Variations in proton transfer pathways and energetics on pristine and defect-rich quartz surfaces in water: Insights into the bimodal acidities of quartz.

HYPOTHESIS: Understanding the mechanisms of proton transfer on quartz surfaces in water is critical for a range of processes in geochemical, environmental, and materials sciences. The wide range of surface acidities (>9 pKa units) found on the ubiquitous mineral quartz is caused by the structural variations of surface silanol groups. Molecular scale simulations provide essential tools for elucidating the origin of site-specific surface acidities. SIMULATIONS: We used density-functional tight-binding-based molecular dynamics combined with rare-event metadynamics simulations to probe the mechanisms of deprotonation reactions from ten representative surface silanol groups found on both pristine and defect-rich quartz (101) surfaces with Si vacancies. FINDINGS: The results show that deprotonation is a highly dynamic process where both the surface hydroxyls and bridging oxygen atoms serve as the proton acceptors, in addition to water. Deprotonation of embedded silanols through intrasurface proton transfer exhibited lower pKa values with less H-bond participation and higher energy barriers, suggesting a new mechanism to explain the bimodal acidity observed on quartz surface. Defect sites, recently shown to comprise a significant portion of the quartz (101) surface, diversify the coordination and local H-bonding environments of the surface silanols, changing both the deprotonation pathways and energetics, leading to a wider range of pKa values (2.4 to 11.5) than that observed on pristine quartz surface (10.4 and 12.1).

Cation hydration by confined water and framework-atoms have crucial role on thermodynamics of clay swelling.

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[Formula: see text], Li[Formula: see text], and H[Formula: see text] ions as the interstitial cations, and performed grand canonical Monte Carlo simulations of water adsorption in mica pores (of widths [Formula: see text] Å). The disjoining pressure ([Formula: see text]), swelling free energy ([Formula: see text]), 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 ([Formula: see text]) due to the smaller size of [Formula: see text] ions having higher hydration energy. However, the counter-intuitive trend of [Formula: see text] (bulk density) [Formula: see text] 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 [Formula: see text] and [Formula: see text] profiles, diminishing to zero for [Formula: see text] Å. The [Formula: see text] for Na-mica is characterised by global minima at [Formula: see text]   corresponding to crystalline swelling with significant and multiple barriers for crystalline swelling to osmotic swelling ([Formula: see text] Å). A shift in the location of global minima of [Formula: see text] towards the higher d values and [Formula: see text] becoming more repulsive is observed in the increasing order of hydration energy of [Formula: see text], [Formula: see text], and [Formula: see text] ions. The [Formula: see text] for all d in the H-mica system thus favours osmotic swelling. We found that the Na[Formula: see text] 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[Formula: see text] ions do hydrate surface oxygen atoms, albeit in lesser numbers, and sharing of hydration shell with nearby Li[Formula: see text] ions is also minimum. Hydration by surface atoms and water sharing, both, are minimum in the H[Formula: see text] 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.

Role of cation size on swelling pressure and free energy of mica pores.

The ion exchange capacity of clay plays an important role in many industrial applications ranging from radioactive waste disposal to cosmetics. However, swelling or shrinking of clay platelets due to water and ions adsorption in the interstitial zone is also a well-known phenomenon. For their applications, it is crucial to understand the stability of these layered materials, especially after exchange of interstitial ions with surrounding ions having different properties. Here, we probed the role of cation size on swelling pressure and free energy profile. We used molecular simulations to investigate the stability of mica pore, having K+, Rb+, and Cs+ ions. We performed a series of grand canonical Monte Carlo simulations at various pore widths. We probed water adsorption in mica pores from which disjoining pressure, grand potential (swelling free energy), and structural properties of confined water and ions were calculated. While the behavior of these three systems is similar qualitatively because of similar hydration properties of ions, significant differences are observed at the quantitative level due to changes in the hydration structure of cations. The global minimum in swelling free energy is found to be at the smaller pore widths (first minimum) for Rb- and K-mica and at bigger pore widths (second minimum) for Cs-mica pores. We find that ±0.1 Å change in the interstitial cation size leads to a -15 to 5% change in equilibrium loading of adsorbed water and -2 to 35% change in swelling. 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.

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

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/ p0) ≤0.2 due to hydration of surface ions; (b) followed by a linear increase between p/ p0 = 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/ p0 = 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.

Role of hydration energy and co-ions association on monovalent and divalent cations adsorption at mica-aqueous interface.

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.