mixtures have been deduced from the results of physico-chemical properties. The values of VE and ∆η were negative over the whole composition range. Thus, the ultimate goal of this work has been to unveil inter-molecular interactions of different binary and ternary mixtures of ILs with different polar molecular solvents.