Water-based functionalization of mesoporous siliceous materials, Part 1: Morphology and stability of grafted 3-aminopropyltriethoxysilane

Surface modification of mesoporous biogenic silica (rice husk ash) by aqueous and ethanol-based 3-aminopropyltriethoxysilane (APTES) grafting solutions has been investigated using 29Si NMR, ESI-MS, TGA, surface-sensitive NH2 titration, and DFT method. Prior to grafting, a rapid formation of ladder-like aminosilane oligomers has been observed in both solvents limiting the grafting time for the mostly desired formation of a monomolecular organosilane layer to few minutes. An excess of water yielded a long-term stable equilibrium between the oligomerized APTES species within the grafting solution promising a reproducible surface modification. After curing at 120 °C, washing studies showed that these oligomeric aminosilane clusters have been successfully grafted but yielding a heterogeneous surface morphology with even pore blocking rather than a dense, uniform aminosilane monolayer.