Kirkwood-Buff Solution Theory

Local Composition Concept (LCC) is combined with Kirkwood and Buff theory to propose a new model in terms of Radial Distribution Function
(RDF) to evaluate the activity and osmotic coefficients of 1-1 and 1-2 strong aqueous electrolyte solutions. The model contains no adjustable parameter and can facilitate the calculations to a great extent. The RDF in the proposed model is obtained by solving integral equations of Percus-Y evick (PY), Hypernetted chain (HNC) and Mean Spherical Approximation (MSA). The effectiveness of these equations in accurately calculating the activity and osmotic coefficients is considered. The activity and osmotic coefficients evaluated by this model show better consistency with experimental data for HNC integral equation. It is concluded that in spite of the simplicity of the proposed model, comparing with similar models in its application to 1-1 and 1-2 strong electrolyte solutions, better agreement with experimental data is observed.

A new analytic statistical mechanical fluctuation solution theory for activity coefficients in multicomponent mixtures is developed. This theory is based on the newly formulated exact relations among the mixture direct correlation function integrals and the closures for cross direct correlation function integrals. One major advantage of this theory is its independence from the nature of intermolecular interaction potentials in solutions which are generally unknown for complex molecules. The theory is successfully used for vapor-liquid equilibria, liquid-liquid equilibria, and phase splitting prediction and correlation of fluid mixtures consisting of polar and associating molecules.

One major obstacle in the development of statistical mechanical models for associating fluids and fluid mixtures has been the lack of accurate intermolecular potential models for such systems. One theory which has shown to be promising in solving this problem without the need for intermolecular potential function is the fluctuation theory of mixtures. This paper consists of recent work of the authors on the development of a new fluctuation theory approaches for calculation of total, partial molar properties, and phase equilibria of associating fluid mixtures. This technique is based upon the newly developed exact relations among the mixture direct correlation function integrals and the closures for unlike-interaction direct correlation function integrals. A new closure for unlike-interaction direct correlation function integrals is suggested and the effect of pressure on the coefficients of two closures is studied.