STRUCTURE OF WATER UNDER SUBCRITICAL AND SUPERCRITICAL CONDITIONS STUDIED BY SOLUTION X-RAY-DIFFRACTION

Structures of water and aqueous electrolyte solutions under sub- and s uper-critical conditions studied mainly by X-ray diffraction and also by neutron diffraction are reviewed and the experimental results are c ompared with those reported by using computer simulations, Some Raman spectroscopic data are included for discussing the existence of hydrog en bonds in water at high temperature and high pressures (HTHPs). The authors propose a classification of supercritical water into three cat egories: (a) low density water, (b) medium density water, and (c) high density water, because density is a very important thermodynamic quan tity to describe properties of sub- and super-critical water. From cha nges in the water-water intermolecular distance and the coordination n umber of water with temperature and pressure, and especially with dens ity at HTHP, the authors conclude that the compact tetrahedral-like wa ter structure is decomposed and-long-distance water-water interactions increase with temperature and pressure, and they propose a model for water: under supercritical conditions water consists of small clusters , much smaller aggregates such as oligomers, and even monomeric gas-li ke water molecules.