A computer simulation study of the effect of molecular mass distribution on mass and momentum transport in molecular liquids

Abstract

Molecular dynamics computer simulation has been used to determine diffusion coefficients and viscosities for four thermodynamical ly identical one component model liquids composed of rigid triatomic molecules of the type XY2 at the same temperature and density. Exactly the same intermolecular potential was used throughout. The molecules comprising these liquids all had the same total mass but the molecular mass distributions were different for each liquid. Differences in the viscosities and the diffusion coefficients clearly show that the molecular mass distribution is a factor which in itself influences transport properties, a point seldom considered in the past. The viscosities were determined by nonequilibrium molecular dynamics and a new method is described for implementing isokinetic shear flow in molecular liquid simulations which employ quaternion algorithms. To obtain satisfactory diffusion coefficients for the equilibrium liquids it was necessary to use simulations extending up to 300 ps. The results are particularly relevant to the phenomenon of rotation–translation coupling and, together with the linear and angular velocity autocorrelation functions, are discussed in this context.