Statistical thermodynamics of the liquid surface

The current theoretical situation regarding the structure of the liquid-vapour transition zone and the associated thermodynamic surface properties of simple liquids is reviewed. The thermodynamic and statistical mechanical treatments of the surface structure are assessed, and it is concluded that the former approach has limiting validity only as T→T _crit. Liquid metal systems are discussed, together with the recent experimental evidence supporting the existence of stable density oscillations at the surfaces of certain liquid metals. The liquid inert gas systems, argon in particular, are considered in some detail, and a discussion of the statistical thermodynamics of the liquid ⁴He surface is given.