(172f) Investigating Mineral-Water Interfacial Dynamics through Zeta Potential Measurements with Mono and Divalent Salts

The ion structure, distribution, and interfacial dynamics at calcite-water interface modulate in various interfacial geological processes such as diffusion, deposition, and dissolution of mineral ions under nanoconfined geometries. Such knowledge is also essential to gain a better understanding of the pressure solution creep and failure at intergranular and intercrystalline boundaries. However, the development of surface charge on calcite in aqueous solutions is not yet fully understood and is more complex than the simple protonation and deprotonation reactions occurring at surface sites. This complexity arises mainly because the calcite is soluble in aqueous solution, allowing ions (Ca²⁺ and CO₃^2-) from the surface to dynamically dissociate into the solution or be precipitated/deposited on the surface. Additionally, there are other lattice sites, such as the protonated anion surface group (CO₃H^o) and hydroxylated cation surface group (CaOH^o), that can react with OH^- or H⁺. These reactions determine the relative abundance of positively and negatively charged sites on the calcite surface, thereby controlling its zeta potential. Employing a custom-designed streaming potential apparatus, we systematically characterized the zeta potentials (ζ) of single-crystal calcite ([1014] plane) surfaces as a function of pH, ion type (monovalent: NaCl vs. divalent: CaCl₂) and ion concentration by applying the modified Helmholtz-Smoluchowski equation. Unlike the ζ measurements taken with the calcite surface in NaCl solution, which show a monotonic decrease with pH, when the type of ions in the solution was switched from monovalent to divalent ions, the pH dependence of ζ became non-monotonic: Initially, it decreases in the same manner as in the case of monovalent ions; however, it then increases with pH, and this charge inversion occurs at the isoelectric point (IEP) of calcite (~8.5). We observe that this charge reversal effect and the surface speciation, dominated by hydrolyzed calcium species on mineral surfaces (both calcite and mica), may require a threshold concentration between 1.5 and 2 of pCa²⁺. To complement the effect of cations (Ca²⁺), the influence of anions (CO₃^2-) on the surface charge and dissolution dynamics of calcite is further explored using Na₂CO₃ solutions. We anticipate that these findings will elucidate the complex interplay among surface chemistry, pH, and ionic strength in dictating mineral-water interfacial behavior. By bridging the fields of surface and interface science, chemical engineering, and geophysics, this study provides a foundation for understanding the fundamental mechanisms that shape mineral-water interfaces and their far-reaching implications for pressure solution dynamics. These insights not only advance our knowledge of interfacial phenomena but also open new avenues for exploring the role of mineral-water interactions in various geological processes and engineering applications.