(675e) First Principles Thermodynamics of Amorphous Silica

Silica nanoparticles have a wide range of potential applications including drug delivery, imaging, electronics, and catalysis. These applications typically utilize large systems of nanoparticles, thus their collective properties are important for optimizing system properties for specific applications. A detailed description of the amorphous silica surface at the atomic level is crucial to developing a complete understanding of the interactions between silica nanoparticles. While properties of silica surfaces have been heavily investigated over past years, an accurate atomistic description is lacking due to the complexity of the amorphous surface.

We have used a novel approach to obtain a detailed atomistic description of the amorphous silica surface, with a central focus on thermodynamic properties of surface silanol groups. First, we constructed model silica slabs via thermal annealing with reactive molecular dynamics and fully relaxed a number of candidate surfaces with accurate density functional theory calculations. We then dehydroxylated the surface by systematically removing neighboring pairs of silanol groups and calculated the energy change associated with each step. We used the resulting energetics in a thermodynamic model to compute the dehydroxylation temperature for each pair of silanols. Both the calculated total silanol concentration as a function of temperature and the relative concentrations of the different silanol types (isolated, geminal, vicinal) are in good agreement with experimental observations, validating our model.