Proximal adsorption of cationic surfactant on silica at equilibrium

The equilibrium force between a glass particle and a silica sheet in aqueous solution was measured as a function of the separation and chemical potential of the surfactant dodecyltrimethylammonium bromide. A Maxwell relation was used to transform data from these force measurements into the change in adsorption of the surfactant as a function of separation. At low concentrations, when the isolated solid-liquid interface is negatively charged, the surfactant adsorbs as the surfaces approach. Under some circumstances, surfactant adsorption exceeds the level necessary to regulate the surface charge. In the range of concentrations (cmc = critical micelle concentration) cmc/130-cmc/10, the interaction energy is much more negative than the expected van der Waals interaction between silica or hydrocarbon surfaces and is approximately constant. In this concentration range, the adsorption is independent of the separation for separations greater than about 7 nm. However, changes in contact adhesion show that there is a large increase in adsorption when the surface separation decreases from 7 nm to contact. From cmc/10 to the cmc, the interaction becomes more repulsive and the surfactant desorbs as the surfaces approach each other. When the surfaces approach in 4 cmc solution, there is first a large increase in surface excess and then a large decrease in surface excess. The large increase in surface excess corresponds to a depletion force, and the large decrease occurs as the surfactant is squeezed off the silica. The fact that the surfactant must be squeezed out from between the surfaces to obtain an adhesive interaction above the cmc requires the adhesion to decrease on the addition of the surfactant to the solution. At even higher concentrations (8 cmc and greater), the energy and the surface excess are oscillatory functions of the separation. The pressure between the surfaces in these concentrated solutions is well described by an osmotic interaction with the concentration derived from the surface excess and a constant osmotic coefficient.