Diffusion of ions in supercritical water: Dependence on ion size and solvent density and roles of voids and necks

We have performed molecular dynamics simulations of alkali metal (Li+, Na+, K+, Rb+, Cs+) and halide (F−, Cl−, Br−, I−) ions in supercritical water at 673 K. The calculations were done for water at three different densities of 1.0, 0.7 and 0.35 g cm−3 to investigate the effects of solute size on the diffusion of ions in supercritical water. On increase of ion size, we observe a maximum for diffusion of ions in supercritical water of higher densities (1.0 and 0.7 g cm−3). However, no such maximum is found for ion diffusion in the supercritical water of low density (0.35 g cm−3) or for diffusion of neutral solutes at all densities. These results are analyzed in terms of passage through voids and necks present in supercritical water. Correlations of the observed diffusion behavior with the sizes of ions and voids present in the systems are discussed.

Research highlights► A simulation study of the diffusion of solutes of varying charge and size in supercritical water is presented. ► Possible reasons for the presence and absence of diffusion maximum observed in varying situations are investigated. ► The diffusion maximum is found to be mainly related to the solvent density although its strength varies with temperature. ► The correlations between the diffusion of the solutes and the void network present in supercritical water are discussed. ► The behavior of solute intermediate scattering functions is also correlated with its diffusion and the void network.