Thermodynamics and structure of molecular clusters in supercritical water

The extent of hydrogen-bonded cluster formation in near- and supercritical water has been studied by application of a hybrid hydrogen-bonding criterion to the analysis of Monte Carlo computer simulation results. Up to 10% of water molecules were found to constitute H-bonded clusters even in dilute supercritical water vapor (T∗=1.04, ρ∗=0.06), and the maximum size of such molecular complexes formed may be as large as seven molecules per cluster under these conditions. Relative abundance and geometric and energetic characteristics of topologically different trimers, tetramers, and pentamers were also examined. Open chain- and tree-like clusters are preferentially formed in supercritical water, while cyclic ring-like structures occur only rarely. Partitioning of molecules between clusters of the same size, but topologically different structure is found to be virtually independent of temperature and density.