A direct method of determining the coupling between internal molecular motions and transport properties: Application to liquid n-butane

Abstract

The coupling between internal molecular motions and transport properties is investigated with particular reference to the influence of torsional motions on the coefficients of self-diffusion and shear viscosity in n-butane. A novel technique, frozen distribution sampling (FDS), is designed to allow comparison of the dynamic properties of two fluids, one in which torsional motions are controlled by a well defined torsional potential, the other in which an equilibrium distribution of torsion angles are held completely rigid. It is shown that, as expected, the two fluids have exactly the same static equilibrium properties. The importance of satisfying the former criteria is illustrated by well chosen counter examples where related models of n-butane were studied in which the intramolecular potential was changed. In these cases interpretation of the relative values of transport properties is ambiguous since alterations of the potential cause significant changes in the equilibrium state. Our results show that there are no significant coupling effects between torsional motion and self-diffusion or shear viscosity at the state point considered.