Self-diffusion coefficients and rotational correlation times in polar liquids. VI. Water

Abstract

A new model for liquid water is presented. As a first approximation (A) the liquid is considered as a quasi-hard-sphere fluid and the scaled particle theory is employed to calculate the work (w) required to create a vacancy of molecular dimension using the observed density of water. In the second approximation (B) a quasilattice model is invoked which is based on the Ice VII structure and the density and isothermal compressibility of water are computed using the values of w calculated in approximation A. A final approximation (C) is based on the Ice Ic structure. In this approximation the temperature and pressure dependence of self-diffusion, viscosity, and nuclear relaxation times are evaluated. The agreement of the model with experiment is good and only a few parameters enter into the calculation, and these have magnitudes that are physically reasonable. Results of molecular dynamics calculations on water obtained by Rahman are presented which are interpreted to show that molecular rotation in water occurs in rather large excursions of relatively short time duration. For times between the large angular reorientations, librational motions occur which do not result in a large amount of net molecular reorientation. It is proposed that the large amplitude reorientations result from hard molecular collisions. These results confirm the conclusions of earlier work on other liquids. The anisotropy of the rotational correlation time of rank 2 (τ2) in an asymmetric top molecule is derived based on the finite step rotation hypothesis. Available experimental values of τ2 for liquid water are summarized and compared with the results of the model.