(583h) Adsorption of Na+, K+, and Ca2+ Ions and Their Hydration Structures on Charged Silica Surfaces: A Molecular Dynamics Study

When a charged surface emerges in an aqueous salt solution, ions in the solution accumulate or deplete from surface, forming the so-called electrical double layer structure (EDL). Silica, one of the most common minerals on earth, can interact with aqueous solution containing various salts, such as Na⁺, K⁺, and Ca²⁺. This interaction has important implications for numerous natural and engineering applications, including hydrocarbon recovery, nanoparticle stability, and gas storage operation, etc.

The importance of EDL had led to numerous studies. The best-known model for EDL structure is Gouy-Chapman theory using the Poisson-Boltzmann (PB) equation, where ions are considered as point charges and water is considered as a continuum. However, surface characteristics are not accounted for in PB equation. Additionally, PB equation becomes less accurate at elevated salt concentrations or with multivalent ions. By using molecular simulations, Bourg et al. (Journal of colloid and interface science 2011, 360 (2), 701-715) investigated the EDL structure on clay surface contacting with mixed NaCl-CaCl₂ solutions. They concluded that the positions of second and third adsorption planes are independent of ion type. In contrast to clay minerals where the surface charge stems from ion exchanges, the deprotonation of silica leads to a highly localized and heterogeneous surface charge distributions. To the best of our knowledge, the impact of silica surface charge heterogeneity on the adsorption and hydration structures of different ion types still remains unclear.

This study employs molecular simulations to explore the adsorption and hydration structures of Na⁺, K⁺, and Ca²⁺ in the vicinity of silica surfaces with heterogeneous surface charge. Two surfaces are specifically designed with the same surface charge density (–0.065 C/m²) but different charge distribution: one surface contains isolated deprotonated OH sites (O_i^-), while the other surface features adjacent deprotonated OH sites (O_a^-). Ions accumulate strongly close to the two surfaces with layered structures. Na⁺ and K⁺ show distinct adsorption behaviors, while Ca²⁺ adsorption is independent of the surface characteristics. Notably, Na⁺ and K⁺ can easily penetrate into the surface with O_a^- but with limited penetration into the surface with O_i^-. However, water molecules strongly hydrate Ca²⁺ ions with specific configurations, which prevents its penetration into the two surfaces, regardless of the surface charge distribution. Furthermore, the ion adsorption and its hydration structure in one adsorption layer can influence the adsorption amount of ion in the adjacent layer. The adsorption of Ca²⁺ as inner-sphere surface complexes (ISSC) and its corresponding hydration shell hinder the formation of outer-sphere surface complexes (OSSC).

Our study elucidates the effect of heterogeneity of silica surface charges on the adsorption and hydration structures of Na⁺, K⁺, and Ca²⁺. The fundamental understanding from this work can provide important insights into ion-water-silica interactions and enhance understanding about the effect of silica charge heterogeneity on the EDL structures from a molecular perspective.