Resolving Two Closely Overlapping 鈭扖N Vibrations and Structure in the Langmuir Monolayer of the Long-Chain Nonadecanenitrile by Polarization Sum Frequency Generation Vibrational Spectroscopy

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• 作者：Zhen Zhang ; Yuan Guo ; Zhou Lu ; Luis Velarde ; Hong-fei Wang
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：February 2, 2012
• 年：2012
• 卷：116
• 期：4
• 页码：2976-2987
• 全文大小：607K
• 年卷期：v.116,no.4(February 2, 2012)
• ISSN：1932-7455

文摘

Polarization sum frequency generation vibrational spectra (SFG-VS) reveals that there are two distinctively different but closely overlapping 鈭扖N vibrations at 2244.5 and 2251.1 cm^鈥?, respectively, in the Langmuir monolayer of the long-chain nonadecanenitrile (CH₃(CH₂)₁₇CN, or C18CN) at the air/water interface. The blue-shifted 鈭扖N group at the 2251.1 cm^鈥? peak is about 1.8 times broader than that of the 2244.5 cm^鈥?. Both the spectral shift and spectral width are consistent with the picture that this blue-shifted peak corresponds to the solvated 鈭扖N group, while the 2244.5 cm^鈥? peak is the signature of the less solvated 鈭扖N group. Polarization dependence of these two peaks further suggests that the 鈭扖N group corresponding to the 2251.1 cm^鈥? peak is tilted with an average angle of 50掳 from interface normal, while that corresponding to the 2244.5 cm^鈥? peak is tilted with an angle around 67掳. The relative population for the 鈭扖N groups corresponding to the 2251.1 cm^鈥? peak is about three times that of the 2244.5 cm^鈥? peak. These results suggest that the 鈭扖N head groups in the C18CN Langmuir monolayer are not aligned uniformly at slightly different depth, in order to avoid the strong repulsive forces between the strong 鈭扖N dipoles. The SFG-VS spectra of the O鈥揌 stretches at C18CN Langmuir monolayer are similar to those of the 4鈥?n-pentyl-4-cyano-p-terphenyl (5CT) monolayer, indicating complete exclusion of the water molecules from the C18CN Langmuir monolayer, but significantly different from those of the 4鈥?n-octyl-4-p-cyanobiphenyl (8CB) monolayer, as well as those of the air/acetonitrile aqueous solution interface. Different from previously held understandings, these results suggest that the structure of the insoluble long-chain C18CN Langmuir monolayer is significantly different from that of the Gibbs adsorption layer of the short-chain soluble acetonitrile or propanenitrile aqueous solution surfaces. These observations not only shed new light on understanding the detailed structure and interactions in the molecular monolayer and films but also suggest the importance of the polarization and spectral resolution in the SFG studies.