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| - A Vibrational Sum Frequency Spectroscopy Study of the Liquid−Gas Interface of AceticAcid−Water Mixtures: 2. Orientation Analysis
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| - Vibrational sum frequency spectroscopy has been used to investigate the surface of aqueous acetic acidsolutions. By studying the methyl and carbonyl vibrations with different polarization combinations, anorientation analysis of the acetic acid molecules has been performed in the concentration range 0−100%.The surface tension of acetic acid solutions was also measured in order to obtain the surface concentration.The orientation of the interfacial acetic acid molecules was found to remain essentially constant in an uprightposition with the methyl group directed toward the gas phase in the whole concentration range. The tilt angle(θCH3) of the symmetry axis of the methyl group with respect to the surface normal was found to be lowerthan 15° when considering a δ distribution of angles or as narrow as 0 ± 11° when assuming a Gaussiandistribution. Further investigations showed that the CO bond tilt (θCO) of the acetic acid hydrated monomerwas constant and close to 55° in the concentration range where it was detected. Finally, the orientationinformation is discussed in terms of different species of acetic acid, where the formation of a surface layerof acetic acid cyclic dimers is proposed at high acid concentrations.
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