Intermolecular Parameters and Combining Rules for the Square Well Potential

Theories for predicting thermodynamic properties of mixtures require parameters describing the interaction between like and unlike molecules. Several theories have been proposed for relating the parameters describing interactions between unlike molecules to the parameters of the pure components of the mixture. In this work, a potential with 3 adjustable parameters is considered. Second virial coefficient data for many pure substances and mixtures having both spherical and nonspherical molecules are utilized to determine the 3 characteristic parameters of the square well intermolecular potential function. This permits consideration to be given to a large quantity of data for systems having nonspherical molecules. It is shown that the effect of the third parameter is important even for inert gases having spherical molecules. Comparisons of the potential parameters calculated from the new rules with those obtained from the data and those calculated from other combining rules now being used show that the new rules give much improved results for mixtures involving substances with nonspherical molecules. (87 refs.)