Molecular dynamics simulation study on sound dispersion and acoustic relaxation of water in wide temperature and density ranges.

Frequency-dependent complex shear and bulk viscosities of water from supercooled to supercritical conditions were evaluated by means of molecular dynamics simulations in order to understand the behavior of sound dispersion in the nm-1 wavenumber region. In the liquid-like density region, the degree of positive sound dispersion decreased with increasing temperature, as was already reported experimentally, which was explained in terms of the viscoelastic relaxation in the THz region. At supercritical temperatures, the degree of positive sound dispersion increased with decreasing density, as was also reported experimentally, and the sound dispersion near the critical density was ascribed to the dispersion of the isothermal compressibility rather than viscoelasticity. The larger degree of positive sound dispersion of water than that of methanol at ambient conditions was related to the larger longitudinal modulus in the THz region, and two mechanisms were found for the larger longitudinal modulus. The first one is the higher density of strong hydrogen bonds, and the second is the stronger coupling of the hydrogen bond to isotropic compression.