Structure and organization of hexadecanol isomers adsorbed to the air/water interface.

The structure and 2D phase behavior of hexadecanol isomers adsorbed to the air/water interface have been studied using surface tension methods and vibrational sum frequency spectroscopy. Isomers include the linear 1-hexadecanol as well isomers with the alcohol functional group in the 2, 3, and 4 positions. Surface-pressure isotherms highlight how the 2D phase behavior of these monolayers depends sensitively on registry and packing efficiency between the alkyl chains whereas vibrational sum frequency spectroscopy, which is vibrational spectroscopy with surface specificity, reveals details about the molecular structure and orientation of molecules within the monolayer films at their equilibrium spreading pressures. At their equilibrium spreading pressures, both 1- and 2-hexadecanol form compact films having a high degree of conformational order and molecular areas of 18.9 and 21.5 A(2)/molecule, respectively. This result for 2-hexadecanol implies that the isomer remains primarily in an all-trans conformation with the methyl group in the C(1) position buried in the water subphase. This conformation leads to significantly reduced intensity in specific vibrational transitions due to partial destructive interference. In contrast, 3-hexadecanol and 4-hexadecanol form more expanded monolayers at their equilibrium spreading pressures, having areas of 28.7 and 40.3 A(2)/molecule, respectively. In these monolayers, the intensities of selected vibrational bands show less evidence of destructive interference, implying that methyl groups on opposite ends of the adsorbates do not adopt strongly correlated orientations.