Mesoscale model of the synthesis of periodic mesoporous benzene-silica

A coarse-grained (CG) model is developed to reproduce the early stages of the templated synthesis of periodic mesoporous organosilicas (PMO), focusing on benzene as the organic linker. Molecular dynamics simulations of hexadecyltrimethylammonium bromide (CTAB) surfactant in aqueous organosilicate solutions were performed to analyze the micelle formation, growth and aggregation during the synthesis of surfactant-templated PMOs. The CG model parameters were calibrated to reproduce radial density profiles of all-atom CTAB spherical micelles in a solution with benzenesilicates (BZS). Our simulations, with over a thousand surfactants, reproduced the experimental micelle aggregation, promoted and driven by the BZS moieties. The micelle sphere-to-rod transition and the subsequent formation of a hexagonally ordered mesophase were observed and characterized, displaying rod diameters (in the range 38-41 angstrom) very close to experimental estimates (38 angstrom). Furthermore, the addition of BZS to a CTAB aqueous solution with spherical micelles at equilibrium promoted the formation of prolate-shaped rods, in accordance with experiments. Subsequent removal of the BZS from the final PMO structure caused the system to revert to the original spherical micelles. In our simulations, the CTAB rods were formed above a 1:5 BZS/CTAB ratio while a ratio of 1:2 was found to be required to induce the hexagonal arrangement of the rods. Overall, this work reinforces the active and cooperative role of organosilicates in the formation of PMO materials.