Association and dissociation of nonpolar solutes in super- and subcriticalwater

The solvent-mediated part of the pair potential of mean force of methane in water was investigated by computer simulations over a wide range of thermo dynamic conditions. It is shown that the solvent effect on the association and dissociation of a pair of methane molecules is weakly dependent on the temperature when the density of solvent water is fixed. The solvent-mediate d attraction between the nonpolar solutes is strengthened when the solvent density increases. This density dependence is in contrast to that of the so lvent-mediated interaction of oppositely charged ions, for which the associ ation is normally strengthened by the density reduction at constant tempera ture. The effect of water on the formation of a chemical bond in a reaction of nonpolar nature is also examined schematically. It is shown that while supercritical water is a worse environment than ambient water with respect to the solvent effect on the reaction rate constant, it isa better medium f or the reaction due to the high-temperature involved.