TEMPERATURE-DEPENDENT ROTATIONAL RELAXATION OF DIPHENYLBUTADIENE IN N-ALCOHOLS - A TEST OF THE QUASI-HYDRODYNAMIC FREE-SPACE MODEL

The rotational correlation times of trans,trans-1,4-diphenylbutadiene in n-alcohol solvents over a wide temperature range are reported. They are well represented as linear functions of eta/T within a single alcohol, but comparison across a series of alcohols indicates that the boundary condition factor in the modified Stokes-Einstein-Debye equation decreases as the solvent size increases. The experimental results are compared with the Dote-Kivelson-Schwartz (DKS) quasihydrodynamic free space model, which is based on regular solution theory. The model in its original form is able to qualitatively predict the trend toward smaller boundary condition factor with increasing solvent size but is unable to quantitatively predict the observed dependence of the rotational correlation time either on eta at fixed temperature across the alcohol series or as a function of temperature within a single alcohol. Substitution of an empirical measure of the solute free space into the DKS theory results in improved agreement between the theoretical and experimental results. This suggests that the general framework of the DKS is appropriate for both regular and associating liquids but that different measures of free volume are required for each solvent type.