MOLECULAR-REORIENTATION DYNAMICS AND MICROSCOPIC FRICTION IN LIQUIDS

Molecular rotation reorientation times are investigated using time res olved fluorescence depolarization studies of three solutes of similar size and shape (nile red, neutral nile blue and cationic nile blue) di ssolved in alcohol and alkane solvents, as well as an extensive compil ation 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 dissolv ed in alcohols experience reduced friction. The results are compared a nd contrasted with slip and stick hydrodynamic predictions, and used t o develop empirical correlations, which can be used to predict molecul ar reorientation times with an uncertainty on the order of a factor of two in virtually any solute-solvent system.