Predicting thermophysical properties of ionic liquids as a function of temperature and pressure

Abstract

Correlation and prediction models to describe density, surface tension and viscosity of ionic liquids are evaluated and extended to new ionic liquids with potential interest for applications in lubrication. Original experimental results on these properties are also reported. The ionic liquids considered are based on alkylammonium cations including hydroxyl functional groups, and on alkylsulfate or alkylsulfonate anions with different lengths of the alkyl side chains. Where possible, predictive schemes are proposed to calculate density, surface tension and viscosity as a function of temperature and up to high pressure, sometimes starting solely from a knowledge of the molecular structure of the ions, but the availability of experimental data at room temperature and atmospheric pressure can greatly improve the predictions away from ambient conditions. For density, a group contribution method was parameterised for the ionic liquids studied here. Parameters for other families of ionic liquids exist in the literature. This model for the density is then used in correlations of surface tension and viscosity. For this last transport property, predictions of viscosity at high pressure are possible from a knowledge of data at atmospheric pressure.