Radial Distribution Functions and their Role in Modeling of Mixtures Behavior

Radial distribution (Pair correlation) functions are the primary linkage between macroscopic thermodynamics properties and intermolecular interactions of fluids and fluid mixtures. Numerous theories of mixtures exist based on the definition of the radial distribution functions. Utilization of such theories require the knowledge about mixture radial distribution functions and their relations. Generally the exact radial distribution functions of real fluid mixtures are not available. In this paper a number of approximations for the mixture radial distribution functions are presented. Application of these approximation can result in analytic mixture theories and mixing rules. A uniform statistical mechanical technique is presented through which one can derive equation of state mixing rules based on different RDF approximations. The relative merits of different sets of mixing rules derived by this technique and their applications for modeling of the behavior of mixtures are discussed. INTRODUCTION AND BACKGROUND Modern theories of fluids and fluid mixtures have benefited a great deal from the concept of radial distribution function (RDF). The RDF theories have been quite successful in describing the behavior of simple pure liquids [Hill, 1956; McDonald, 1973; McQuarrie, 1975; Haile and Mansoori, 19831. However, the RDF theories of mixtures have lacked sufficient progress due to complexity of such theories and the increasing number of radial distribution functions in a multicomponent mixture [Matteoli and Mansoori, 19901. Here we introduce a technique by which it is possible to use approximations for the mixture RDFs in order to develop analytic mixture theories. Recently, constraining relations between RDFs of multicomponent mixtures were derived using statistical mechanical theory of canonical ensembles [Hamad & Mansoori, 19891. According to these constraints mixture RDFs are not all independent from one another, contrary to the common belief. In here we first introduce the 0378-3812193606.00 81993 Blsevier Science Publishers B.V. All rights re.save,j