IMPLICATIONS OF THE PHASE RULE FOR CAPILLARY SYSTEMS CONTAINING SURFACES AND 3-PHASE CONTACT LINES WITH SURFACE AND LINEAR CONSTRAINT RELATIONS

The phase rule is generalized to heterogeneous systems with moderately curved surfaces and linear or line-phase boundaries. It will be shown that the number of degrees of freedom or variance f of a capillary sysem is, in general, larger than that predicted by the classical Gibbs' phase rule. Restrictions on the value of f for capillary systems is intimately connected with the imposition of mechanical constraints like the Laplace and Young equations of capillarity. As a direct consequence, it may be shown that the variance for an r-component, three-phase solid-liquid-vapour system (i.e., a liquid drop on a solid surface) is different from that of an r-component, liquid-liquid-vapour system. This conclusion has important implications for the existence or possible existence of equation-of-state type relations.