Two simple correlations to predict viscosities of pure and aqueous solutions of ionic liquids

Abstract

Because of the special nature of ionic liquids, consisting of large heterogeneous organic cations and either an organic or inorganic anion, they possess interesting physical properties, such as negligible vapor pressure, low melting temperature, and high viscosity. The high viscosities of ionic liquids can be useful in some industries and for certain applications, such as lubricants. In this study, the viscosities of eight different ionic liquids and their binary mixtures with water have been investigated. Two easy-to-use correlations have been proposed, one for the estimation of viscosities of pure ionic liquids, and the other for their aqueous mixtures. The simple correlations are applicable at various temperatures at atmospheric pressure, and in the case of the mixture correlation, at various molar compositions. For the pure component correlation, an absolute average relative deviation of 2.97% was obtained for a total of 245 pure ionic liquid (and pure water) data, while for the mixture correlation, an absolute average relative deviation of 6.72% was obtained for a total of 512 viscosity data points for mixtures of ionic liquids with water. Such error values show the good agreement of both the proposed pure component and aqueous mixture correlations with respect to the corresponding experimental values. The proposed pure ionic liquid viscosity model was also compared to the two common literature correlations of Vogel and Daubert and Danner, after optimizing these two literature models to obtain the necessary equation coefficients for ionic liquids. Also, the newly-proposed mixture viscosity correlation was compared to the Grunberg and Nissan method for mixtures, coupled with three different pure viscosity models, to calculate the viscosities of mixtures of ionic liquids with water. Comparison of error values indicated the two correlations of this work to be more accurate than the above literature models for both pure and mixture viscosities.