Structural features in ethanol-water mixtures revealed by picosecond fluorescence anisotropy

When an ensemble of molecules is excited with polarized light an anisotropic orientational distribution with respect to the transition dipole moment is produced. This anisotropy can decay in time due to the rotational motion of the molecules and consequently leads to depolarization of the fluorescence^1-6. The rate of this rotational motion has been successfully predicted from hydrodynamic theory. How much the rotational relaxation depends on molecular geometry and how much on specific solvent-solute interactions has been studied by picosecond spectroscopy^1-6 and other techniques^7-9. In all cases so far reported, the rotational behaviour seems to be accounted for by the Debye-Stokes-Einstein (DSE) equation τ_R = f/kT. This relates the rotational relaxation time τ_R (inversely related to the rotational diffusion coefficient) to the frictional coefficient, f, which is proportional to the product of the shear viscosity, the molecular volume and a constant dependent on the 'stick' or 'slip' boundary conditions^3,10,11. We report here, however, that large deviations from DSE behaviour have been observed in the rotational diffusion of the dye cresyl violet in ethanol-water mixtures. Different rotational relaxation times are observed in solutions of the same viscosity but differing composition. This behaviour can be rationalized using previously proposed models for water-ethanol mixtures.