Institute of Physics and Materials Science, University of Natural Resources and Life Sciences, Peter Jordan Straße 82, A-1190 Vienna, Austria.
Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szt. Gellért tér 4, H-1111 Budapest, Hungary.
J Phys Chem B. 2020 Nov 5;124(44):9884-9897. doi: 10.1021/acs.jpcb.0c05547. Epub 2020 Oct 21.
The surface tension of all aqueous alkali halide solutions is higher than that of pure water. According to the Gibbs adsorption equation, this indicates a net depletion of these ions in the interfacial region. However, simulations and experiments show that large, soft ions, such as I, can accumulate at the liquid/vapor interface. The presence of a loose hydration shell is usually considered to be the reason for this behavior. In this work, we perform computer simulations to characterize the liquid-vapor interface of aqueous alkali chloride and sodium halide solutions systematically, considering all ions from Li to Cs and from F to I. Using computational methods for the removal of surface fluctuations, we analyze the structure of the interface at a dramatically enhanced resolution, showing that the positive excess originates in the very first molecular layer and that the next 3-4 layers account for the net negative excess. With the help of a fictitious system with charge-inverted ion pairs, we also show that it is not possible to rationalize the surface affinity of ions in solutions in terms of the properties of anions and cations separately. Moreover, the surface excess is generally dominated by the smaller of the two ions.
所有水溶液中碱金属卤化物的表面张力均高于纯水。根据吉布斯吸附方程，这表明这些离子在界面区域的净消耗。然而，模拟和实验表明，大的软离子，如 I，可在液/气界面处聚集。通常认为疏松的水合壳层是这种行为的原因。在这项工作中，我们通过计算机模拟系统地研究了水溶液中碱金属氯化物和卤化钠溶液的液/气界面，考虑了从 Li 到 Cs 和从 F 到 I 的所有离子。我们使用去除表面波动的计算方法，以显著提高的分辨率分析界面结构，表明正超额源于最内层，接下来的 3-4 层解释了净负超额。借助于具有电荷反转的离子对的虚拟系统，我们还表明，不可能根据阴离子和阳离子的性质单独合理化溶液中离子的表面亲和力。此外，表面超额通常由两个离子中的较小者主导。
