Order–disorder transition as a function of surface coverage for n-hexadecanoic acid chemisorbed on aluminium

Fourier transform infrared multiple specular reflectance (FTIR-MSR) spectroscopy has been applied to study the chemisorption of n-hexadecanoic or palmitic acid CH3(CH2)14COOH on aluminium as a function of surface coverage. The intensity of the symmetric carboxylate stretching vibration has been used to estimate the surface coverage. The intensities and band positions of the CH stretching modes are presented as a function of surface coverage. A sudden transition at about 80% of monolayer coverage is observed for the intensity as well as the position of the CH2 stretching modes. The intensity changes correspond to a change from a more or less planar orientation of the polymethylene chains at low surface coverage to an almost perpendicular orientation at high surface coverage. The asymmetric CH3 stretching vibration exhibits a shift opposite to and smaller than what is observed for the CH2 vibrations. No significant changes are observed for the position of the symmetric CH3 stretching vibration. However, for the symmetric carboxylate stretching vibration a sudden shift of the band position is observed of the same magnitude, but opposite to the shifts of the CH2 vibrations.