Molecular Shape Dependent Model of Self-Diffusion in, and the Viscosity of, Large Molecule Liquid Systems: Viscosity, Temperature, and Pressure Relationships for Model Liquid Hydrocarbons

Our earlier molecular shape dependent theoretical model for self-diffusion in, and the viscosity of, liquids of large (nonpolymeric) molecules was modified to incorporate the effect of an applied pressure. In this model, the pressure acts to increase the activation energy for the fundamental molecular hopping event. For large molecule liquids, significant pressure is viewed as causing molecules to crowd against each other as the free volume available for site-to-site hopping decreases and intermolecular interactions become stronger. In this way, the number of translational and rotational degrees of freedom decreases and a decrease in the coefficient of self-diffusion and an increase in viscosity result. Theoretical predictions compared favorably with existing experimental viscosity data for flexible linear and rigid bulky hydrocarbon molecule liquids as a function of both pressure and temperature