Dipolar solute rotation in a supercritical polar fluid.

Fluorescence anisotropy measurements reveal a non-monotonic density dependence for average rotation time (τ(R)) of a polar solute, coumarin153 (C153) in polar supercritical fluoroform (CHF(3)). The conventional Stokes-Einstein-Debye model, relating τ(R) to the solvent viscosity, fails to explain the observed density dependence, because the experimental viscosity increases monotonously with density for a fluid, in general. Here, the density-dependent τ(R) is calculated by incorporating the wave vector-dependent viscosity of the solvent and the solute-solvent interaction. A molecular hydrodynamic description is used for the wave vector-dependent viscosity which is verified by molecular dynamics (MD) simulation. A justification for the applicability of the present prescription is provided by reproducing the experimental viscosity of supercritical (SC) CHF(3). Solute-solvent interaction has been included via the fluctuating torque acting on the rotating solute. Incorporation of wave vector-dependent viscosity leads to a qualitative description of the experimental density dependence of τ(R) which is further improved upon inclusion of solute-solvent interaction.