Molecular interactions of organic molecules at the air/water interface investigated by sum frequency generation vibrational spectroscopy.
Molecular interactions of organic molecules at the air/water interface investigated by sum frequency generation vibrational spectroscopy.
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DOI：
10.1039/c6cp07827c
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发表时间：
2017-02
期刊：
影响因子：
--
通讯作者：
Wenting Wang;Shuji Ye
中科院分区：
文献类型：
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作者：
Wenting Wang;Shuji Ye
有机分子在水界面上的分子结构和动力学由于其重要性和特殊性而引起了许多研究。然而，直接表征空气/水界面分子相互作用的研究相对较少，因为它们在实验中极难测量。本研究以二丁基酯分子（R1CO2R2O2CR1）为有机分子模型，利用和频产生振动谱法研究其分子结构和相互作用。我们证明了分子相互作用可以通过测量甲基的对称拉伸（ν1）和费米共振带（ν 2）的强度比来估计。在这里，二丁基酯分子被广泛用作聚合物中的增塑剂，以改善塑料和聚合物的性能。当R2链长从0增加到8时，气/水界面上尾部甲基的取向角从34°减小到19°。随着R2链长的增加，二丁基酯分子的分子间相互作用总量减小，这是由于烃链之间的范德华相互作用增加，而羰基与水分子之间的氢键相互作用减少。我们的研究表明，酯基增塑剂在聚合物中的稳定性可以由甲基ν1和2ν2波段的强度比很好地预测。因此，这种强度比可以用作表征增塑剂和聚合物之间分子相互作用的有效振动光学标尺。
The molecular structure and dynamics of organic molecules at the aqueous interface have attracted a number of investigations owing to their importance and specific nature. However, there are relatively few studies on the direct characterization of the molecular interactions at the air/water interface because they are extremely difficult to measure in experiments. In this study, we use dibutyl ester molecules (R1CO2R2O2CR1) as a model of organic molecules, and investigate their molecular structure and interactions using sum frequency generation vibrational spectroscopy. We demonstrate that the molecular interactions can be estimated by measuring the intensity ratio of the symmetric stretching (ν1) and Fermi resonant bands (2ν2) of methyl groups. Here, dibutyl ester molecules are widely used as plasticizers in polymers to improve the properties of the plastics and polymers. It is found that the orientation angles of the tailed methyl groups at the air/water interface decrease from 34° to 19° when the chain length of R2 increases from 0 to 8. The total intermolecular interactions of the dibutyl ester molecules decrease as the chain length of R2 increases because the van der Waals interactions between the hydrocarbon chains increase, while the hydrogen bond interactions between the carbonyl group and water molecules decrease. Our study demonstrates the stability of ester-based plasticizers in polymers can be well predicted from the intensity ratio of the ν1 and 2ν2 bands of methyl group. Such an intensity ratio can be thus used as an effective vibrational optical ruler for characterizing molecular interactions between plasticizers and polymers.