Spontaneously Organized Molecular Assemblies. 2. Quantitative Infrared Spectroscopic Determination of Equilibrium Structures of Solution-Adsorbed n-Alkanoic Acids on an Oxidized Aluminum Surface

Infrared reflection spectroscopy has been applied to the determination of the structures of adsorbed monolayer films of n-alkanoic acids on oxidized aluminum substrates. Acids of 16-22 carbons, terminated by methyl, vinyl, or propargyl groups, were adsorbed from hexadecane solution at 25 degree C, using immersion times up to several days. These results, together with those for perdeuterated acids, indicate that close-packed assemblies are formed with extended alkyl tails oriented with their chain axes tilted away from normal to the surface with a value of approximately equals 10 degree for the longer chains. Frequency shifts to higher values are observed for CH stretching modes of the vinyl and propargyl terminal groups measured in monolayer films compared to the bulk acids.