Structural changes in ethanol–water mixtures: Ultrasonics, Brillouin scattering and molecular dynamics studies

Mixing of two hydrogen-bonded liquids may lead to significant composition dependent changes of liquid microscopic structure, which in turn will be reflected by changes of the corresponding thermodynamic, thermophysical and spectroscopic properties. For the liquid system {xEtOHC2H5OH + (1 − xEtOH)H2O} at ambient temperatures and pressures, we show that major structural rearrangements occur in the composition range of 0.15 < xEtOH < 0.20, where xEtOH denotes the ethanol mole fraction. Most interestingly, in this region, the excess enthalpy shows a minimum, the excess heat capacity at constant pressure shows a maximum, and both the ultrasonic speed and the hypersonic speed exhibit a maximum. In fact, molecular dynamics (MD) simulation results indicate the saturation of ethanol clustering in this low ethanol mole fraction region. The corresponding change of structural/vibrational dynamics is detected by Brillouin scattering, i.e. it is reflected by the observed sound dispersion in the composition region 0.15 < xEtOH < 0.65, where simulations show micro-segregated distributions of water and ethanol domains that form a bi-continuous mixture. At larger ethanol mole fractions, we found chain-like hydrogen-bonded water clusters.