Confinement-induced phase behavior and adsorption regulation of ionic surfactants in the aqueous film between charged solids

Self-consistent field theory for adsorption and association is utilized to calculate the adsorption of an ionic surfactant (C16P3+) to two identical, negatively charged, slightly hydrophobic solids separated by a thin film and immersed in an infinite reservoir of aqueous 1:1 electrolyte. This theory explicitly accounts for short-ranged "chemical" forces as well as electrostatic and polarization forces. A change in the separation between the solids leads to a change in interaction energy, which leads to a change in both the amount and the organization of the adsorbed surfactant (proximal adsorption). When the separation between the solids is much greater than the thickness of the adsorbed layers, the general features of the proximal adsorption can be predicted from the adsorption at infinite separation; the adsorption depends on whether each surface is above or below the charge compensation point (ccp). At the ccp, the net charge of the solid plus the adsorbed surfactant is zero. There are few other ions at the interface so the net charge is also approximately zero. Within the confines of our model, there is not much force or adsorption until the adsorbed layers are in close proximity. When the concentration is below the ccp, the net charge of the solid plus adsorbed surfactant is negative, so the cationic surfactant adsorbs to reduce the electrostatic repulsion when the solids are brought together. At concentrations above the ccp, the net charge is positive and the surfactant desorbs. In both cases, the proximal adsorption drives the surfactant layer toward a structure similar to that obtained at the ccp. We find, however, that it is not appropriate to assume that the potential (in any plane) is constant. We describe an example in which the electrostatic potential decreases with separation. At solid-solid separations where the surfactant layers on each surface begin to merge with each other, there is a confinement-induced phase-transition leading to complex adsorption and interaction behavior at very small separations. In addition, calculations show that the interaction between surfactant tails is a key contributor to both the magnitude of proximal adsorption and the exchange between surfactant and co-ions at the surface. The consequences of our findings are discussed in the context of experimental studies of interaction forces and images of adsorbed structures with atomic force microscopy. In addition, we comment on the effects of centrifuging when measuring adsorption isotherms by the depletion method.