Balancing hydrophobic and hydrophilic forces at the Water/Vapor interface: Surface structure of soluble alcohol monolayers

Surface tension and surface specific vibrational techniques were used to study the structure of soluble alcohol monolayers adsorbed to the air/water interface. Surface tension measurements were used to determine terminal monolayer coverages of different alcohol isomers whereas the surface specific vibrational sum frequency generation probed the interfacial structure and organization. The alcohols studied ranged from heptanols to dodecanols 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 similar to 20 A(2)/molecule, a value close to the calculated cross section of linear, all-trans alkyl chains. The vibrational spectra of these monolayers consistently show features associated with monolayers having very little conformational disorder. For the 2- and 3-position isomers, surface tension measurements show surface coverages that were only half those of the linear isomers. Quantitative analyses of the vibrational band intensities in the CH stretching region show that methyl groups in the C, and Q, positions contribute constructively to observed spectral intensities. Together with calculations of areas corresponding to different conformers, monolayers formed by these isomers are predicted to adopt structures with two gauche defects. Such structures require that the repulsive hydrophobic interactions of shorter alkyl segments with the water subphase be strong enough to offset the cohesive van der Waals interactions of the longer alkyl segments. Unlike the 2- and 3-position alcohols, 5-position isomers have surface areas and spectral band intensities that show a systematic variation over the range of alkyl chain lengths. Experiments are unable to quantitatively address the question of preferred conformer structures for these branched alcohols having pairs of longer alkyl segments. Nevertheless, the vibrational spectra of these 5-position alcohols suggest a complex organization of monomers adsorbed to the air/water interface.