Density Correlations, Critical Opalescence, and the Free Energy of Nonuniform Fluids

Abstract

We show the equivalence of three prior approaches to the theory of critical opalescence: (a) the direct study of the long range behavior of the radial distribution function, due to Ornstein and Zernike, (b), the indirect approach by Rocard, who obtained the local pressure in a system of nonuniform density, and (c), the indirect approach by Debye, who obtained the free‐energy density in a system of nonuniform density. The molecular aspects of the theory are relieved to some extent from the confines of regular solution theory. The consequences of the Kirkwood superposition approximation, and its necessary revision, are examined.