ORDER AND DISORDER IN N-ALKYLCARBOXYLIC ACID MONOLAYERS - CHAIN-LENGTH DEPENDENCE AND LATERAL INTERACTION EFFECTS

The chain-length dependence of order and disorder phenomena in n-alkylcarboxylic acids, CH3(CH2)n-COOH, chemisorbed on oxidized aluminum, was studied by infrared reflection-absorption spectroscopy (IRRAS). The frequencies of the CH2 stretching modes decrease continuously with increasing chain length, which implies a gradual transition from conformationally disordered chains with many gauche conformation to all-trans ordered chains. The band position of the symmetric carboxylate stetching vibration in a fully packed monolayer exhibits a shift to higher frequencies for chain lengths between 10 and 14 methylene groups. This suggests a transition from a rather open structure for short chains to a densely packed structure for longer chains. The observed maximum integrated intensities for the symmetric carboxylate stretching vibration, which have a lower value for the shorter chain molecules, support this suggestion. A comparison has been made between the coverage-dependent shifts of the symmetric and the asymmetric carboxylate vibration for methylene chain lengths of n = 0, 2, and 14. The data are compared with computations based on dipole-dipole coupling theory for modes perpendicular and parallel to the metal surface. A discussion is given about the possibility of an ordered adlayer structure for the n = 14 chain at low coverage.