A Comparative Study of the CD and CH Stretching Spectral Regions of Typical Surfactants Systems Using VSFS: Orientation Analysis of the Terminal CH3 and CD3 Groups

作者：Tyrode Eric^*; Hedberg Jonas

来源：Journal of Physical Chemistry C, 2012, 116(1): 1080-1091.

DOI：10.1021/jp210013g

A systematic comparison between the vibrational spectra in the CD and CH stretching regions of four deuterated and perprotonated surfactants adsorbed at the liquid/vapor interface has been carried out using vibrational sum frequency spectroscopy (VSFS). Bulk IR and polarized Raman spectra of hexane and its deuterated analogue have also been obtained for reference. Due to differences in the Fermi resonances, the relative positions and intensities of the spectral features, including conformational order indicators vary between the CD and CH regions. The commonly used sum frequency (SF) order indicator for perprotonated molecules, which involves the amplitude ratio of the symmetric CH3 r(+)/ symmetric CH2 d(+) bands is not directly transposable to the CD region. Instead, the ratio between the asymmetric CH3 r(-)/ antisymmetric CD2 d(-) bands is proposed as a more suitable alternative. Using as a starting point the SF spectra from the upright all-trans monolayer of dodecanol and its deuterated analogue, the theoretically modeling of the terminal methyl group orientation has been critically evaluated. The orientational analysis in the CD region gives significantly more reliable and consistent results, mainly because of the relative simplicity in unambiguously resolving the concerned peaks in the SF spectra.

• 出版日期2012-1-12