Phthalate adsorption on gamma-alumina: a spectroscopic study

Sammanfattning: Adsorption of phthalic acid/phthalate has been investigated on anodically oxidized aluminium foils - used in wet electrolytic capacitors -, by different spectroscopic methods like UV, infrared, Raman and surface- enhanced Raman spectroscopy. In addition to the capacitor foils the adsorption was studied utilizing a high surface area, synthetic Y-aluminium oxide powder as a model material. The capacitor foils were first characterized comprising BET, SIMS, XRF and electron microscopic (SEM) measurements in order to determine the composition and structure of the oxide surface. Adsorption on the two different substrates has been compared and concluded to lead to the same type of surface complexes approving the applicability of the model material for further adsorption studies. In the adsorption experiments, the influence of time, pH and ionic media on the adsorbed amount, as well as on the structure of the surface complex has been studied. Based on the analyses of the different spectra the following conclusions can be drawn: The adsorption is a fast process driven by electrostatic forces. Complexation of the aromatic compound starts with either inner- or outer sphere co-ordination of the hydrogen phthalate ion. The ratio of the two types of complexes shows pH dependence which can be correlated to the different type of surface sites preferred at different pHs. Surface-enhanced Raman spectroscopy (SERS) was applied directly on the capacitor foils for detecting the adsorbed species. Besides of the analytical aspects of the method, a detailed investigation has been performed to explore of the mechanism of the SER effect in this particular experimental approach. It was concluded that SER spectra originate from a silver-phthalate complex rather than from the phthalate adsorbed on the oxide. The structure and orientation of the final SER species is discussed. Although the aluminium oxide surface of the capacitor foil is not involved in the SER complex, the roughness of the surface, and other properties of the substrate can have positive contribution to the enhancement.