A Molecular Dynamics Investigation of Hydrogen Bonding in Supercritical Water

Abstract

The extent, distribution, and temporal and structural aspects of hydrogen bonding in supercritical water were investigated through molecular dynamics simulations at 773 K and densities between 115 and 659 kg/m3 using a flexible water potential. Pair energy distributions in supercritical water do not have the bimodal nature of liquid water distributions. Supercritical water molecular pairs are energetically disposed towards parallel alignment of dipoles in the first solvation shell whereas liquid water pairs favor a near-orthogonal orientation. An energetic criterion was used to identify hydrogen bonded molecular pairs. The number of hydrogen bonds per water molecule in supercritical water is one-sixth to one-half of that present in ambient water. Unlike in ambient water in which almost all the molecules belong to a hydrogen bonded gel, hydrogen bonded clusters in supercritical water typically consist of fewer than five members. The persistence time constant for hydrogen bonds is 0.1 ps in supercritical water in contrast to 0.6 ps in ambient water.