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    Friction Force Microscopy of Alkylphosphonic Acid and Carboxylic Acids Adsorbed on the Native Oxide of Aluminum

    Year: 2006

    Journal: Langmuir, 2006, 22 (22), pp 9254–9259, 20111221

    Authors: Trevor T. Foster, Morgan R. Alexander, Graham J. Leggett, and Eoghan McAlpine

    Organizations: Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K., School of Pharmacy, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., and Innoval Technology, Beaumont Close, Banbury OX16 1TQ, U.K.

    Monolayers of alkylphosphonic acids (APA) and alkylcarboxylic acids (ACA) on magnetron-sputtered aluminum films have been investigated by friction force microscopy (FFM), contact angle measurement, and polarization-modulation infrared reflection−absorption spectroscopy (PM-IRRAS). Clear evidence has been provided from PM-IRRAS that friction coefficients, determined from FFM data, may be correlated directly with variations in adsorbate molecular structure. The friction coefficient increased with the length of the adsorbate molecule, but reached a limiting value when the alkyl chain of the adsorbate contained eight carbons in the case of APA or 12 carbons in the case of ACA. For a given alkyl chain length, APA monolayers yielded coefficients of friction that were similar to those of monolayers of alkylthiols of the same length, but smaller than those of ACA. These data indicate that APA monolayers are better ordered than ACA monolayers. These inferences were supported by PM-IRRAS data, which enabled the density of gauche defects to be estimated and correlated with variations in the coefficient of friction.