Adsorption of ions at the solid - aqueous interface is the primary mechanism in fast biological processes to very slow geological transformations. Despite, little is known about role of ion charge, hydration energy and hydration structure on competitive adsorption of ions, their structure and coverage at the interface. In this report, we investigate the structure and adsorption behavior of monovalent (Rb+ and Na+) and divalent (Sr2+ and Mg2+) cations ranging from 0–4.5 M of bulk concentrations on the muscovite mica surface. Divalent ions have stronger adsorption strength compared to monovalent ions due higher charge. However, we observed counter-intuitive behavior of lesser adsorption of divalent cations compared to monovalent cations. Our detailed analysis reveals that hydration structure of divalent cations hinders their adsorption. Both, Na+ and Rb+ ions exhibits similar adsorption behavior, however, the adsorption mechanism of Na+ ions is different from Rb+ ions in terms of redistribution of the water molecules in their hydration shell. In addition, we observed surface mediated RbCl salting out behavior, which is absent in Na+ and divalent ions. We observed direct correlation in hydration energy of cations and their adsorption behavior. The obtained understanding will have tremendous impact in super-capacitors, nanotribology, colloidal chemistry and water purifications.

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.

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The swelling capacity and stability of clay play a crucial role in various areas ranging from cosmetics to oil extraction; hence change in their swelling behaviour after cation exchange with the surrounding medium is important for their efficient utilisation. Here we focus on understanding the role of different hydration properties of cation on the thermodynamics of clay swelling by water adsorption. We have used mica as the reference clay, Na$$^+$$ + , Li$$^+$$ + , and H$$^+$$ + ions as the interstitial cations, and performed grand canonical Monte Carlo simulations of water adsorption in mica pores (of widths $$d = 4-40$$ d = 4 - 40 Å). The disjoining pressure ($$\Pi$$ Π ), swelling free energy ($$\Delta \Omega ^{ex}$$ Δ Ω ex ), and several structural properties of confined water and ions were calculated to perform a thermodynamic analysis of the system. We expected higher water density in H-mica pores ($$\rho_{ \hbox{H}}$$ ρ H ) due to the smaller size of $$\hbox {H}^+$$ H + ions having higher hydration energy. However, the counter-intuitive trend of $$\rho _{\hbox{Li}}> \rho _{\hbox{Na}} > \rho_b$$ ρ Li > ρ Na > ρ b (bulk density) $$> \rho_{\hbox{H}}$$ > ρ H was observed due to adsorption energy, where the interaction of water with mica framework atoms was also found to be significant. All three mica systems exhibited oscillatory behaviour in the $$\Pi$$ Π and $$\Delta \Omega ^{ex}$$ Δ Ω ex profiles, diminishing to zero for $$d \ge 11$$ d ≥ 11 Å. The $$\Delta \Omega ^{ex}$$ Δ Ω ex for Na-mica is characterised by global minima at $$d=6 {\hbox {\AA}}$$ d = 6 Å   corresponding to crystalline swelling with significant and multiple barriers for crystalline swelling to osmotic swelling ($$d > 12$$ d > 12 Å). A shift in the location of global minima of $$\Delta \Omega ^{ex}$$ Δ Ω ex towards the higher d values and $$\Delta \Omega ^{ex}$$ Δ Ω ex becoming more repulsive is observed in the increasing order of hydration energy of $$\hbox {Na}^+$$ Na + , $$\hbox {Li}^+$$ Li + , and $$\hbox {H}^+$$ H + ions. The $$\Delta \Omega ^{ex} > 0$$ Δ Ω ex > 0 for all d in the H-mica system thus favours osmotic swelling. We found that the Na$$^+$$ + ions hydrate more surface oxygens, act as anchors, and hold the mica pore together (at smaller d), by sharing hydrating water with ions of the opposite side, forming an electrostatically connected mica-Na-water-Na-mica bridge. The Li$$^+$$ + ions do hydrate surface oxygen atoms, albeit in lesser numbers, and sharing of hydration shell with nearby Li$$^+$$ + ions is also minimum. Hydration by surface atoms and water sharing, both, are minimum in the H$$^+$$ + ion case, as they are mostly present in the center of the pore as diffusive ions, thus exerting a consistent osmotic pressure on the mica frameworks, favouring swelling.

