Self-consistent field analysis of ionic surfactant adsorption regulation in the aqueous film between two neutral solids

Self-consistent field theory for adsorption and association is utilized to calculate the adsorption of the ionic surfactant (C16P3+) on two neutral, slightly hydrophobic solids separated by a thin aqueous film and immersed in an infinite reservoir of aqueous 1:1 electrolyte. The change in surfactant adsorption as a function of the separation between the solids is discussed in relation to the forces between the solids as well as the shape of the adsorption vs concentration curve for an isolated surface. Surfactants desorb from the surface as the electrostatic potential in the gap between the surfaces is increased due to the overlap of electric potential profiles. Changes in adsorption are most sensitive to solid-solid separation when the surfactant concentration is near the step in the adsorption isotherm. Adsorbed surfactant layers also become thinner as the surfaces approach, and the adsorbed amount can change in more complex ways when the adsorbed layers come into contact