Solvation shell dynamics studied by molecular dynamics simulation in relation to the translational and rotational dynamics of supercritical water and benzene.

The solvation shell dynamics of supercritical water is analyzed by molecular dynamics simulation with emphasis on its relationship to the translational and rotational dynamics. The relaxation times of the solvation number (tau S), the velocity autocorrelation function (tau D), the angular momentum correlation function (tau J), and the second-order reorientational correlation function (tau 2R) are studied at a supercritical temperature of 400 degrees C over a wide density region of 0.01-1.5 g cm(-3). The relaxation times are decomposed into those conditioned by the solvation number n, and the effect of the short-ranged structure is examined in terms of its probability Pn of occurrence. In the low to medium-density range of 0.01-0.4 g cm(-3), the time scales of water dynamics are in the following sequence: tau D>tau S approximately or > tau J approximately or > tau 2R. This means that the rotation in supercritical water is of the "in-shell" type while the translational diffusion is not. The comparison to supercritical benzene is also performed and the effect of hydrogen bonding is examined. The water diffusion is not of the in-shell type up to the ambient density of 1.0 g cm(-3), which corresponds to the absence of the transition from the collision to the Brownian picture, whereas such transition is present in the case of benzene. The absence of the transition in water comes from the fast reorganization of the hydrogen bonds and the enhanced mobility of the solvation shell in supercritical conditions.