The radial distribution functions for pairs in a fluid mixture can be closely approximated from known data on pair distribution functions for pure components evaluated at a mean density determined by a properly defined average of molecular interaction parameters in the mixtures. This approximation has been used to replace the assumption about structure in mixtures inherent in the traditional regular solution theory. The result is a modified regular solution theory which gives an improvement in predicting excess thermodynamic functions and activity coefficients in mixtures involving appreciable differences in molecular size and shape. This study was conducted to investigate the usefulness of some new theoretical knowledge about the structure of fluid mixtures. The term structure in mixtures of simple fluids means a description of the probability of finding the center of one molecule at various distances from the center of another and the probability that the molecules found at a given separation belong to various species among those present in the mixture.This new theoretical development was used to improve the description of structure inherent in the Hildebrand regular solution theory using solubility parameters. The simplicity of this theory has made it attractive for engineering computations, but, in its original form, it was quite limited in its range of applicability. bne of the early improvements in the theory was the incorporation of the Flory-Huggins equation for the entropy of mixing to replace the ideal solution entropy of mixing asaumed by the original theory.?his work is an attempt to make another improvement in this theory by improving one of the basic assumptions while retaining as much as possible its simplicity. The objective is to remove the unrealistic assumption in the original theory that the probability of a particular pair of molecules having a specified center-to-center separation distance in the mixture is independent of composition and is exactly the same as that of the pair in a pure component at the temperature and pressure of the mixture.The correct pair distribution function in a mixture of dissimilar molecules, however, is very much composition dependent, and the improvement here accounts for this dependence by means of an assumption called the mean density approximation. The effectiveness of this approximation is demonstrated in this paper by comparison of pair distribution functions in an ideal mixture of Lennard-Jones fluids calculated by this procedure with those obtained from computer simulations of this mixture.The new approximation was incorporated into the regular solution theory and tested by making excess free energy calculations in real binary mixtures with varying degrees of dissimilarity of molecular size and character. CONCLUSIONS AND SIGNIFICANCEThe results for the new modified regular solution theory (MRST) show in every case an improvement in calculated values of the excess Gibbs free energy and excess enthalpy of mixtures when the molecules in the mixture differ appr...
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