Does polar interaction influence medium viscosity? A computer simulation investigation using model liquids

Abstract

Molecular dynamics simulations of model liquids interacting via Lennard–Jones (L–J) and Stockmayer (SM) interactions have been carried out to explore the effects of the longer-ranged dipole–dipole interaction on solvent viscosity and diffusion. Switching on of the dipolar interaction at a fixed density and temperature has been found to increase the viscosity over that of the LJ liquid, the extent of increase being a few percent to as large as ∼60% depending on the magnitude of the solvent dipole moment used in the SM potential. The simulated translational and rotational diffusion coefficients show strong dipole moment and temperature dependences, eventhough effects of these parameters on solvent–solvent radial distribution function are moderate. Interestingly, a partial solute–solvent decoupling is observed when the simulated translational and rotational diffusion coefficients are connected to the simulated viscosity coefficients via the Stokes–Einstein (SE) and Stokes–Einstein–Debye (SED) relations. In the limit of large dipole moment, simulated self-part of the van Hove correlation function at intermediate times reveals a departure from the Gaussian distribution with particle displacement. This suggests that dynamic heterogeneity is one of the reasons for the departure of centre-of-mass diffusion from the SE relation in these model systems.