Rotational dynamics of nonpolar solutes in different solvents: Comparative evaluation of the hydrodynamic and quasihydrodynamic models

Abstract

The rotational reorientation times (τr) of substituted quaterphenyl and quinquephenyl molecules (neutral nonpolar solutes) of 4–5 Å radius have been measured in a series of n alkanes, n nitriles, and n alcohols, by picosecond fluorescence depolarization technique. The experimental data were compared with hydrodynamic and quasihydrodynamic models of Gierer Wirtz (GW) and Dote Kivelson Schwartz (DKS). Our studies reveal that these molecules rotated faster in alcohols compared to alkanes and nitriles. A nonlinear variation of τr vs viscosity (η) in alcohols was also observed, being more prominent for the smallest solute. The transition towards the stick hydrodynamic limit with increase in solute size was evident in the chosen set of solute probes. Both GW and DKS theories reproduce the nonlinear variation of τr vs η; however, DKS theory alone correctly predicted the faster rotation in alcohols. A test for quantitative prediction of τr of nonpolar solutes in alkanes and alcohols, using both GW and DKS models were made by compiling the existing data in literature. Our study indicates that DKS model agrees with experimental data in both solvents up to solute radius of ∼4.5 Å but fails beyond that. The GW model is seen to be adequate only for very small molecules. Thus, the ability of the quasihydrodynamic microfriction models to show a transition from microscopic to macroscopic hydrodynamic behavior with increase in solute size is lacking.