Molecular dynamics study of screening in ionic fluids

Molecular-dynamics simulations of molten and gaseous NaCl are used to probe the entire range of screening behaviors including weakly and strongly-coupled regimes as well as the transition between them. Our simulations confirm that, in accordance with the Debye–Hückel theory, in the weakly-coupled regime (corresponding to the ionic gas) the spatial correlations between the ions decay monotonically, with a screening length, ${\lambda }_Q,$ given by the Debye length, ${\lambda }_D,$ which decreases with increasing ionic coupling strength. By contrast, in the strongly-coupled regime (corresponding to the melt) the spatial correlations between the ions decay in an oscillatory manner, ${\lambda }_Q$ then being given by the envelope of the damped oscillations. In accordance with theoretical predictions, in the strongly-coupled ${\lambda }_Q$ increases with increasing coupling strength. Also in agreement with existing theories our simulations show that the transition from monotonic to oscillatory screening occurs at the shortest possible screening length, which is on the order of the average ion diameter, σ; also, the simulated values of ${\lambda }_Q$ across the entire range can be represented by a universal function of the ionic-strength parameter, ${\mathrm{\sigma /\lambda }}_D.$