The Effects of the Water Content, Acidity, Temperature and Alcohol Content on the Acidic Sol-Gel Polymerization of Tetraethoxysilane (TEOS) with Monte Carlo Simulation

Facile Preparation of Mechanically Robust and Functional Silica/Cellulose Nanofiber Gels Reinforced with Soluble Polysaccharides

Nanoporous silica gels feature extremely large specific surface areas and high porosities and are ideal candidates for adsorption-related processes, although they are commonly rather fragile. To overcome this obstacle, we developed a novel, completely solvent-free process to prepare mechanically robust CNF-reinforced silica nanocomposites via the incorporation of methylcellulose and starch. Significantly, the addition of starch was very promising and substantially increased the compressive strength while preserving the specific surface area of the gels. Moreover, different silanes were added to the sol/gel process to introduce in situ functionality to the CNF/silica hydrogels. Thereby, CNF/silica hydrogels bearing carboxyl groups and thiol groups were produced and tested as adsorber materials for heavy metals and dyes. The developed solvent-free sol/gel process yielded shapable 3D CNF/silica hydrogels with high mechanical strength; moreover, the introduction of chemical functionalities further widens the application scope of such materials.

Sodium hydroxide catalyzed monodispersed high surface area silica nanoparticles

Understanding of the synthesis kinetics and our ability to modulate medium conditions allowed us to generate nanoparticles via an ultra-fast process. The synthesis medium is kept quite simple with tetraethyl orthosilicate (TEOS) as precursor and 50% ethanol and sodium hydroxide catalyst. Synthesis is performed under gentle conditions at 20 °C for 20 min Long synthesis time and catalyst-associated drawbacks are most crucial in silica nanoparticle synthesis. We have addressed both these bottlenecks by replacing the conventional Stober catalyst, ammonium hydroxide, with sodium hydroxide. We have reduced the overall synthesis time from 20 to 1/3 h, ~60-fold decrease, and obtained highly monodispersed nanoparticles with 5-fold higher surface area than Stober particles. We have demonstrated that the developed NPs with ~3-fold higher silane can be used as efficient probes for biosensor applications.

Nearly monodisperse silica microparticles form in silicone (pre)elastomer mixtures

The formation of silica from a tetraalkoxysilane in a sol-gel process usually requires a highly polar, typically aqueous, medium that aids in the hydrolysis of the silane and leads to electrostatic stabilization of the growing silica particles. Formation of such silica particles in a hydrophobic medium is much more challenging. We report the formation of silica microspheres within silicone oils (hydroxy-terminated poly(dimethylsiloxane), HO-PDMS) during elastomer cure using atmospheric humidity in a one-pot and one-step synthesis. Using tetraethyl orthosilicate (TEOS) as both cross-linker and silica precursor, and aminopropyl-terminated dimethylsiloxane oligomer (AT-PDMS) as a catalytic surfactant, silica particles of low polydispersity formed near or at the air interface of the elastomer: the presence of a hydrophilic polymer, poly(ethylene glycol) (PEG), had an indirect effect on the particle formation, as it assisted with water transmission into the system, which resulted in particle formation over a wider range of parameters and facilitated silicone elastomer cure further away from the air interface. Depending on the relative humidity during cure, the sizes of particles presenting at the air interface varied from ~6-7 μm under ambient conditions (20-30%RH) to ~7-9 μm at high relative humidity (90% RH). The origin of the controlled particle synthesis is ascribed to the relative solubility of the catalyst and the efficiency of water permeation through the silicone matrix. AT-PDMS preferentially migrates to the air interface, as shown by ninhydrin staining, where it both catalyzes alkoxysilane hydrolysis and condensation, and stabilizes the growing silica particles prior to aggregation. Since reactions in the presence of this catalyst are slow, TEOS can migrate from within the pre-elastomer body to the interface faster than water can penetrate the silicone, such that the main locus of hydrolysis/condensation leading both to silica formation and elastomer cross-linking is at the air interface.