MODEL OF ADSORPTION AT A FLUID-FLUID INTERFACE

The three-component penetrable-sphere model is used to study the adsor ption of a third component, b, at the surface of two fluid phases, alp ha which is rich in component a, and gamma which is rich in component c. The system has a tricritical point at a reduced pressure of pi(tc) = 15/4, in a mean-field approximation. At and below this pressure the adsorption of b, GAMMA(b), first rises as the activity of b, Zeta(b), increases and then falls rapidly as the tricritical or ordinary critic al point of the alpha-gamma system is reached. Above pi(tc) the adsorp tion diverges to an infinite value as Zeta(b) approaches Zeta(b)sat, t he value at which component b saturates both the alpha and gamma phase s, and so first forms a new bulk phase, beta, which wets the alpha-gam ma interface. It is shown that Gamma(b) diverges as - 1n(Zeta(b)sat -- Zeta(b)), a divergence that had been derived previously by Widom on t he basis of a phenomenologically constructed free energy.