(626h) Role of Surface Forces on Spatiotemporal Dynamics of Calcite Pressure Solution and Ion Adsorption Under Nanoconfinement

The structure and dynamics of electric double layer and hydration layer 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.

In this work, we investigate the interactions between single-crystal calcite and mica mineral surfaces (asymmetric case) and between calcite surfaces (symmetric case) as a function of separation distance (i.e., a degree of nanoconfinement and thus, pressure) as well as pH and electrolyte concentration using the surface forces apparatus (SFA) coupled with multiple beam interferometry (MBI). The force-distance profiles were obtained for the separation distances below 500 nm till the molecular contact due to the overlap of the electron clouds of the approaching surfaces. For both symmetric and asymmetric cases, the range and magnitude of electrostatic and hydration forces were found to be strongly dependent on solution pH and Ca²⁺ ion concentration. Furthermore, it was found that the dissolution of calcite under nanoconfinement could only be enhanced with pressures above a critical pressure value that was also dependent on the electrolyte concentration and pH. As a complementary study, the spatiotemporal dynamics of dissolution morphology was probed using in situ and ex situ atomic force microscope (AFM) technique. It was observed that the pressure (nanoconfinement)-induced dissolution of calcite pursued the formation of local, discontinuous nanodomains instead of having continuous, uniform dissolution throughout calcite surface. The time-dependent variation in root-mean-square roughness and fractal dimension was analyzed using the kinetical models to capture the quantitative trends of the pressure solution process. Zeta potential of calcite and mica surfaces were also evaluated at the conditions relevant to the SFA and AFM studies to gain insights into the role and mechanisms of surface complexation processes using a custom-made streaming potential apparatus (SPA). In combination of these unique surface characterization approaches, it was established that the valency and concentration of electrolytes strongly controlled the asymptotic behaviors of zeta potential versus pH curves.

We anticipate these findings may bring about a new perspective into the complex interfacial processes and dynamics of calcite-water-calcite and calcite-water-mica nanoconfinement geometries, which is of theoretical and practical value in geoscience, geochemistry, and surface science.