Water-silanol interactions on the amorphous silica surface: A dispersion-corrected DFT investigation

Interpreting the interaction between the amorphous silica surface and water is a key step to understand its physicochemical properties. However, due to the flexibility of the structure and the distribution of types and geometries of the silanols one can obtain a broad range of interaction energies, as was already shown in former studies. This time we were able to investigate the distribution of silanols in relation with the calculated interaction energies, and thus designate those different silanols sites. Different dispersion-correction PBE-D (PBE + D2, D3, D3-BJ, TS, TS-HI, MBD, and FI-MBD) and meta-GGA SCAN methods has been used to quantify the interactions between water and defined silanols sites of the surface. All the methods give similar interaction energies, showing equivalent performances of SCAN and PBE-D methods to describe week interactions in our system. Following various routes, we identified a protocol of calculation in order to compute the interaction energies more accurately, taking into consideration the van der Waals (vdW) forces. Once different silanols are correctly described within the calculation level, it is clear that the geometry and environment determine its chemistry. Furthermore, the possible deformation of the silica surface affected by the water interaction is studied. The quantification of the interaction energies is important in order to correctly scale the results and confront with the experiments. With this information in mind, one can think about synthesis techniques that modify the silanols distribution of the silica surface in a way to tune its hydrophobicity and acidity.