Assessment of the temperature-dependent viscosity of diverse functionalized ionic liquids: Revisiting the fluidity equation

Abstract

Abstract The equation that has already been developed to describe the temperature dependence of viscosity of ionic liquids (ILs) with good accuracy, makes use of a preferential feature of fluidity and importantly represents a smooth function of temperature, both at low and high temperatures. It can be applied as two- and three-parameter modes. In this work, in order to further demonstrate the wide applicability of this model, viscosities of imidazolium-, pyridinium-, pyrrolidinium-, piperidinium-, ammonium- and phosphonium-based pure ILs, made available to date, are correlated. The fluidity equation is also applied for the temperature dependence of viscosity of various alkyl functionalized ionic liquids, which not yet published until very recently, for the first time. The capability of the fluidity equation to reproduce the experimental viscosity data is compared with the most commonly used equation in the field of the viscosity of ionic liquids with a semiempirical basis, i.e. Vogel–Fulcher–Tammann (VFT) equation. For the new ILs, the dynamic crossover temperature is estimated by the parameters of the fluidity equation for the first time. Moreover, an apparent trend is found between exponent of the fluidity equation and cation alkyl chain length, which is attributed to the competition between cation-anion coulombic interactions and the cation-cation alkyl chain van der Waals interactions.