In Memoriam: George Amer (1891–1985)
The thermodynamics, dynamics and structure of any condensed phase depends ultimately on the interatomic or intermolecular interactions; an adequate theory of the liquid state would be able to calculate structure and properties at a given thermodynamic state point using only a knowledge of the elementary potential function. The structure of ‘simple’ liquids such as inert gases is determined essentially by the repulsive core of the potential function; the details of the structure can be considered as perturbations from an ‘ideal’ structure, which to zeroth order approximation relates to Bernal's random packing of hard spheres.
The (relative) simplicity of ‘simple’ liquids is due mainly to the isotropic nature of the potential function Φ. Neglecting three-body effects – which for these systems can be considered as a perturbation – the potential energy of a pair of particles is determined solely by their separation r, so that
Φ = Φ (r only). (1)
This spherical symmetry, together with the sufficient hardness of its repulsive core, means that the structures of the condensed phases (solid and liquid) are determined largely by packing considerations. Thus, we can immediately explain qualitatively such ‘normal’ liquid behaviour as contraction on freezing, expansion on heating, and the increase in viscosity with pressure.
For molecular liquids, this simplicity is lost, as the potential function becomes a more complex function of relative position and orientation of a pair of molecules. The departure of the behaviour of molecular liquids from ‘normality’ will depend upon the nature of the intermolecular interactions. Most still behave ‘normally’ in terms of volume changes on freezing and expansion on heating.