Saturation adsorption at interfaces of surfactant solutions

Abstract

Plots of the surface or interfacial tension of aqueous surfactant solutions versus the logarithm of the bulk concentration of the surfactant frequently become linear at concentrations from 10 to 30% of the critical micelle concentration. According to the Gibbs equation, the surface excess concentration of the surfactant in the linear region is constant; saturation adsorption is said to exist. However, the surface or interfacial tension continues to decrease considerably with increasing bulk concentration of the surfactant throughout the saturation adsorption region until the critical micelle concentration is reached. This decrease is explained as follows. While the surface excess concentration of the surfactant reaches a constant upper limit at the onset of saturation adsorption, the total surfactant concentration in the surface layer, which consists of the surface excess concentration plus the surfactant concentration present in an equivalent volume of bulk solution, continues to increase slightly with increasing bulk concentration throughout the saturation adsorption region. As the bulk concentration approaches the critical micelle concentration, the total surface concentration of the surfactant exceeds its surface excess concentration by small but increasing amounts. Because the surfactant monolayer in the saturation adsorption region is densely packed, slight increases in the total surface concentration produce disproportionately large decreases in the surface or interfacial tension. This explanation is illustrated with experimental data.