ADSORPTION OF IONIC SURFACTANTS ON CONSTANT CHARGE SURFACES - ANALYSIS BASED ON A SELF-CONSISTENT FIELD LATTICE MODEL

Adsorption isotherms of ionic surfactants onto surfaces with a constant charge density have been calculated using the self-consistent field lattice theory for adsorption and/or association, initiated by Scheutjens, Fleer, and Leermakers. At low salt concentrations the adsorption isotherms show a two-step behavior; after the first step the surface charge is compensated and a surfactant monolayer is formed on the surface. At about 0.1 of the critical micelle concentration a second step occurs and bilayer formation takes place. At higher salt concentrations the initial adsorption is reduced, the distinction between the two steps tends to disappear, and the final plateau values are slightly higher than those at low salt concentrations. The isotherms at different salt concentrations have a common intersection point at the isoelectric point, which is in agreement with thermodynamic analysis and experiments. The predicted effects of salt concentration, surface charge, and surfactant chain length are also in agreement with experimental findings.