Physical properties and solid-liquid equilibria for hexafluorophosphate-based ionic liquid ternary mixtures and their corresponding subsystems

Abstract

Mixing ionic liquids is a simple and economical method of exploiting their tunability and allows to use ionic liquids with high melting temperatures for low-temperature applications through the formation of eutectic mixtures. In this study, the phase diagrams of the [C4mpy][PF6]-[C4mpip][PF6]-[C4mpyrr][PF6] ternary system (where [C4mpyrr] = 1-butyl-1-methylpyrrolidinium; [C4mpy] = 1-butyl-3-methylpyridinium; [C4mpip] = 1-butyl-1-methyl-piperidinium) and all of its unary and binary subsystems were measured and modelled using the Modified Quasichemical Model and the Compound Energy Formalism for the liquid and relevant solid solutions, respectively. The phase diagram determination allowed for density and viscosity measurements over the entire composition range, from temperatures close to the liquidus up to about 110 °C. In addition, the thermal and physical properties of the ionic liquid [C4mim][PF6] ([C4mim] = 1-butyl-3-methylimidazolium) were measured. A new viscosity model was proposed to describe mixtures and was compared to the Grunberg-Nissan mixing law. The proposed model exhibited a better predictive ability for the viscosity data of ternary mixtures compared to the Grunberg-Nissan mixing law with the same number of adjustable parameters. The limits of the proposed viscosity model were analyzed in light of the Gibbs-Adam theory, using viscosity and configurational entropy data for [C4mim][PF6].