Composition in the Interface between Clay Mineral Surfaces and Divalent Cation Electrolytes

The interfacial free energy of a solid, which determines its adsorption properties, depends on interactions between the surface and the fluid. A change in surface composition can completely change the behavior of the solid. Decades of work have explored adsorption and its effects at solid–fluid interfaces from the macroscopic perspective and using molecular modeling, so the concept of the electric double layer (EDL) is well established in the community. However, direct, molecular level, experimental observations of the composition within the interface region, and its change with time and conditions, are not abundant. We used cryogenic X-ray photoelectron spectroscopy (cryoXPS) to observe the composition in the clay mineral–solution interface region as a function of bulk solution composition, on illite and chlorite in MgCl2 and CaCl2 electrolytes, over a range of concentrations (1–125 mM), in situ, on vitrified samples. These samples were prepared from very thin smears of centrifuged wet paste that were in...