Calorimetric evidence for the similarity between the mechanisms of cationic and anionic surfactant adsorption on oppositely charged crystalline oxide surfaces

Abstract

Adsorption of one cationic surfactant, benzyldimethyldode-cylammonium bromide (BDDAB), and two anionic surfactants, sodium heptylbenzene sulfonate (SHBS) and sodium dodecyl sulfate (SDS), from aqueous solutions on the oppositely charged crystalline oxide surfaces, negatively charged quartz and positively charged zirconium dioxide, has been studied. Differential molar enthalpies of adsorption, supplemented by adsorption isotherms and electrophoretic data, are believed to reflect the same general scheme of the process, imposed by the particular geometry and properties of a crystalline solid surface. Some quantitative differences, due to the specific properties of every system, do not change this impression. The following three sequential mechanisms can be proposed in each case: 1) electrostatic adsorption, involving ion-exchange with the pre-adsorbed counterions or ion pairing at oppositely charged surface sites, accompanied by the displacement of interfacial water molecules, 2) formation of the two-dimensional, monolayer aggregates, producing discrete and highly hydrophobic areas on the surface, and 3) process of growing of the transient monolayer aggregates into extended three-dimensional-like micelles of surface.