Effect of nanoconfinement and pore geometry on point of zero charge in synthesized mesoporous siliceous materials

Abstract

Fluid-rock interactions may differ in mesopores (2–50 ​nm diameter) compared to macropores (>50 ​nm diameter). This study evaluates the influence of nano-confinement and pore geometry on point of zero charge (PZC) and hence surface charge and adsorption. PZC was determined for a suite of synthetic mesoporous amorphous silica materials and compared to the PZC of macroporous and non-porous amorphous silica. Reactive molecular dynamics simulations using ReaxFF determined surface equilibrium constants (pK) and clarified the large range of PZC values reported for macroporous and non-porous amorphous silica. Potentiometric titrations measured PZC and pK values of three mesoporous amorphous silica materials (SBA-15, SBA-16, and MCM-41) possessing a range of pore diameters (∼4–13 ​nm) and distinct pore geometries (cylindrical pores in a hexagonal lattice, spherical pores in a body-centered-cubic lattice, and hexagonal pores arranged in a hexagonal lattice); fluids were pre-saturated with silica to inhibit reaction with pore walls. Results are integrated with data from more than 150 published experimental studies. Neither nano-confinement nor pore geometry affects the PZC of the mesoporous amorphous silica materials. These results have implications for adsorption chemistry under confinement in mesoporous siliceous natural systems such as shales.

Keywords

• Mesoporous silica
• Point of zero charge
• MCM-41
• SBA-15
• SBA-16
• ReaxFF molecular dynamics