Molecular dynamics model of the vapour–liquid interface of molten potassium chloride

Abstract

A model of the liquid–vapour interface of potassium chloride at ≈ 1050 and 1294 K has been explored using the method of molecular dynamics. The sample box of edge ≈ 2 nm, containing either 288 or 504 ions, is replicated in two dimensions with two free surfaces in the third dimension. The vapour pressure is so low that it was unnecessary explicitly to include vapour in the simulation. Density profiles have been determined and are discussed. The profile of the interface shows a smooth transition over ≈ 0.7 nm from liquid to vapour. There is no tendency towards surface crystallinity and no detectable surface absorption of either ion. Thermodynamic properties of the laminae are discussed. The computed surface tension is within 30% of the experimental value just above the melting point. Pair distribution functions for the direction tangential to the surface reveal a tendency towards clustering of ions at the surface. Self diffusion at the interface is enhanced by 50% or more by comparison with the bulk and particularly for the direction perpendicular to the interface. Velocity autocorrelation functions for tangential and perpendicular motion at the interface are discussed.