Equilibrium and Nonequilibrium Phenomena Associated with the Adsorption of Ionic Surfactants at Solid–Water Interfaces

Abstract

The evolution of interfacial phenomena involving physisorbing ionic surfactants on oxides in aqueous systems is presented. Four distinct regions in the adsorption isotherm are associated with the aggregation mode of adsorbed ions at the solid–water interface. As the adsorbed surfactant begins to associate into hemimicelles at the interface, colloidal particles begin to coagulate and become hydrophobic, exhibiting maximum coagulation and hydrophobicity at conditions where the ζ potential is zero. Upon further increase in surfactant concentration, the zeta potential is reversed and the particles redisperse and regain their hydrophilicity. Upon reversal of the ζ potential, the adsorbed surfactant ions appear to adsorb in reverse orientation, forming reverse hemimicelles or bilayers, at surfactant concentrations significantly lower than the critical micelle concentration (CMC). Coarser particles remain dispersed and hydrophobic up to the CMC because their kinetic energy allows them to overcome energy barriers acting between the particles. In systems involving dewetting after surfactant adsorption at the solid–water interface, excess adsorption of the surfactant at the gas–solid interface is reflected in the behavior of the system, such as in the measurement of advancing and receding contact angles, consecutive dewetting–rewetting, reflotation, and capillary rise.