Viscosity Modeling of Ionic Liquids Using the Friction Theory and a Simple Cubic Equation of State

In the present work, the well-known friction theory (FT) based on friction concepts of classical mechanics and the van der Waals theory of fluids has been modified to accurately represent the dynamic viscosity of pure ionic liquids in a rather simplified manner. Unlike previous FT applications to pure ionic liquids, the dilute gas limit for viscosity was excluded from the present model since it is negligible for ionic liquids which exhibit extremely low vapor pressures; the friction term thus prevailed for viscosity calculations. The latter was expressed in terms of new temperature-dependent friction coefficients whereas the repulsive and attractive pressure terms were in turn estimated from two simple cubic equations of state (Soave or Peng-Robinson) rather than using sophisticated multiparameter equations of state such as SAFT- or CPA-based expressions previously used by other authors. The resulting model was successfully validated during the representation of experimental dynamic viscosities of three families of imidazolium-based ionic liquids ([C_Xmim][BF₄], [C_Xmim][PF₆], and [C_Xmim][Tf₂N]) within a temperature range varying from 0 to 80 °C and at pressures from 1 up to 3000 bar.