Microscopic frictional forces on molecular motion in liquids. Picosecond rotational diffusion in alkanes and alcohols

The rotational diffusion times of the nonpolar solutes p-terphenyl, p-quaterphenyl, and 9,10-diphenylanthracene dissolved in n-alkane and n-alcohol solvents are reported. The observed microscopic frictional coefficients approximate the predictions of ‘‘slip’’ hydrodynamics in n-alkanes. Substantially lower friction is found in n-alcohols of comparable viscosity. Large deviations are also observed in branched alkane and dialcohol solvents. These differences are interpreted as an indication of nonhydrodynamic behavior in the microscopic rotational friction. Comparison with the results of other recent studies supports this interpretation and also indicate that charged solutes experience higher friction than the neutral solutes used in this study. The difference between charged and uncharged solutes is attributed to the larger effective volume of charged species, as well as to dielectric friction and solvation environment effects. A new picosecond spectroscopic technique based on stimulated emission gain anisotropy relaxation is used to perform the experiments.