Effect of surface free energy and surface stress on phase equilibria

Abstract

Due to a call for modification of the graphical method of describing phase equilibria for systems with a second-phase inclusion using a common tangent, as previously presented, the thermodynamic basis for such diagrams has now been examined in detail but only for isotropic surface properties. For an incompressible second phase with composition independent surface properties there seems to be no need of modification. For compressible liquids and solids under constant pressure the common tangent construction can still be used but the evaluation of how much the Gibbs energy curve for the second phase should be lifted is more complicated. For spherical inclusions of compressible solids there will be an additional factor containing the compressibility. The equilibrium condition is modified for all cases if the surface properties change with composition and that will cause severe complications for the common tangent construction. There may be a particular composition dependence of the specific surface free energy for an interstitial phase but it can be treated as any other phase.Although the partial volumes appear in the equilibrium conditions for each component, they play only a minor role for the two-phase equilibrium in binary systems. An exception is the equilibrium between a particle and its own vapor.