ADSORPTION FORCES BETWEEN HYDROPHOBIC MONOLAYERS

Thermodynamic analysis of energy vs distance plots for the interaction of silica and mica surfaces in solutions of CTAB (cetyltrimethylammon ium bromide) and similar surfactants at different concentrations shows that the long range attraction observed around the point of zero char ge (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 w ith decreasing separation. This is due to an electrostatic contributio n to the free energy of adsorption which increases as surfaces approac h. An additional adsorption energy gain arises from association of hyd rophobic tails when a sufficiently large adsorption density is reached and two-dimensional micellization is enhanced in the gap. A small inc rease of the electrochemical potential with decreasing separation give s rise to a large increase of adsorption. The double-layer repulsion a t long distances corresponds to the ordinary DLVO result as long as ch anges in adsorption with separation remain small. At smaller separatio ns the potential falls below its initial value because of the cooperat ive adsorption of the potential-determining ion, which results in the shift of the pzc to lower concentrations at close separations. The int eraction is more attractive than would be expected from the constant p otential approximation. The interaction pattern determined by equilibr ium 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 dep osited monolayers of insoluble surfactants.