Theoretical study of the molecular motion of liquid water under high pressure

Abstract

The pressure effects on the molecular dynamics of liquid water are investigated using the site–site generalized Langevin modified mode-coupling theory. The calculations are performed for temperatures from 273 to 373 K and densities from 0.9 to 1.2 g/cm3. The static structure factor required as input is obtained from the reference interaction-site model hypernetted chain integral equation. The shear viscosity, the dielectric relaxation time, the translational diffusion coefficient, and the first-rank reorientational relaxation times are evaluated. All these quantities show unusual pressure dependence in the low-density, low-temperature region in that the molecular mobility is enhanced by applying the pressure. The magnitude of the enhancement is larger on the reorientational motions than on the translational ones. These tendencies are consistent with experimental observations, although the quantitative agreement is not so good. An analysis of the theory indicates that the decrease in the dielectric friction on the collective polarization at small wave numbers upon increasing pressure is the principal reason for the pressure-induced enhancement of the dielectric relaxation and the decrease in the dielectric relaxation time affects other motions. The decrease in the dielectric friction is caused by the decrease in the number-density fluctuation around the low-wave-number edge of the first peak of the structure factor by compression. The comparison between the results for water and acetonitrile extracts two characteristic features of water that are important for the anomalous pressure effect on its molecular motion. The first one is the small collisional friction on the reorientation due to the spherical repulsive core, and the second one is the strong short-range Coulombic interaction caused by the formation of the hydrogen bonding. A theoretical calculation on a model diatomic liquid consisting of oxygen and hydrogen atoms proposes that the above two characteristic properties of water are sufficient for the emergence of the anomalous pressure dependence. This conclusion is also supported by the molecular dynamics simulation performed on the same model diatomic liquid.