Adsorption of organic acids on metal oxides. The umbrella effect

Simultaneous interpretation of electrokinetic and adsorption data for organ ic ions at metal oxide surfaces is introduced. It is shown that, for the sa licylic acid/hematite system, the singly charged salicylate ions react with surface -OH, groups releasing one water molecule. The salicylate ions, whe n bound to the surface, exhibit a pronounced >>Umbrella effect<<, i.e. each relatively large organic ion covers about four to six surface -OH, excludi ng them from the further adsorption process. Within the electrical interfac ial layer, the salicylate ions are located in the plane near the onset of t he diffuse layer. It is demonstrated that the interpretation of adsorption data solely may lead to erroneous conclusions regarding the mechanism of bi nding and the structure of the interfacial layer. Electrokinetic measuremen ts provide useful information, enabling the solution of the above problem.