Spectroscopic Characterization of Salt Dissolution in Microhydrated Cluster Ions and at the Water / Vapor Interface

我们对盐在水介质中的溶解及其界面的理解还很不完整。例如，发生在海洋气溶胶表面的过程对大气化学有重大影响，但它们的表面形态很难用实验来探测，也很难用计算模型来可靠地预测。对模型系统的光谱研究提供了关键的分子水平的洞察力，以了解水/蒸汽界面的特定性质。在这里，我们建议结合两种强大且高度互补的光谱技术来研究大气相关离子，如Cl-， (NO3)-, (SO4)2-, Na+， Mg2+和(NH4)+，是如何在水表面水化的，这取决于盐浓度，反离子的性质，酸度和有机表面活性剂的存在。用中性碰撞离子散射光谱法测定二元和三元盐水溶液的水/蒸气界面的浓度深度分布，由此可以确定界面的热力学性质。低温离子阱振动光谱将用于研究较小的、尺寸选择的微水合离子、离子对和较大的离子配合物的结构和稳定性。这些方法的协同作用允许改进表面模型，因为气相团簇适用于更高水平的计算，并批判性地评估当前团簇/界面方法的通用性。我们的实验最终旨在揭示水/蒸汽界面的分子水平结构及其对大气相关过程的影响。

Our understanding of salt dissolution in aqueous media and at their interfaces remains far from complete. Processes occurring at marine aerosol surfaces, for example, have a major impact on atmospheric chemistry, but their surface speciation is challenging to probe experimentally and to predict reliably using computational models. Spectroscopic studies on model systems provide crucial molecular level insight into specific properties of the water/vapor interface. Here, we propose to combine two powerful and highly complementary spectroscopic techniques to study how atmospherically-relevant ions, like Cl-, (NO3)-, (SO4)2-, Na+, Mg2+ and (NH4)+, are hydrated at aqueous surfaces in dependence of the salt concentration, the nature of the counter ion, the acidity and the presence of organic surfactants. Concentration depth profiles at the water/vapor interface of binary and ternary aqueous salt solutions will be determined using Neutral Impact Collision Ion Scattering Spectroscopy from which interfacial thermodynamic properties can be determined. Cryogenic Ion Trap Vibrational Spectroscopy will be used to study the structure and stability of smaller, size-selected microhydrated ions, ion pairs and larger ion complexes. The synergy of these approaches allows improving surface models, because gas phase clusters are amenable to higher level calculations, and critically evaluating the generality of the present cluster/interface approach. Our experiments are ultimately aimed at shedding new light on the molecular level structure of the water/vapor interface and its implications for atmospherically-relevant processes.