Understanding the structure and reactivity of quartz–water interfaces is critical for a wide spectrum of applications in geological, environmental, and biological sciences. Here, we were motivated by an inability to model recent experimental results accurately to model a set of defective quartz surfaces having various surface silicon (Si) vacancies. We first used density functional tight-binding (DFTB)-based molecular dynamics to simulate a series of quartz (101) surfaces having different types and densities of surface defects in water and compared them with the structures determined by X-ray reflectivity. We found that the rotation of surface [SiO<sub>4</sub>] tetrahedra near Si vacancies can lead to outward displacements of Si atoms similar to those observed in the experiments. The presence of additional surface Si vacancies caused inward relaxations of terminal oxygens through the formation of hydrogen bonds. Second, we used DFTB-Plumed metadynamic simulations to calculate the deprotonation energies from various surface ≡Si–OH groups that are near the Si defect sites. Preliminary results indicated that the deprotonation is a highly dynamic process that involves participation of interfacial water molecules and other surface hydroxyl groups. Approximately two to three collective variables were needed in order to understand the deprotonation mechanisms. The calculated pKa values will be compared with experimental data and the results are expected to show that defective surfaces can have a major impact on deprotonation energies at the quartz/water interface.

Understanding interfacial structure and protonation at the quartz/water interface by DFTB simulations

ACS Spring 2022

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