Liquid/air interfacial structure of alcohol-octyl hydroxamic acid mixtures: a study by sum-frequency spectroscopy

The molecular structure of the liquid/air interfaces of 1-octanol, 1-decanol, n-decane and the branched decyl alcohol EXXAL 10 has been studied by sum-frequency spectroscopy (SFS) in the C-H stretching vibrational region. The data suggest that the interfaces consist of ordered molecules with closely packed alkyl tails, in close to all-trans conformation with some gauche defects. The degree of surface ordering for the branched alcohol is much higher than for octanol and decanol. When octyl hydroxamic acid (OHA) is dissolved in 1-octanol it increases the gauche conformational defects in the interfacial chains, possibly due to mixing with the surface alcohol molecules and disrupting their ordering. In contrast, we suggest that when octyl hydroxamic acid is dissolved in EXXAL 10, the surface ordering of the alcohol chains does not change. We put forward the hypothesis that the appearance of new bands, belonging to the asymmetric methylene group vibrations and to the asymmetric methyl modes in the SF spectra of the mixture suggests that the surface OHA molecules are arranged with their hydrocarbon tails tilted very close to the interface.