Diameters of the Liquid–Liquid Coexistence Curves of Aqueous Solutions of Tetrahydrofuran

The diameters of the coexistence curves of aqueous solutions of tetrahydrofuran and two quasibinary, isotopically related mixtures near their lower consolute points are analyzed in terms of different composition variables including mole, mass, and volume fractions, using various possible definitions of the reduced temperature. A (1−a) anomaly is observed for all choices of the temperature and concentration variables. In terms of mole fraction the diameters are free from a regular contribution as well as from a spurious 2b contribution arising from the inappropriate choice of the order parameter. The mass and volume fractions lead to an apparent symmetrization of the coexistence curve, but cause significant 2b contributions to the diameter that could mask the 1−a anomaly. A reduced temperature that accounts for the presence of both upper and lower critical consolute points is found to be preferable, although the second critical point is 80 K away.