On the Statistical Mechanical Theory of Solutions

A perturbation expansion of the excess free energy of a binary mixture based on the theory of composition fluctuations in an open system is presented. The theory is applied to classical mixtures and quantum mechanical isotope mixtures, and comparisons with conformal solution theory and Chester's theory of isotope mixtures are made. The advantage of the present theory is that it contains in summed form, in each order of perturbation, terms which in the direct expansion of the partition function would be considered to be higher order.