Ethanol-water mixtures: ultrasonics, Brillouin scattering and molecular dynamics

New ultrasonic and hypersonic measurements of the speed of sound in aqueous ethanol mixtures are presented, particularly in the temperature and ethanol-concentration domains of the maximum of sound. Molecular Dynamics studies indicate that these mixtures show aggregation of ethanol molecules in the low ethanol mole fraction x<0.2, bicontinuous-like phase around x = 0.5, and weak water clustering above x = 0.8. The resulting micro-heterogeneity is seen to affect the long range part of all the correlation functions of the aggregating species. Nevertheless, simulation results help understanding the general trends seen in the experimental results: it is the structural change between ethanol's clustering and water-ethanol micro-segregation that corresponds to the maximum observed in the speed of sound around x = 0.15. (C) 2011 Elsevier B.V. All rights reserved.