Relationship between viscosity coefficients and volumetric properties using a scaling concept for molecular and ionic liquids

In this work, a scaling concept based on relaxation theories of the liquid state was combined with a relation previously proposed by the authors to provide a general framework describing the dependency of viscosity on pressure and temperature. Namely, the viscosity-pressure coefficient (partial differentialeta/partial differentialp)T was expressed in terms of a state-independent scaling exponent, gamma. This scaling factor was determined empirically from viscosity versus Tvgamma curves. New equations for the pressure- and temperature-viscosity coefficients were derived, which are of considerable technological interest when searching for appropriate lubricants for elastohydrodynamic lubrication. These relations can be applied over a broad range of thermodynamic conditions. The fluids considered in the present study are linear alkanes, pentaerythritol ester lubricants, polar liquids, associated fluids, and several ionic liquids, compounds selected to represent molecules of different sizes and with diverse intermolecular interactions. The values of the gamma exponent determined for the fluids analyzed in this work range from 1.45 for ethanol to 13 for n-hexane. In general, the pressure-viscosity derivative is well-reproduced with the values obtained for the scaling coefficient. Furthermore, the effects of volume and temperature on viscosity can be quantified from the ratio of the isochoric activation energy to the isobaric activation energy, Ev/Ep. The values of gamma and of the ratio Ev/Ep allow a classification of the compounds according to the effects of density and temperature on the behavior of the viscosity.