Perturbation theory and conformal solutions

A two-parameter family of perturbation theories is examined for conformal solutions whose molecules differ only in size; it resembles an earlier perturbation theory due to Henderson and Barker. It is shown that the optimum choice of parameters leads to expressions which are identical, in zeroth order of the theory, to those of the one-fluid van der Waals model of Leland, Rowlinson and co-workers. The theory is examined to the second-order terms in the case of binary hard-sphere mixtures and numerical comparisons are made with experimental results for such systems. The approach shows promise for treating more complex types of conformal mixtures.