THERMODYNAMIC ANALYSIS OF PHASE COEXISTENCE AND STABILITY IN TEXTUREDAMPHIPHILIC MONOLAYERS AT THE AIR WATER INTERFACE/

Starting from an expression for the Gibbs energy derived previously fo r a monolayer consisting of a single amphiphilic component present in the (LE/LC) coexistence region, the general thermodynamic criteria for internal equilibrium are derived. It is assumed that the solid phase is present as a dispersion of equally sized circular islands. The assu mption of isodispersity is justified by the results of an earlier stud y. The necessary and sufficient conditions for stability of the equili bria are formulated within the constraints imposed by the model. All o f these are stated in a general form, independent of the specific anal ytical expressions for the various thermodynamic quantities. It is poi nted out that a more or less complete analysis will only be possible o nce an explicit expression for the line tension and an equation of sta te for a 2D fluid of rigid disks with smeared-out dipole moment become available. The latter would allow the calculation of the free energy of mixing, G(mix),, as well as the formulation of a 2D equation of sta te for the film within the coexistence region of the phase diagram. A generic functional form of these functions is given that is consistent with the requirements set by dimensional analysis. Finally, generaliz ations of the thermodynamic formalism to mixed monolayers and noncircu lar domain shapes are suggested.