Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China.
Institute of Science and Technology, University of Sanya, Sanya, Hainan 572022, China.
Phys Chem Chem Phys. 2020 May 13;22(18):10106-10115. doi: 10.1039/d0cp00974a.
Understanding the interfacial structure of aqueous electrolyte solutions is important and relevant to a wide range of systems, ranging from atmospheric aerosols to electrochemistry, and biological environments. Though significant efforts have been made to unravel the interfacial structure of water molecules, the structure and dynamics of ions at the interface have not yet been fully elucidated. Here, the interfacial structure of the aqueous solution was investigated directly by monitoring the thiocyanate (SCN-) anions using surface-specific sum frequency generation (SFG) vibrational spectroscopy. The molecular orientation of the SCN- anions and their adsorption behavior at the air/water interface were systematically determined by quantitative polarization analysis. The transition dipole of the CN stretching of the SCN- anion is oriented around 44° from the surface normal of the NaSCN aqueous solution surface and remained unchanged with the bulk concentration varying from 1 mol kg-1 to 13 mol kg-1. The free energy of adsorption of SCN- anions at the air/water interface was determined to be -1.53 ± 0.04 kcal mol-1. Furthermore, a new SFG peak positioned at 2080 cm-1 in the ppp polarization combination was observed at the air/15.0 mol kg-1 NaSCN aqueous solution interface for the first time. Concentration-dependent SFG analysis and density functional theory (DFT) calculation further revealed that the SCN- anions form an ion clustering structure at the air/water interface. The subtle and specific Na+ and K+ counter-cation effects on the interfacial structure of the SCN- anions at the aqueous solution interface were also observed, which showed that ion cooperativity plays an important role in affecting the interfacial structure of ions at the air/water interface. The results are expected to yield significant insights into the understanding of the structure of aqueous solution surfaces and the molecular level mechanism of the cationic Hofmeister effect.
了解水相电解质溶液的界面结构对于从大气气溶胶到电化学和生物环境等广泛的体系都很重要。尽管已经做出了巨大努力来揭示水分子的界面结构，但离子在界面处的结构和动力学仍未完全阐明。在这里，通过使用表面特定的和频发生（SFG）振动光谱直接监测硫氰酸根（SCN-）阴离子来研究水相溶液的界面结构。通过定量偏振分析系统地确定了 SCN-阴离子的分子取向及其在气/水界面上的吸附行为。SCN-阴离子的 CN 伸缩跃迁的偶极矩相对于 NaSCN 水溶液表面的表面法线方向约为 44°，并且随着从 1 mol kg-1 到 13 mol kg-1 的体相浓度变化而保持不变。SCN-阴离子在气/水界面上的吸附自由能被确定为-1.53 ± 0.04 kcal mol-1。此外，首次在气/15.0 mol kg-1 NaSCN 水溶液界面上观察到在 ppp 偏振组合中位于 2080 cm-1 的新 SFG 峰。浓度依赖性 SFG 分析和密度泛函理论（DFT）计算进一步揭示了 SCN-阴离子在气/水界面上形成离子簇结构。还观察到 Na+和 K+抗衡阳离子对水相溶液界面上 SCN-阴离子界面结构的细微和特定的影响，这表明离子协同作用在影响气/水界面上离子的界面结构方面起着重要作用。该结果有望为深入了解水相溶液表面的结构以及阳离子 Hofmeister 效应的分子水平机制提供重要的见解。
