Coexistence lines phase coupling

A free energy analysis of the region where three two-phase couples (e.g., solid-liquid, solid-vapor, and liquid-vapor) coexist leads, under the above model, to the consideration of equilibrium of the contact line formed by the intersection of three surfaces associated with two-phase couples. The free energy analysis of the contact line region most frequently invoked in wetting discussions is that for an inert system consisting of a liquid resting on a locally flat, solid surface (Fig. 3). 

Figure 5.4. The concentration distribution in a diffusion couple of a fictitious system A/B that has two compounds, a line compound ABj and one with considerable nonstoichiometry AjBj. The reactants have some solubility in each other. If both compounds form during annealing at (dotted line in the phase diagram) a line profile over the diffusion couple shows a distribution of [B] as indicated. In the vertical parts of this graph (in the coexistence regions) the concentration signal oscillates between two values, depending on which of the two crystallites happens to be measured (by SEM).

We consider phase coexistence in a one-component system—for instance between gas (g) and liquid (1). We envision two coupled subsystems—one on either side of the saturation line. Each of the subsystems is represented by a simulation box containing Ng and Ni particles, respectively. By exchanging particles between the boxes and by varying their volumes, Vg and V), we attempt to generate the proper thermodynamic states for both gas and liquid at coexistence. Our result wiU be two densities, Pg T) and pi T), at coexistence as functions of temperature. 

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