The Theory of Critical Opalescence in Binary Mixtures

The present theory of X-ray scattering by simple fluids and binary mixtures is applied to the phenomenon of critical opalescence in binary mixtures. Firstly, the condition for phase separation is expressed in terms of the three radial distribution functions for a binary mixture, and then it is shown that, at phase separation, these functions are all of the same form, although they differ in scale. The form of these functions can then, in principle, be determined from experimental observations of visible opalescence, although the normalization requires further discussion. The theory is then applied to the work of Furth and Williams, and of Quantie, who suggested analytic representations of two types of radial distribution function. Only one of these is found to be acceptable.