Density Fluctuation and Hydrogen-Bonded Clusters in Supercritical Water. A Molecular Dynamics Analysis Using a Polarizable Potential Model

Abstract

Abstract A series of molecular dynamics calculations of supercritical (SC) water along an isotherm at 600 K have been performed using a polarizable potential model. A relatively large system composed of 1000 molecules was adopted to reproduce long-ranged density fluctuations in SC water. The oxygen–oxygen partial static structure factor has been calculated for various densities. The strong intensity of the static structure factor in the small wave number region has successfully been reproduced for the fluid near the critical point. The density fluctuation was investigated using a concept of “cluster,” which was defined by a set of molecules connected by “bonds.” In this paper, two kinds of bonds were adopted. One is a bond defined simply using the oxygen–oxygen distance (D-bond) and the other is a hydrogen bond (H-bond). Size distribution of both the D- and H-bond clusters was described well by Fisher’s droplet model. These clusters have bulky structures compared with closely packed low temperature clusters, and can be regarded as “fractal." The larger the D- and H-bond clusters, the longer the relaxation times. Together with the size distribution of the clusters, this may give a molecular picture of critical slowing down.