Negi Anjali, Malik Ravi, Chandra Amalendu
Department of Chemistry, Indian Institute of Technology Kanpur, Uttar Pradesh 208016, India.
J Chem Phys. 2025 Feb 7;162(5). doi: 10.1063/5.0249942.
We have investigated the liquid-vapor interface of aqueous acetic acid solution through calculations of the structure and vibrational sum frequency generation (VSFG) spectra of the interfaces for varying concentrations of the acetic acid (AA). Our findings reveal the surface propensity of the AA molecules. As the concentration of AA increases, more AA molecules are found to be present in the interfacial region. The AA molecules at the interface are found to be oriented in a manner where the hydrophobic part (methyl chain) is oriented toward the vapor phase and the hydrophilic part is pointed toward the liquid phase. The total VSFG spectrum reveals that the intensity of the peak of dangling OH groups of water around 3750 cm-1 decreases when the concentration of AA increases, while the intensity of the peak around 3550 cm-1 due to OH groups having hydrogen bonds between water and AA molecules increases. Furthermore, the latter peak is redshifted as the AA concentration increases, which is due to the partial cancellation between the positive response of water OH modes and the negative response of the OH modes of AA molecules. It is also noted that the AA molecules make the interfacial water more structured by making hydrogen bonds with its otherwise dangling OH modes at the surface.
我们通过计算不同浓度乙酸（AA）的界面结构和振动和频产生（VSFG）光谱，研究了乙酸水溶液的液-气界面。我们的研究结果揭示了AA分子的表面倾向。随着AA浓度的增加，发现更多的AA分子存在于界面区域。发现界面处的AA分子以疏水部分（甲基链）朝向气相、亲水部分指向液相的方式取向。总的VSFG光谱表明，当AA浓度增加时，3750 cm-1附近水的悬垂OH基团峰的强度降低，而由于水与AA分子之间存在氢键的OH基团导致的3550 cm-1附近峰的强度增加。此外，随着AA浓度的增加，后一个峰发生红移，这是由于水OH模式的正响应与AA分子OH模式的负响应之间的部分抵消。还注意到，AA分子通过与表面原本悬垂的OH模式形成氢键，使界面水的结构更加有序。
