Adapa Sai, Swamy Dhananjay R, Kancharla Samhitha, Pradhan Swastik, Malani Ateeque
Department of Chemical Engineering , Indian Institute of Technology Bombay , Mumbai 400076 , India.
Department of Chemical Engineering , Institute of Chemical Technology , Mumbai 400019 , India.
Langmuir. 2018 Dec 4;34(48):14472-14488. doi: 10.1021/acs.langmuir.8b01128. Epub 2018 Nov 15.
Understanding solid-water(vapor) interfacial systems is relevant for both industrial and academic scenarios for their presence in wide areas ranging from tribology to geochemistry. Using grand canonical Monte Carlo simulations, we have investigated the role of monovalent (lithium, Li; sodium, Na; and potassium, K) and divalent (magnesium, Mg; calcium, Ca) cations on the structure and adsorption behavior of water on mica surface. The water density adjacent to the surface exhibits (a) oscillations due to hydration of surface cations (interfacial layer), (b) followed by a thick liquidlike layer. The thickness of the interfacial layer is strongly dependent on the hydration shell size and hydration energy of surface ions. Water molecules immediately next to the surface (contact layers) adsorb on ditrigonal (hexagonal) cavities of mica surface and form an ordered structure. The Li, Na, Mg, and Ca surface ions are coadsorbed with water molecules on the ditrigonal cavities due to their smaller hydration shell. Majority of water molecules in the second contact layer hydrate the surface ions and, together with the rest of the water molecules, form hydrogen bonds among themselves. The structure of the water molecules in the third and subsequent layer is random and more bulk liquidlike, except those molecules that hydrate the surface ions. The adsorption isotherm of water on all ion-exposed mica surface exhibits three regimes: (a) an initial rapid increase in water loading for relative vapor pressure ( p/ p) ≤0.2 due to hydration of surface ions; (b) followed by a linear increase between p/ p = 0.2 and 0.7, where the hydrogen bond formation between the water molecules of the interfacial layer occurs; and (c) exponential growth beyond p/ p = 0.7 due to thickening of the liquidlike layer. The water loading on divalent-ion-exposed mica surface is higher compared to the monovalent ions case. Although the divalent ions have higher hydration energy, the fraction of water molecules hydrating the surface ions is less compared to nonhydrating water molecules. We found that ion hydration energy and size of hydration shell play a crucial role in their structure adjacent to mica surface. At lower water loadings, the surface ions form a hydration shell with surface bridging oxygens, whereas at higher water content, the hydration preference is shifted toward mobile water molecules. The detailed understanding obtained from this work will be useful in identifying the role of ions in cloud formation, nanotribological studies, and activities of biological molecules and catalysts.
理解固 - 水（蒸汽）界面系统在工业和学术场景中都具有重要意义，因为它们广泛存在于从摩擦学到地球化学等众多领域。通过巨正则蒙特卡罗模拟，我们研究了单价阳离子（锂，Li；钠，Na；钾，K）和二价阳离子（镁，Mg；钙，Ca）对云母表面水的结构和吸附行为的作用。靠近表面的水密度呈现出：（a）由于表面阳离子水化（界面层）而产生的振荡，（b）随后是一个较厚的类液态层。界面层的厚度强烈依赖于表面离子的水合壳大小和水合能。紧邻表面的水分子（接触层）吸附在云母表面的双三角（六边形）空洞上并形成有序结构。Li、Na、Mg和Ca表面离子由于其较小的水合壳而与水分子共吸附在双三角空洞上。第二接触层中的大多数水分子使表面离子水化，并与其余水分子一起在它们之间形成氢键。除了使表面离子水化的那些分子外，第三层及后续层中的水分子结构是随机的且更像 bulk 液体。水在所有离子暴露的云母表面上的吸附等温线呈现出三个阶段：（a）对于相对蒸汽压（p / p）≤0.2，由于表面离子水化，水负载量最初快速增加；（b）随后在p / p = 0.2和0.7之间线性增加，此时界面层水分子之间形成氢键；（c）超过p / p = 0.7后呈指数增长，这是由于类液态层变厚。与单价离子情况相比，二价离子暴露的云母表面上的水负载量更高。尽管二价离子具有更高的水合能，但与非水化水分子相比，使表面离子水化的水分子比例较小。我们发现离子水合能和水合壳大小在它们与云母表面相邻处的结构中起着关键作用。在较低水负载量下，表面离子与表面桥连氧形成水合壳，而在较高水含量下，水合偏好转向移动的水分子。从这项工作中获得的详细理解将有助于确定离子在云形成、纳米摩擦学研究以及生物分子和催化剂活性中的作用。
