Adsorption Forces between Hydrophobic Monolayers

Thermodynamic analysis of energy vs distance plots for the interaction of silica and mica surfaces in solutions of CTAB (cetyltrimethylammonium bromide) and similar surfactants at different concentrations shows that the long range attraction observed around the point of zero charge (pzc) is a charge regulation effect enchanced by the cooperativity of the cationic surfactant adsorption at anionic hydrophilic surfaces. The application of the Gibbs adsorption equation to experimental data shows that in the low concentration regime the adsorption increases with decreasing separation. This is due to an electrostatic contribution to the free energy of adsorption which increases as surfaces approach. An additional adsorption energy gain arises from association of hydrophobic tails when a sufficiently large adsorption density is reached and two-dimensional micellization is enhanced in the gap. A small increase of the electrochemical potential with decreasing separation gives rise to a large increase of adsorption. The double-layer repulsion at long distances corresponds to the ordinary DLVO result as long as changes in adsorption with separation remain small. At smaller separations the potential falls below its initial value because of the cooperative adsorption of the potential-determining ion, which results in the shift of the pzc to lower concentrations at close separations. The interaction is more attractive than would be expected from the constant potential approximation. The interaction pattern determined by equilibrium surfactant adsorption is different from that for nonpolar surfaces in pure water. An enhancement of adsorption in confined geometries is typical of condensation phenomena. A similar mechanism occurs for deposited monolayers of insoluble surfactants.