Bodycote Prize Paper Characterisation of alkyl phosphonic acid monolayers self assembled on hydrated surface of aluminium

Self assembly of alkyl phosphonic acids (APAs) on magnetron sputtered aluminium (MS-Al) has been investigated with particular attention on the role of the aluminium surface chemistry produced by conditioning through ambient atmospheric and ethanoic methyl phosphonic acid solution immersion. For the longest molecule investigated, decyl phosphonic acid, contact angle measurement revealed rapid formation of relatively disordered self assembled monolayers (SAMs) from a 5 mM ethanoic solution. Evidence of a reaction of the phosphonic acid head group in a condensation reaction with the hydroxyls at the alumina surface to form a phosphonate was obtained using polarisation modulation infrared reflection absorption spectroscopy (PM-IRRAS). The concentration of hydroxyl groups in the oxide at the surface of the MS-Al, determined by X-ray photoelectron spectroscopy (XPS), increased on immersion in ethanol owing to the presence of dissolved impurity water. Adsorption of hydrocarbon impurities is also evident. Preadsorption of methyl phosphonic acid inhibited oxide growth and stabilised the wettability of the MS-Al stored in ethanol, although it did not increase the order of self assembled decyl phosphonic acid (DPA) compared with conditioning of the surface by atmospheric exposure.