THERMODYNAMIC ANALYSIS OF SOLUTE EFFECTS ON SURFACE FORCES - ADHESION BETWEEN SILICATES IN SOLUTIONS OF CATIONIC SURFACTANTS

The very large increase in adhesion between silicate surfaces observed on addition of small amounts of cationic surfactants is typically attributed to the formation of hydrophobic layers at each of the two surfaces. This implies that adsorption does not change with separation. An analysis based on the Gibbs adsorption equation shows that the usual explanation is valid only when adsorption equilibrium is not maintained during a force measurement. Equilibrium adsorption is a function of distance and changes reversibly when the surfaces approach one another or are separated. Stronger attraction at higher solute concentration corresponds to an increase of adsorption with decreasing intersurface separation, and vice versa. Thermodynamic analysis of existing data at low surfactant concentrations reveals that the increase of adsorption on contact compared to adsorption at isolated interfaces is dramatic. The effect is reversed at higher concentrations when bilayers are formed at the free interfaces. The thermodynamic results are exact for equilibrium systems and explain kinetic phenomena accompanying adsorption reequilibration. The significance of the Gibbs adsorption equation as a universal tool for rigorous analysis of short-range surface forces and various solute effects is briefly discussed.