Paper OF+TF-ThP15
Comparison of Alkyl-phosphonic Acid (APA) and Alkyl-carboxylic Acid (ACA) Self Assembled Monolayers on the Hydrated Alumina Surface of Aluminium
The interaction of organics with metal oxide surfaces is important in many areas including corrosion protection and adhesion promotion. Good corrosion protection has been obtained using APA SAMs deposited on Al prior to painting.@footnote 1@ The phosphonic acid head-group interacts with the surface of alumina to form a phosphonate bond.@footnote 2@ It is presumed that resin components react with the phosphonic acid tail-group when di-functional molecules are used. Detailed characterisation of ACAs on Al has been reported but not of APAs.@footnote 3@ The formation of alkyl-thiol SAMs on Au has been extensively studied.@footnote 4@ In contrast to the stable Au metal surface Al forms an oxide, the outer region of which is readily hydrated on exposure to ambient conditions. Two approaches have been employed to obtain reproducible Al surface chemistry for self-assembly: solution pre-treatment or atmospheric exposure. Here, SAMs have been formed on sputtered Al by immersion in 5 mM ACA and APA solutions in n-hexadecane and ethanol respectively. Surface pre-treatment with acetic or methyl-phosphonic acid was compared with 24 h atmospheric exposure. Both XPS and contact angle measurement of SAMs on Al indicate relationships between assembly, time and alkyl chain length similar to alkyl-thiols on Au. Notable differences include changes in the oxide surface, found by XPS to occur in ethanoic solutions, and the importance of pre-treatment. FTIR provides information on the orientation of the SAM and the bonding to the surface. This enables a model of the APA-Al system to be proposed, which may be compared with existing models of ACA-Al. The electrochemical properties of uncoated SAMs are investigated using cyclic voltammetry and rationalised in terms of the models. @FootnoteText@ @footnote 1@I Meage et al. Prog Org Coat 34(1998)1 @footnote 2@RD Ramsier et al. Surf Sci 203(1998)72 @footnote 3@DL Allara, RG Nuzzo Langmuir 1(1985)52 @footnote 4@GM Whitesides, PE Laibinis Langmuir 6(1990)87.