Densities of binary liquid mixtures near their consolute points

Abstract

Starting from a formula of Gaw and Scott, we show by a scaling argument that the volume–composition isotherm of a binary liquid mixture at its consolute temperature should be of the same algebraic degree at the consolute point as the volume–composition coexistence curve and, by continuity, tangent to it there. Our data on 2,4-lutidine + water, and our data together with those of Woermann and Sarholz on isobutyric acid + water, show conspicuously the non-classical vanishing of the curvature of the volume–composition critical isotherm at the consolute point. The predicted tangency of the critical isotherm and coexistence curve is confirmed for isobutyric acid + water within the uncertainties in the estimates.