ADSORPTION OF SODIUM LAURATE FROM ITS AQUEOUS-SOLUTION ONTO AN ALUMINA SURFACE - A DYNAMIC STUDY OF THE SURFACE SURFACTANT INTERACTION USING ATTENUATED TOTAL-REFLECTION FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

The interaction dynamics and the structure of the adsorbing sodium laurate from its aqueous solution onto an alumina surface were studied in situ using infrared attenuated total internal reflection spectroscopy. For pH values under the isosteric point of alumina, electrostatically bound laurate is irreversibly adsorbed on the surface. With time, the laurate ion reacts with the alumina, through a water abstraction reaction, forming a unidentate aluminum salt. Desorption experiments show that the latter structure is reversibly bound to the surface. Spectroscopic evidence indicates that the electrostatically bound layer has C2v symmetry. The aluminum laurate does not exhibit such symmetry, and no organization on the surface is observed upon formation of the salt. At higher pH values, when the alumina surface is not positively charged, surface aluminum atoms and the laurate ions form a chelating structure, through a hydroxyl abstraction reaction.