Boundary tension between amphiphilic layers

Abstract

The structure and tension of the region of contact where an oil phase, a water phase, and a microemulsion phase meet, are theoretically investigated. The analysis has as a starting point a Landau theory-like expression for the free energy in which, besides the usual gradient of the density squared term, a term proportional to the second derivative of the density squared is present, while the coefficient of the squared gradient tern is taken to be negative in the microemulsion phase. It is shown that a first-order wetting transition exists at which point infinitely many (n=0,1,2,...) surface phases can coexist, each described by the presence of a diferent thickness of the microemulsion phase between the oil and the water phase. This situation physically describes the coexistence of layers of amphiphilic molecules with different thicknesses between a water and an oil bulk phase, the thickness of one amphiphilic layer being equal tol0. We present the calculation of the density profile and the boundary tension of the contact region of a coexistingn = 0 andn = 1 surface phase, e.g., between the oil-water interface with a single amphiphilic layer present and the oil-water interface where three amphiphilic layers are present.