Calculated electrocapillary curve for a molten salt

If bulk properties of simple molten salts may be reasonably well understood in terms of the primitive model, the situation with respect to surface properties is less satisfactory. It has been shown that a simple model for the distributions at the free surface of a molten salt can give surface tension and surface energy in reasonable accord with experiment, provided that a factor guaranteeing local electroneutrality is introduced. In this model, properties are given in terms of bulk-salt distribution functions, for which the primitive model is used. The present work extends this model to the electrocapillary curve, i.e., variation of surface tension with surface charge density. The calculations are like those for the free surface, corresponding to the point of zero charge. The local electroneutrality correction, while extremely important for the magnitude of the surface tension, is much less important for its variation with surface charge, and hence the electrical capacitance. Our capacitances, derived from surface charges and potential drops derived from our model, are much too small, whereas the Gouy-Chapman model gives values which are much too large. The calculated variations of surface tension and potential drop with surface charge do not satisfy the thermodynamically derived Lippmann equations; neither does one obtain the same surface tension from different thermodynamically equivalent formulas. In order to understand the reasons and to improve the situation, we show how thermodynamic consistency may be restored to our model. Capacitances are still numerically much smaller than those reported experimentally