Simulation of the Na⁺Cl^- ion pair in supercritical water

Potentials of mean force (pmf) for the Na⁺Cl^- ion pair in supercritical water were obtained from Monte Carlo simulations at 400°C and pressures of 350 and 1000 atm. The usual oscillatory behavior for ion pairs in ambient water is found to be much less structured in supercritical water, which is accompanied by disappearance of the minimum corresponding to the solvent-separated ion pair and a much deeper well for the contact ion pair. Solvent clustering is predicted to be significant for this attractive supercritical solution from analyses of the pmf and radial distribution functions. The results should be useful for developing models of ionic interactions in supercritical water.