MOLECULAR-REORIENTATION DYNAMICS AND MICROSCOPIC FRICTION IN LIQUIDS

Molecular rotation reorientation times are investigated using time resolved fluorescence depolarization studies of three solutes of similar size and shape (nile red, neutral nile blue and cationic nile blue) dissolved in alcohol and alkane solvents, as well as an extensive compilation of previous results for neutral and charged solutes dissolved in non-polar, polar and associated solvents. A universal correlation is found between reorientation time, solvent viscosity, and solute volume for solutes dissolved in alkanes, while strongly interacting solutes experience relatively enhanced friction, and non-polar solutes dissolved in alcohols experience reduced friction. The results are compared and contrasted with slip and stick hydrodynamic predictions, and used to develop empirical correlations, which can be used to predict molecular reorientation times with an uncertainty on the order of a factor of two in virtually any solute-solvent system.