Brillouin light scattering in low temperature water–ethanol solutions

Brillouin light scattering spectra of water–ethanol mixtures in a wide temperature and concentration range are reported and analyzed on the basis of a generalized hydrodynamic theory. The obtained sound velocities and longitudinal kinematic viscosity coefficients are compared with the analogous results obtained from ultrasonic measurements. In particular, light scattering spectra of 23% molar water–ethanol solution show direct evidence of a relaxation process in the GHz region which appears to be well separated from the lower frequency relaxation process determining the anomalous ultrasonic properties of these solutions. This high frequency relaxation involves both shear and volume viscosities and closely resembles the viscoelastic relaxation found in many associated liquids. The role played by the microscopic variables driving the relaxation is briefly discussed.