(515e) Molecular Dynamics Studies on the Effect of Silica Surface Characteristics on Ion Adsorption and Their Hydration Structures

Ion adsorption on charged surfaces is a ubiquitous phenomenon of natural and engineered systems containing water and colloids, nanoparticles, or nanopores, which plays crucial roles in nanofiltration, colloidal assembly, and biological processes, etc. In subsurface porous media, silica is one of the most abundant rocks, which often carries negative charges due to the deprotonation of surface hydroxyl (-OH) groups. Ion adsorption on negatively-charged silica surfaces and their hydration structures at elevated salt concentrations are in the heart of various energy and environmental engineering applications, such as hydrocarbon recovery, CO₂ and H₂ storage, and hydrogeology.

The adsorbed ions on perfectly-smooth structureless charged surfaces are often conceptually divided into two regions, the so-called electric double layer: a Stern layer in which ions adsorb as inner- and outer-sphere surface complexes (ISSC, OSSC); a diffuse layer which screens the remaining uncompensated surface charge. Classical electrical double layer theories based on mean-field Poisson-Boltzmann equation are only valid for dilute solutions, arising from the negligence of ion size and ion-ion correlation effects. In addition, implicit solvent (water) models are often used. Therefore, near surface ion hydration structures cannot be revealed from these theories. By using molecular simulations, Bourg et al. ( Journal of colloid and interface science 2011, 360 (2), 701-715) confirmed that surface counterions close to smectite surfaces can form ISSC, OSSC, and Diffuse species with zero, one and two full hydration shells, respectively. Unlike layer-type clay minerals, deprotonation of silica leads to a highly localized and heterogeneous surface charge distributions. To the best of our knowledge, the effect of silica surface charge heterogeneity on ion adsorption and their hydration structures still remains unclear.

In this work, molecular simulations are conducted to study Na⁺ and Cl^- adsorption and their hydration structures in the vicinity of silica surface at the salinity of 0.6 mol/L. The surface charge heterogeneity is represented by three adsorption sites, including adjacent sites (O_a^-), isolated sites (O_i^-), and hydroxyl groups (-OH). In line with Bourg et al., ions can form ISSC, OSSC, and Diffuse species. However, we find that ion adsorption exhibits distinct structures around different adsorption sites. As ISSC1 and ISSC2 species, Na⁺ ions exhibit preference for adsorbing around O_a^- than O_i^-, but Na⁺ ions are more likely to adsorb around O_i^- as OSSC species. -OH groups are the less favored sites for Na⁺ adsorption. On the contrary, the adsorption of Cl^- is mainly due to the tri-OH – Cl^- structure. Additionally, the ion pairs formed around O_a^- and O_i^- also contribute to Cl^- adsorption. Water molecules in the hydration shells of ions can form water-surface hydrogen bonds (HBs) and in-shell water-water HBs, which can facilitate the stabilization of adsorbed ions. The hydration structures of Na⁺ and Cl^- ions near silica surfaces are highly dependent on water-surface HB interactions, resulting in several preferred hydration spots in the first/second hydration shells. The direct electrostatic interactions between ions and surface and water-surface HB interactions collectively influence ion mobility near surface. Our study from molecular perspectives emphasizes the importance of heterogeneity of silica surface charges and surface functional group characteristics on ion adsorption and their hydration structures. Our work can shed important insights into the fundamental understanding about ion-water-silica interactions which have paramount importances in gas storage and geohydrology.