Understanding the structure and reactivity of quartz–water interfaces is critical for a wide range of applications in geological, environmental, and biological sciences. Here, we modeled the deprotonation energetics with an approximate density functional theory method on pristine and defective quartz (101) surfaces in 2 nm confined water film, following our previous work by comparing experimental to simulated quartz (101)/water interfaces [1]. We used density-functional tight-binding (DFTB)-based molecular dynamics combined with rare-event metadynamic simulations to calculate the deprotonation energies from various surface ≡Si–OH groups found on pristine and Si defect-rich quartz surfaces. Preliminary results indicated that the deprotonation is a highly dynamic process that involves participation of multiple interfacial water molecules and surface hydroxyl groups. The deprotonation of the outer and inner surface -OH groups on the pristine quartz surface showed two different pKa values where the inner -OH requires higher energy to be deprotonated. The -OH groups near the Si vacancy site required less energy for deprotonation, giving rise to lower pKavalues. The calculated one-dimensional potential of mean force (PMF) was reweighted to a two-dimensional PMF, to include additional collective variables, such as bond length and coordination number in order to understand the deprotonation mechanisms. The calculated pKa values are expected to show that defective surfaces can have a major impact on proton affinities at the quartz/water interface.

Understanding deprotonation at pristine and defective quartz/water interfaces with approximate density functional theory simulations

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