Molecular theory of phase equilibria in model associated mixtures. I. Binary mixtures of water and a simple fluid

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Abstract
The phase equilibrium behavior of a basic statistical mechanical model of a binary mixture of water and a simple fluid are investigated. For water we use a primitive model which incorporates no attractive forces other than hydrogen bonds within a hard‐sphere core, and the simple fluid is modeled by a fluid of hard spheres with an attractive mean‐field term. The qualitative behavior of the phase equilibria and P−T critical lines of this model are calculated using an accurate theoreticalequation of state for primitive water obtained in analytical form using the Wertheim thermodynamic perturbation theory, for a range of values of the mixture’s two underlying parameters, the hard‐sphere size ratio and the mean field strength. The model exhibits type IIId and type IV* phase behavior of three qualitatively different types, characterized by the existence of either one or two three‐phase lines. The model exhibits a range of phenomena, including tricritical points and double critical end points. The implications of the model results for corresponding real fluid mixtures are also discussed.

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