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The swelling capacity and stability of clay play a crucial role in various areas ranging from cosmetics to oil extraction; hence change in their swelling behavior after cation exchange with the surrounding medium is important for their efficient utilization. Here we focus on understanding the role of different hydration properties of cation on the thermodynamics of clay swelling by water adsorption. We have used mica as the reference clay, Na+, Li+, and H+ ions as the interstitial cations, and performed grand canonical Monte Carlo simulations of water adsorption in mica pores (of widths d = 4−40 Å). We found that water adsorption in Na-, Li- and H-mica pores is qualitatively similar; however significant quantitative differences are observed, especially at smaller d. Higher water density in H-mica pores (ρH) was expected due to the smaller size of H+ ions having higher hydration energy. However, a counter-intuitive trend of ρLi > ρNa > ρb (bulk density) > ρH was observed due to adsorption energy where the contribution of mica framework atoms was also found to be significant. The disjoining pressure (Π), swelling free energy (∆Ωex), and several structural properties of confined water and ions were calculated to perform thermodynamic analysis of the system. Our detailed calculations have captured the structural evolution of ions and water, especially the transitions from mono- to bi- and multilayer as a function of d. Oscillatory behavior in the Π and ∆Ωex profiles with diminishing to zero for d ≥ 11 Å is observed in all three mica systems. A shift in the location of global minima of ∆Ωex towards the higher d values and ∆Ωex becoming more repulsive is observed in the increasing order of hydration energy of Na+, Li+, and H + ions. The ∆Ωex for Na-mica is characterized by global minima at d = 6 Å corresponding to crystalline swelling, a significant barrier for crystalline swelling from d = 6 to 9 Å and lower for crystalline (d = 9 Å) to osmotic swelling (d > 12 Å). For Li-mica, the energy barrier for crystalline to osmotic swelling is lesser compared to the Na-mica system, whereas for H-mica the ∆Ωex > 0 for all d thus favoring osmotic swelling. We found that the hydration of cations by surface atoms plays a key role in the thermodynamics of clay swelling. The Na+ ions hydrate more number of surface oxygens, act as anchors, and hold the mica pore at d = 6 Å by sharing hydrating water with ions of opposite sides forming an electrostatically connected bridge of mica Na-water-Na-mica. The Li+ ions do hydrate surface oxygen atoms, albeit lesser number and sharing of hydration shell with nearby Li+ ions is also minimum. Both, surface hydration and water sharing, is minimum in the H+ ion case, as they are mostly present in the center of the pore as diffusive ions; thus exerting a consistent osmotic pressure on the mica frameworks, favoring swelling.

<div>Clay minerals in contact with aqueous bulk reservoir undergo a geological transformation of swelling or shrinking by exchanging interstitial cations. For geological applications, it is crucial to understand the stability of these layered materials. Here, we demonstrate that a sub-angstrom change in the interstitial cation size with similar hydration characteristics is enough to destabilize the optimum spacing of layered materials. We used molecular simulations to investigate the stability of water layers in the K-, Rb-, and Cs-mica pores. We find that ±0.1 Å, change in the size of interstitial cation - from Rb⁺ to K⁺ or Cs⁺ ion - leads to -15 to 5 % change in equilibrium loading of adsorbed water and 2 to 35 % change in interlayer spacing. Our thermodynamic analysis reveals an intricate interplay between enthalpic and entropic contributions caused by the structural change of water in the pores due to the hydration of interstitial cations. The understanding from this work has direct implications in designing clay swelling inhibitors in the oil/gas recovery using fracking and sealing materials for radioactive waste. </div>

<div>Clay minerals in contact with aqueous bulk reservoir undergo a geological transformation of swelling or shrinking by exchanging interstitial cations. For geological applications, it is crucial to understand the stability of these layered materials. Here, we demonstrate that a sub-angstrom change in the interstitial cation size with similar hydration characteristics is enough to destabilize the optimum spacing of layered materials. We used molecular simulations to investigate the stability of water layers in the K-, Rb-, and Cs-mica pores. We find that ±0.1 Å, change in the size of interstitial cation - from Rb⁺ to K⁺ or Cs⁺ ion - leads to -15 to 5 % change in equilibrium loading of adsorbed water and 2 to 35 % change in interlayer spacing. Our thermodynamic analysis reveals an intricate interplay between enthalpic and entropic contributions caused by the structural change of water in the pores due to the hydration of interstitial cations. The understanding from this work has direct implications in designing clay swelling inhibitors in the oil/gas recovery using fracking and sealing materials for radioactive waste. </div>