Physics Department, University of Texas at El Paso, United States.
Chemical and Biological Engineering Department, The State University of New York at Buffalo, United States.
Adv Colloid Interface Sci. 2017 Jun;244:90-99. doi: 10.1016/j.cis.2016.06.006. Epub 2016 Jun 14.
The distribution of ions in the vicinity of the air/water interface is still a matter of strong debate, with numerous calculations and experiments providing contradictory results, even regarding the preference of simple ions (such as H and OH) for interfacial or bulk water. When short range interactions between ions and the interface are assumed independent of bulk concentrations, if they are compatible with the surface tension data, they underpredict the experimental Zeta potentials by orders of magnitude. If they are compatible with Zeta potential data, they are in strong disagreement with surface tension experiments. It is suggested that these observations might be a result of the relatively low number of interfacial water molecules available to hydrate the ions and the competition between various ions for adsorption sites. Therefore, whereas at low bulk concentrations, the Structure-Breaking ions prefer the interface, at sufficiently large bulk concentrations the surface adsorptions of these ions become saturated, and their interfacial concentrations may become lower than in the bulk. Consequently, the total interactions of ions with the interface can be strongly attractive at low bulk concentrations, and less attractive (or even repulsive), at high concentrations. To model this effect, the interactions between ions and interface are taken into account via modified Langmuir adsorption expressions for OH and Cl, while the H ions are considered to be attached to any interfacial water molecule, even if the latter participate in the hydration of anions. The simple model of adsorption employed here is in agreement with both experiments on Zeta potential and on surface tension, and might reveal the conditions under which a given ion exhibits propensity for either the air/water interface, or for bulk water.
离子在空气/水界面附近的分布仍然是一个激烈争论的问题，许多计算和实验提供了相互矛盾的结果，甚至对于简单离子（如 H 和 OH）优先进入界面或体相水也存在争议。当假设离子与界面的短程相互作用与体相浓度无关时，如果它们与表面张力数据相兼容，那么它们会将实验 Zeta 电位预测低几个数量级。如果它们与 Zeta 电位数据相兼容，那么它们与表面张力实验强烈不一致。有人认为，这些观察结果可能是由于可用于水合离子的界面水分子数量相对较少，以及各种离子对吸附位的竞争。因此，尽管在低体相浓度下，结构破坏离子优先进入界面，但在足够大的体相浓度下，这些离子的表面吸附会达到饱和，其界面浓度可能会低于体相浓度。因此，离子与界面的总相互作用在低体相浓度下可能具有很强的吸引力，而在高浓度下吸引力较小（甚至可能具有排斥力）。为了模拟这种效应，通过对 OH 和 Cl 的 Langmuir 吸附表达式进行修正，考虑了离子与界面之间的相互作用，而 H 离子被认为附着在任何界面水分子上，即使后者参与阴离子的水合作用。这里采用的简单吸附模型与 Zeta 电位和表面张力实验都相吻合，并且可能揭示了给定离子表现出优先进入空气/水界面或体相水的条件。
