Percolation transitions of physically and hydrogen bonded clusters in supercritical water

Molecular Dynamics simulations were employed to locate the densities and pressures at which infinitely spanning networks form for both physically and hydrogen bonded water clusters along the 650, 700, 750 and 800 K supercritical isotherms. The SPC/E water model was used and the percolation thresholds were determined by examining the cluster size distributions. Average hydrogen bond numbers for the entire system are reported and are near the critical number of 1.55 expected at the hydrogen bonded percolation threshold. Percolating physical clusters appeared at lower densities than infinite hydrogen bonded networks. Physical and hydrogen bonded percolation thresholds were plotted on the P-rho and P-T planes. Neither of the lines of percolation transitions match the extension of the vapor-liquid coexistence line into the supercritical region. Empirical relationships were obtained for each transition threshold as functions of temperature as well as density. Activation volumes of diffusion increase rapidly until they reach their maximum at the threshold densities where water molecules are connected via percolating network of hydrogen bonds. (C) 2019 Elsevier B.V. All rights reserved.