Specific Na+ and K+ cation effects on the interfacial water molecules at the air/aqueous salt solution interfaces probed with nonresonant second harmonic generation
Specific Na+ and K+ cation effects on the interfacial water molecules at the air/aqueous salt solution interfaces probed with nonresonant second harmonic generation
复制标题
使用非共振二次谐波产生探测空气/盐水溶液界面处特定的 Na 和 K 阳离子对界面水分子的影响
DOI：
10.1063/1.3104609
复制
发表时间：
2009-04-07
影响因子：
4.4
通讯作者：
Wang, Hong-fei
中科院分区：
文献类型：
--
作者：
Bian, Hong-tao;Feng, Ran-ran;Wang, Hong-fei
在这里，我们报告的极化相关的非共振二次谐波产生（SHG）测量的界面水分子在以下盐的水溶液：氟化钠，氯化钠，NaBr，KF，KCl，和KBr。通过对二次谐波数据的定量极化分析，确定了这些水溶液表面的取向参数D（D= < cos theta >/< cos（3）θ>）值和界面水分子的相对表面密度。从这些结果中，我们发现，添加六种盐中的每一种都会在一定程度上导致表面处的界面水层厚度的增加。阳离子和阴离子对界面水层厚度的影响均为：KBr>NaBr>KCl>NaCl，NaF>KF。由于这些变化不能分解为单个阴离子和阳离子的贡献，因此可能存在离子配对或缔合效应，特别是对于NaF的情况。我们还发现，界面水分子的取向参数D值的变化方向相反的三种钠盐的水溶液与三种钾盐的水溶液。这些发现清楚地表明了在水溶液表面的意想不到的特定Na+和K+阳离子效应。这些影响是没有预期从最近的分子动力学模拟结果，其结论是，Na+和K+阳离子可以被视为小nonpolarizable硬离子，他们从水界面排斥。这些结果表明，电解质水溶液表面比目前流行的理论和实验的理解更复杂。
Here we report on the polarization dependent nonresonant second harmonic generation (SHG) measurement of the interfacial water molecules at the aqueous solution of the following salts: NaF, NaCl, NaBr, KF, KCl, and KBr. Through quantitative polarization analysis of the SHG data, the orientational parameter D (D=< cos theta >/< cos(3) theta >) value and the relative surface density of the interfacial water molecules at these aqueous solution surfaces were determined. From these results, we found that addition of each of the six salts caused an increase in the thickness of the interfacial water layer at the surfaces to a certain extent. Noticeably, both the cations and the anions contributed to the changes, and the abilities to increase the thickness of the interfacial water layer were in the following order: KBr>NaBr>KCl>NaCl similar to NaF>KF. Since these changes cannot be factorized into individual anion and cation contributions, there are possible ion pairing or association effects, especially for the NaF case. We also found that the orientational parameter D values of the interfacial water molecules changed to opposite directions for the aqueous solutions of the three sodium salts versus the aqueous solutions of the three potassium salts. These findings clearly indicated unexpected specific Na+ and K+ cation effects at the aqueous solution surface. These effects were not anticipated from the recent molecular dynamics simulation results, which concluded that the Na+ and K+ cations can be treated as small nonpolarizable hard ions and they are repelled from the aqueous interfaces. These results suggest that the electrolyte aqueous solution surfaces are more complex than the currently prevalent theoretical and experimental understandings.