Some interesting new phase diagrams in hydrogen-bonded liquid mixtures

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A decorated lattice model of hydrogen-bonded mixtures that exhibits closed-loop coexistence curves and lower critical solution temperatures is shown to exhibit a low-temperature phase separation with an upper critical solution point lying below the lower critical solution point of the closed-loop curve. The model is exactly soluble in terms of the three-dimensional spin–1/2 Ising model and therefore exhibits nonclassical critical behavior. The model also exhibits critical double points at which two coexistence curves merge at a common critical point and the critical exponent β is renormalized to 2β. For a special choice of the parameters the model exhibits a new type of phase diagram in the T–X plane in which the shape of the coexistence curve near the critical solution point is described by the exponent 3β. As a result the phase boundaries appear to approach the critical point nearly as straight lines. Coexistence curves are presented exhibiting the variety of behavior possible in the model, including critical double points and the critical inflection point. We also discuss the possibility of observing the critical inflection point in real mixtures and the behavior to be expected in closed systems in which the overall mole fractions are fixed.