Role of Mono- and Divalent Surface Cations on the Structure and Adsorption Behavior of Water on Mica Surface

Abstract

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, Mg2+; calcium, Ca2+) 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+, Mg2+, and Ca2+ 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) ≤0.2 due to hydration of surface ions; (b) followed by a linear increase between p/p 0 = 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 = 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.

Ion-Specific and pH-Dependent Hydration of Mica–Electrolyte Interfaces

“…Despite this pronounced positional order, water molecules in the second and third layers display a much weaker preferential orientation (Figure 4e), as also reported elsewhere. 69 In addition to decreasing orientational order, the maxima in the water density are much less pronounced and broader for Cs + (Figure 4c). Thus, adsorbed Cs + ions disrupt both the orientational and the positional order of interfacial water beyond the first water layer.…”

An investigation on hygroscopic properties of 15 black carbon (BC)-containing particles from different carbon sources: roles of organic and inorganic components

“…The observed hysteresis effect of herbal BCPs and soot likely stemmed from their relatively high contents of OC and/or dissolved minerals (such as wheat and household soot). Sorbing water molecules could cause strong and irreversible hydration of organic acids (Petters et al, 2017) and dissolution or phase change of minerals (Adapa et al, 2018), consequently leading to hysteresis effect due to nonidentical structures of BCPs between the sorption and desorption branches even at the same RH. The negligible hysteresis effect observed on the two woody BCPs could be attributed to their very low contents of OC and dissolved minerals.…”

An investigation on hygroscopic properties of 15 black carbon (BC) from different carbon sources: Roles of organic and inorganic components

“…The observed hysteresis effect of herbal BC and soot likely stemmed from their relatively high contents of OC and/or dissolved minerals (such as wheat BC and Household soot). Sorbing water molecules could cause strong and irreversible hydration of organic acids (Petters et al, 2017) and dissolution or phase change of minerals (Adapa et al, 2018), consequently leading to hysteresis effect due to non-identical structures of BC between the sorption and desorption branches even at the same RH. The negligible hysteresis effect observed on the two woody BC could be attributed to their very low contents of OC and dissolved minerals.…”