Interaction of carboxylic acids with the oxyhydroxide surface of aluminium: poly(acrylic acid), acetic acid and propionic acid on pseudoboehmite

High sensitivity and spectral resolution XPS, polarisation-modulation reflection FTIR and static SIMS have been used in this investigation of the interfacial chemistry between poly(acrylic acid) (PAA) and an aluminium oxyhydroxide surface. The pseudoboehmite oxyhydroxide surface is used as a model for the chemistry found at the surface of the air-formed film after ambient exposure of aluminium metal. Conversion of carboxylic acids to monodentate carboxylate species at the PAA–AlOOH interface has been identified using reflection absorption FTIR. By virtue of the very low concentration of surface carbon on hydrothermally formed AlOOH, quantification of the interfacial carboxylate concentration was achieved by curve fitting the C1s core level. Through analysis of a range of PAA overlayer thicknesses, a limit to interfacial carboxylate formation with the surface was identified. To determine the reason for this limit, monoacids that chemisorb to the AlOOH surface exclusively through carboxylate bonds were analysed. It is proposed that steric and/or conformational hindrance, associated with the polymer backbone, prevent reaction of all carboxylic acid groups in PAA with surface hydroxyl functionalities to form carboxylate complexes. The implications of this finding for the application of PAA as an adhesion promotion pretreatment are considered.