Adsorption of Organic Acids on Metal Oxides. The Umbrella Effect

Simultaneous interpretation of electrokinetic and adsorption data for organic ions at metal oxide surfaces is introduced. It is shown that, for the salicylic acid / hematite system, the singly charged salicylate ions react with surface –OH, groups releasing one water molecule. The salicylate ions, when bound to the surface, exhibit a pronounced »Umbrella effect«, i.e. each relatively large organic ion covers about four to six surface –OH, excluding them from the further adsorption process. Within the electrical interfacial layer, the salicylate ions are located in the plane near the onset of the diffuse layer. It is demonstrated that the interpretation of adsorption data solely may lead to erroneous conclusions regarding the mechanism of binding and the structure of the interfacial layer. Electrokinetic measurements provide useful information, enabling the solution of the above problem