Phase Equilibria, Thermodynamic Behavior, and Transport Properties of Aqueous Solutions of [BMPYR] Trifluoromethanesulfonate and [BMPYR] Tricyanomethanide

Abstract

In this work, continuing our systematic studies on mixtures of 1-ethyl-3-methylimidazolium- or 1-butyl-1-methylpyrrolidinium (BMPYR)-based ionic liquids (ILs) with water, we thoroughly examine the thermodynamic and transport behavior of binary aqueous systems of two BMPYR-based ILs containing trifluoromethanesulfonate (OTF) and tricyanomethanide (TCM) as counterions. We performed accurate measurements of the phase equilibria, mixing enthalpy, density, viscosity, and electrical conductivity for these mixtures. The condensed phase equilibria were determined by employing dynamic measurements of freezing or cloud point temperatures and static solubility measurements for the mixtures, as well as DSC measurements on the neat ILs. The mixing enthalpy of [BMPYR]­[OTF] with water was determined at high IL dilutions and four temperatures in the range T = (288.15–318.15) K. Measurements of other properties were performed over the entire composition range (apart from the narrow miscibility gap in the dilute [BMPYR]­[TCM] region). The vapor–liquid equilibrium was determined by measuring water activity at seven equidistant temperatures in the range T = (288.15–318.15) K. The activity data were simultaneously correlated with those on either excess enthalpy (for [BMPYR]­[OTF]) or mutual liquid–liquid solubilities (for [BMPYR]­[TCM]) using an extended nonrandom two-liquid equation. This thermodynamic description of the systems was shown to be of outstanding global performance and allowed us to gain a deeper insight into their energetics. Density, viscosity, and conductivity for the mixtures, as well as of neat ILs, were measured at 10 temperatures covering the range T = (288.15–333.15) K and were adequately represented by suitable empirical relations. A consistent molecular-level interpretation of various facets of the observed behavior is presented in terms of an interplay of anion–water and water–water H-bonding, cation hydrophobic hydration, and ion pairing. Due comparison of new data with those available in the literature enabled us to discriminate their quality and identify some of the latter as incorrect.