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Surface structure at hexadecane and halo-hexadecane liquid/vapor interfaces

2004-07-29
Esentürk, Okan
WALKER, ROBERT A
Vibrational spectra in the CH stretching region have been acquired from the n-hexadecane, 1-chlorohexadecane, and 1-bromohexadecane liquid/vapor interfaces using broadband vibrational sum-frequency spectroscopy. Different polarization conditions of the incident and generated fields are used to assess surface structure based on functional group alignment along the Surface normal (SsumSvisPIR) and within the plane of the surface (SsumPvisSIR). The n-hexadecane liquid/vapor interface evinces a surprising degree of conformational order, given that X-ray scattering studies imply an absence of any surface organization for alkanes above their surface freezing temperature. Replacing a terminal hydrogen of n-hexadecane with either a chlorine or bromine substituent significantly changes the out-of-plane surface structure, but the in-plane surface structure remains qualitatively unchanged. These effects are attributed to dipole-dipole interactions between the halogenated ends of the C-16 alkaries and are indicative of a higher surface activity of the halogenated ends, compared to the unsubstituted ends of the molecule.