Effect of “Local Viscosity” on Translational Friction and Viscosity B Coefficients for a Spherical Molecule in Solution

Abstract The translational friction and the viscosity B coefficients for a spherical molecule in solution are calculated from the Navier–Stokes equation for a continuum with space-dependent viscosity. We test three functions for the space dependence of the viscosity as models for the “local viscosity”. Each function is a smooth function of distance from the solute center and approaches the viscosity in the bulk at an infinite distance. For all the three functions studied, the translational friction and the viscosity B coefficients decrease with decreasing viscosity in the vicinity of the solute, and the effect of the space-dependent viscosity is larger for the B coefficient than for the translational friction coefficient. These results can explain the observed shortcomings of the ordinary continuum model with space-independent viscosity. This confirms the validity of the use of the viscosity B coefficient as a measure of the structure-breaking effect which is closely related to the idea of the “local viscosity”.