Rotational motion of charged molecules in aqueous solutions. A change in the diffusion mechanism

The rotational motion of the anions polyphenyl 2 (PP2) and pyrene tetrasulfonate (PTS) was studied as a function of the solvent viscosity by time-resolved fluorescence depolarization spectroscopy using the single photon counting technique. The viscosity was varied by changing the temperature and the composition of the water–glycerol mixtures, respectively. At low viscosity the reorientational behavior of PP2 and PTS is well described by the Stokes–Einstein–Debye (SED) equation under stick boundary conditions. With increasing viscosity (η≥25 cP) however the rotational motion did no longer follow the SED predictions. Instead, we observed a much faster relaxation time, approaching an asymptotic value at very high viscosities. It seems that in the high viscosity regime microscopic collisional effects rather than the macroscopic hydrodynamic drag of the solvent determines the reorientational dynamics of the solute molecules.