Theories and facts about liquids

Abstract

A theory of liquids, to be valid, must not disagree with any pertinent experimental fact such as:

(1) The entropy of vaporization of all simple liquids to the same concentration of vapour is the same. It is larger for associated liquids, hence simple liquids are in a state of maximum disorder. They have no structure.

(2) Simple liquids flow freely when expanded by only a few percent over their intrinsic volume. There is no room for “holes”, or “lattices”, or jumps longer than a small fraction of a molecular diameter. Self-diffusion occurs because thermal motion prevents any molecule from remaining in the same spot.

(3) A parameter has been discovered and evaluated for molecules that are not hard spheres. It is the molal volume V_t at which soft molecules begin to be sufficiently separated between collisions to acquire fractions of their momentum in free space. At volumes smaller than V_t they are in fields of force not appropriately described by pair-potentials. The role of temperature is only to determine volume.

Values of V_t depend upon the capacity of a molecular species to absorb collision momentum by bending, vibrating or rotating. The kinetic theory equation for the viscosity of dilute gases becomes applicable to polyatomic vapours when σ² is replaced by V_t^2/3.