The Term ‘Regular Solution’

Abstract

IN 19201 I directed attention to the fact that the solubility of solids, plotted as a logarithm of mole fraction against reciprocal temperature, gives a family of curves in the case of solutions “where no molecular changes are involved”. In 19272 I designated such solutions as ‘regular’ and in 19293 I discussed the thermodynamic significance of this regularity, stating, in part: “From our picture of a regular solution as one in which orienting and chemical effects are absent, and in which the distribution and orientations are random, just as in an ideal solution, we may conclude that the probability of X2 [species 2] is the same in the two solutions [ideal and actual] and, therefore, that the difference in entropy is zero. We cannot expect this conclusion to hold unless the random distribution of molecules persists. We may expect further that a small correction should be applied to take care of the change in entropy accompanying changes in volume, given by (∂S/∂V)T = (∂P/∂T)V; or we may state our principle in the following form. A regular solution is one involving no entropy change when a small amount of one of its components is transferred to it from an ideal solution of the same composition, the total volume remaining unchanged.”