ReaxFF Molecular Dynamics Simulations of Hydroxylation Kinetics for Amorphous and Nano-Silica Structure, and Its Relations with Atomic Strain Energy

We performed reactive force field molecular dynamics simulation to observe the hydrolysis reactions between water molecules and locally strained SiO₂ geometry. We optimized the force field from J. Fogarty et al. 2010, to more accurately describe the hydroxylation reaction barrier for strained and nonstrained Si - O structures, which are about 20 and 30 kcal/mol, respectively. After optimization, energy barrier for the hydroxylation shows a good agreement with DFT data. The observation of silanol formation at the high-strain region of a silica nanorod also supports the concept that the adsorption of water molecule: hydroxyl formation favors the geometry with higher strain energy. In addition, we found three distinct hydroxylation paths - H₃O⁺ formation reaction from the adsorbed water, proton donation from H₃O⁺, and the direct dissociation of the adsorbed water molecule. Because water molecules and their hydrogen bond network behave differently with respect to temperature ranges, silanol formation is also affected by the temperature. The formation of surface hydroxyl in an amorphous silica double slit displays a similar tendency: SiOH formation prefers high-strain sites. Silanol formation related with H₃O⁺ formation and dissociation is observed in hydroxylation of amorphous SiO₂, similar to the results from silica nano wire simulation. These results are particularly relevant to the tribological characteristics of surfaces, enabling the prediction of the attachment site of the lubrication film on silica surfaces with a locally strained geometry.