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    Study of the Resistance of SAMs on Aluminium to Acidic and Basic Solutions Using Dynamic Contact Angle Measurement

    Year: 2007

    Journal: Langmuir, 2007, 23 (3), pp 995–999, 20111221

    Authors: Ioannis L. Liakos, Roger C. Newman, Eoghan McAlpine, and Morgan R. Alexander

    Organizations: EC-Joint Research Centre-IHCP, Biomedical Materials and Systems Unit, 21020 Ispra, Varese, Italy, University of Toronto, Department of Chemical Engineering and Applied Chemistry, 200 College Street, Toronto, Ontario M5S 3E5, Canada, Innoval Technology, Beaumont Close, Banbury OX16 1TQ, U.K., and Laboratory of Biophysics and Surface Analysis, School of Pharmacy, University of Nottingham NG7 2RD, U.K.

    We report the development of a method to determine the aqueous stability of self-assembled monolayers (SAMs) using the Wilhelmy plate dynamic contact angle (DCA) experiment. The DCA is measured in solutions over a range of pH values for alkyl carboxylic and alkyl phosphonic acid SAMs formed on magnetron-sputtered aluminum. The change in DCA on repeated immersion is used as a measure of the degradation of the SAMs by hydrolytic attack. The short and intermediate chain length alkyl acids are not stable in water of neutral pH, whereas molecules with the longest alkyl chains show considerably greater stability in neutral and both high and low pH solutions. The packing density inferred from the DCA and the contact angle hysteresis suggests the C18CO2H monolayer to be slightly less well packed than that of the C18P(O)(OH)2; this is consistent with related friction force microscopy and infrared reflection absorption spectroscopy findings published elsewhere (Foster, T. T.; Alexander, M. R.; Leggett, G. J.; McAlpine, E. Langmuir 2006, 22, 9254−9259). The resistance of the SAMs to acid and alkaline environments is discussed in the context of aluminum oxide solubility, SAM packing density, and the resistance of the interfacial phosphate and carboxylate functionalities to different aqueous conditions.