Reactive molecular dynamics studies of DMMP adsorption and reactivity on amorphous silica surfaces

By using molecular dynamics (MD) computer simulations in conjunction with the ReaxFF reactive force fields, the interaction of dimethyl methylphosphonate (DMMP) with amorphous silica as a function of surface hydration was examined. Surface hydroxylation densities of 2.0, 3.0, 4.0, and 4.5 hydroxyl/nm ² were modeled. The amorphous silica surface used in our simulations is quantified structurally and compares well to experimental findings. At the higher OH densities, binding of DMMP to the hydroxylated silica was found to occur through a combination of van der Waals interactions and hydrogen bonding. In addition to these types of interactions, at the lower OH surface coverages, strong covalent bonding between the phosphonyl (P - O) oxygen of DMMP and 3-coordinate Si defects on the surface was observed. Finally, at extremely low hydroxyl coverages (2.0 nm^-2), DMMP fragmentation was found to occur. The binding energy of DMMP on amorphous silica with a hydroxyl density of 4.5 OH/nm² was calculated to be -4.7 kcal/mol. Addition of a water layer to the silica-supported DMMP system showed that water can displace and/or hydrolyze the adsorbed DMMP molecules. To validate the ReaxFF/MD findings, we performed MP2 and DFT quantum chemical studies of reactions predicted by the MD/ReaxFF by using small silica clusters. The quantum chemistry results support the MD/ReaxFF results, providing further verification of our findings and indicating the viability of ReaxFF/MD to study complex surface chemistry.