Molecular dynamics simulation study of growth regimes during polycondensation,of silicic acid: From silica nanoparticles to porous gels

Molecular dynamics simulations based on a reactive force field with charge transfer were used to model the sol-gel synthesis of nanoporous silica gels in an aqueous environment. Three distinct growth regimes emerge, depending on the solvent-to-silica ratio: compact nanoparticles, percolated silica networks, and branched clusters. These growth regimes can be identified on the basis of distinctive structural features. In the case of compact particles, the radial distribution functions exhibit a broad maximum that coincides with the radius of gyration of the aggregates, whereas in continuous networks the radial distribution function increases steadily beyond the near-range structural features. Furthermore, these growth regimes can be distinguished on the basis of the concentrations of structural defects, such as dangling bonds and residual OH groups. The growth kinetics of individual regimes are characterized by different relative contributions of atomic diffusion to the overall aggregation rate. Finally, the resulting gel structures possess different mechanical stability, as can be assessed by quantifying the extents of structural collapse during simulated. supercritical drying.