| Abstract
| - Surface tension and surface specific vibrational techniques were used to study the structure of soluble alcoholmonolayers adsorbed to the air/water interface. Surface tension measurements were used to determine terminalmonolayer coverages of different alcohol isomers whereas the surface specific vibrational sum frequencygeneration probed the interfacial structure and organization. The alcohols studied ranged from heptanols tododecanols with constitutional isomers having the −OH group in the 1, 2, 3, and 5 positions. Not surprisingly,linear isomers form tightly packed monolayers having molecular areas of ∼20 Å2/molecule, a value close tothe calculated cross section of linear, all-trans alkyl chains. The vibrational spectra of these monolayersconsistently show features associated with monolayers having very little conformational disorder. For the2- and 3-position isomers, surface tension measurements show surface coverages that were only half those ofthe linear isomers. Quantitative analyses of the vibrational band intensities in the CH stretching region showthat methyl groups in the C1 and Cn positions contribute constructively to observed spectral intensities. Togetherwith calculations of areas corresponding to different conformers, monolayers formed by these isomers arepredicted to adopt structures with two gauche defects. Such structures require that the repulsive hydrophobicinteractions of shorter alkyl segments with the water subphase be strong enough to offset the cohesive vander Waals interactions of the longer alkyl segments. Unlike the 2- and 3-position alcohols, 5-position isomershave surface areas and spectral band intensities that show a systematic variation over the range of alkyl chainlengths. Experiments are unable to quantitatively address the question of preferred conformer structures forthese branched alcohols having pairs of longer alkyl segments. Nevertheless, the vibrational spectra of these5-position alcohols suggest a complex organization of monomers adsorbed to the air/water interface.
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