Solvation in supercritical water

Abstract

Cochran, H.D., Cummings, P.T. and Karaborni, S., 1992. Solvation in supercritical water. The aim of this work is to determine the solvation structure in supercritical water compared with that in ambient water and in simple supercritical solvents. Molecular dynamics studies have been undertaken of systems that model ionic sodium and chloride, atomic argon, and molecular methanol in supercritical aqueous solutions using the simple point charge model of Berendsen for water. Because of the strong interactions between water and ions, ionic solutes are strongly attractive in supercritical water, forming large regions of increased local water density around each ion comparable to the solvent structures surrounding attractive solutes in simple supercritical fluids. Likewise, the deficit of water molecules surrounding a dissolved argon atom in supercritical aqueous solutions is comparable to that surrounding repulsive solutes in simple supercritical fluids. Methanol appears to be a weakly attractive or even repulsive solute in supercritical water. Only a small number of excess water molecules (if any) surround a methanol molecule in supercritical water, and this becomes a deficit at higher density. The number of hydrogen bonds per water molecule in supercritical water was found to be about one-third the number in ambient water. The number of hydrogen bonds per water molecule surrounding a central particle in supercritical water was only weakly affected by the identity of the central particle -atom, molecule or ion. These results should be helpful in developing a qualitative understanding of important processes which occur in supercritical water.

Molecular modelling of rare earth element complexation in subduction zone fluids

“…5), i.e. a decrease in dielectric constant from 85 to 20, caused by a decrease in the number of hydrogen bonds due to temperature and density effects (Cochran et al, 1992). The hydration free energy becomes less negative with decreasing dielectric constant.…”

Reactions at supercritical conditions: Applications and fundamentals

“…The early mechanistic modeling papers previously cited suggested that the inability of accepted combustion models to describe SCWO implied that there must be some other, poorly understood effects, perhaps attributable to the presence of an SCF, responsible for the discrepancy between their models and experiments. These papers were cited by others (e.g., Cochran et al, 1992) as indicative of the need to understand the effects of local composition and solution microstructure, in this case by performing molecular dynamics calculations. The failure of well-established combustion mechanisms and rate constants is somewhat surprising, however, when one considers that many of the experiments performed were at sufficiently high temperatures that the fluid phase could be approximated as being an ideal gas.…”

Solubility of Cellulose in Supercritical Water Studied by Molecular Dynamics Simulations

“…The box expanded and although the contribution to enthalpy was only +1.6 kJ/mol AGU , it showed that the density augmentation was diminished, similar to what has been reported for methanol in high density SCW. 47 Water-water Coulomb potential energy increased like in ambient water (Figure 3) suggesting a reduced water-water bonding around solutes compared with the bulk solvent phase.…”

“…When the cellulose-water bonds were excluded, the number of hydrogen bonds was lower in the vicinity of the chains than in bulk water, indicating a reduced density augmentation around a solute. 47 The shape of the hydrogen bonding distribution in ambient water resembled that in high density SCW but the number of lost hydrogen bonds was larger. This was in a good agreement with Figure 3 regarding the number of lost hydrogen bonds upon cellulose dissolution.…”