Self-consistent field model of inhomogeneous adsorption of nonionic surfactants onto polystyrene latex

The structure of a nonionic surfactant layer adsorbed on a hydrophobic surface is studied by means of a molecular realistic self-consistent field theory. The formation of discrete surface aggregates is considered. Combining the one-gradient self-consistent field theory with the two-gradient one, which allows for lateral inhomogeneities in the adsorbed layer, we predict the adsorption isotherm and the surface structure for surfactants with a branched hydrocarbon tail and several lengths of the PEO head. Analyzing the surface free energy of the system, it is found that the structure of the adsorbed layer changes along the isotherm. At very low surfactant concentrations a homogeneous dilute "gas"-like layer of individual surfactants is formed. Above a certain value (still below the cmc) hemispherical aggregates develop on the surface, coexisting with the "gas"-like homogeneous layer. A method is suggested to take interaggregate interactions into account. For the short-headed surfactant, hemicylindrical aggregates appear at higher concentrations. Finally, near the cmc the homogeneous adsorbed layer is recovered. Theoretical adsorption isotherms are compared to the experimental ones for Triton X-100 and Triton X-405 onto a polystyrene Latex dispersion. The model that includes the inhomogeneous adsorption (two-gradient SCF) is significantly closer to the experimental results than the one-gradient SCF theory that has the lateral homogeneous layer as a constraint.