Hua Wei, Verreault Dominique, Allen Heather C
J Phys Chem Lett. 2013 Dec 19;4(24):4231-6. doi: 10.1021/jz402009f. Epub 2013 Nov 27.
Air/aqueous interfaces provide a unique environment for many chemical, environmental, and biological processes. To gain insight, molecular-level understanding of the interfacial water organization and ion distributions at these interfaces is required. Here, the air/aqueous interface of NaClO4 salt solutions was investigated by means of conventional and heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. It is found that perchlorate (ClO4(-)) ions exist in the interfacial region and prefer to reside on average above their counterions. This finding is inferred from the average orientation of the OH transition dipole moment of interfacial water molecules governed by the direction of the net electric field arising from the interfacial ion distributions. At the air/aqueous interface of NaClO4 salt solutions, the net dipole moments of hydrogen-bonded water molecules are oriented preferentially toward the vapor phase. Contrary to some other salts (e.g., sulfates), the presence of ClO4(-) may cause a full reversal in the direction of the interfacial electric field at a higher concentration (≥1.7 M). Another interpretation for the positive Im χ((2)) spectra of NaClO4 salt solutions could be an increase in the population of water species contributing positively to the net OH transition dipole moment. Regardless of the mechanism, this effect becomes even more pronounced with increasing salt concentration.
气/水界面为许多化学、环境和生物过程提供了独特的环境。为了深入了解，需要从分子水平上理解这些界面处的界面水结构和离子分布。在此，通过常规和外差检测振动和频产生（HD-VSFG）光谱对NaClO4盐溶液的气/水界面进行了研究。研究发现，高氯酸根（ClO4(-)）离子存在于界面区域，并且平均而言更倾向于位于其抗衡离子上方。这一发现是根据界面水分子的OH跃迁偶极矩的平均取向推断出来的，该取向由界面离子分布产生的净电场方向决定。在NaClO4盐溶液的气/水界面处，氢键合水分子的净偶极矩优先指向气相。与其他一些盐（如硫酸盐）不同，在较高浓度（≥1.7 M）时，ClO4(-)的存在可能会导致界面电场方向完全反转。对NaClO4盐溶液正的Im χ((2))光谱的另一种解释可能是对净OH跃迁偶极矩有正向贡献的水物种数量增加。无论其机制如何，随着盐浓度的增加，这种效应会变得更加明显。
