Water at supercritical conditions: A first principles study

We analyze, via first principles molecular dynamics, the structural and ele ctronic properties of water close to and above the critical point. Contrary to the ordinary liquid state, at supercritical conditions the hydrogen bon d network is destabilized to various extents and the continuous breaking an d reformation of hydrogen bonded structures allow large density and dipole fluctuations that, in turn, can significantly affect the dielectric propert ies of the solvent. Close to the critical point, where the density is very low, small clusters, mainly dimers and trimers, are the dominant features, but many molecules exhibit no H-bond. On the other hand, at higher densitie s, more extended structures appear, but still a continuous network cannot f orm. In both cases, H-bond configurations that are anomalous with respect t o the normal liquid phase appear. These features strongly affect the solven t properties of supercritical water with respect to those of ambient water. They most likely vary continuously as a function of temperature, pressure and density and, hence, can be tuned to optimize the desired chemical proce ss. (C) 2001 American Institute of Physics.