Thermophysical Properties of 1-Ethyl-3-methylimidazolium Dicyanamide + Water: Understanding Interactions and Structure of Ionic Liquid + Water Systems

Abstract

Ionic liquids with the dicyanamide anion, namely, with 1-alkyl-imidazolium cations have been receiving attention recently due to their potential applications. The utilization of these liquids as heat-transfer fluids, specifically in small heat exchangers and microchannels for microprocessor cooling, is presently deemed highly feasible, as it can be both more efficient and environmentally acceptable. The heat-transfer equipment design that uses fluids requires knowledge of their thermophysical properties. If one or more fluid properties are considered inadequate, additives may be introduced to improve the performance in key aspects. Water can be used to lower viscosity and thermal conductivity, for example, if the working temperature range allows for it. In this paper, we report the most comprehensive measurements of thermophysical properties of the pure 1-ethyl-3-methylimidazolium dicyanamide, [C₂mim][N(CN)₂], and of its mixtures with water, in the whole concentration range, at P = 0.1 MPa. The properties measured were the density and speed of sound, electrical conductivity (293.15 < T/K < 343.15), thermal conductivity (293.15 < T/K < 353.15), and refractive index (293.15 < T/K < 343.15). Water-free values of the properties of the ionic liquid were obtained, following procedures previously developed. All the results obtained were analyzed and discussed in light of the current knowledge of the molecular structure and its effect on macroscopic behavior. Analysis of the ionicity of this aqueous ionic liquid mixture in the MacFarland scale shows that it is similar to that of [C₂mim][CH₃SO₃] but much higher than that of [C₂mim][CH₃COO]. For the pure ionic liquid, ΔW = 0.18, 82% of that of 0.01 M KCl, with an ionicity of 66%. The thermal conductivity of the mixture and its variation with composition at a given temperature was analyzed using the Filaments Model, which was not capable of reproducing the experimental points. The structural behavior of the water mixtures was found to be qualitatively similar to previous molecular simulation data and experimental data on other ionic liquid + water mixtures, solidifying the previously proposed picture of the structure of these binary mixtures, as composition and temperature vary. This type of macroscopic behavior can pave the way to several applications, namely, in the fields of heat transfer/storage and battery electrolytes.