Monitoring Silane Sol-Gel Kinetics with In-Situ Optical Turbidity Scanning and Dynamic Light Scattering

Abstract

Organosilanes (e.g., R’-SiOR3) provide hydrophobic functionality in thin-film coatings, porous gels, and particles. Compared with tetraalkoxysilanes (SiOR4), organosilanes exhibit distinct reaction kinetics and assembly mechanisms arising from steric and electronic properties of the R’ group on the silicon atom. Here, the hydrolysis and condensation pathways of n-propyltrimethoxy silane (nPM) and a tri-fluorinated analog of nPM, 3,3,3-trifluoropropyl trimethoxy silane (3F), were investigated under aqueous conditions at pH 1.7, 2.0, 3.0, and 4.0. Prior to hydrolysis, 3F and nPM are insoluble in water and form a lens at the bottom (3F) or top (nPM) of the solutions. This phase separation was employed to follow reaction kinetics using a Turbiscan instrument to monitor hydrolysis through solubilization of the neat silane lens while simultaneously tracking condensation-induced turbidity throughout the bulk solution. Dynamic light scattering confirmed the silane condensation and particle aggregation processes reported by the turbidity scanning. Employing macroscopic phase separation of the starting reactants from the solvent further allows for control over the reaction kinetics, as the interfacial area can be readily controlled by reaction vessel geometry, namely by controlling the surface area to volume. In-situ turbidity scanning and dynamic light scattering revealed distinct reaction kinetics for nPM and 3F, attributable to the electron withdrawing and donating nature of the fluoro- and organo-side chains of 3F and nPM, respectively.

Multicomponent oxide microspheres with designed macroporosity (MICROSCAFS®): a customized platform for chemicals immobilization

“…S11†) show a longer induction period and a slower gelation process for a higher GPTMS/TEOS molar ratio (ST/50C/GPTMS+). Steric hindrance and the hydrophobic character from the relatively long glicidyloxypropyl chain of GPTMS may be the cause for slower condensation, 34,39 but also for higher tendency for phase separation, which results in the coarsening of the skeleton domains and larger pore sizes and volumes, typical of ST/50C/GPTMS+. Corroborating this finding, the hydrolyzate for ST/50C/GPTMS+ exhibits a slightly lower viscosity than the reference ST/50C.…”

Silica Nanoparticles Synthesized from 3,3,3-Propyl(trifluoro)trimethoxysilane or n-Propyltrimethoxysilane for Creating Superhydrophobic Surfaces

“…The trifluoropropyl moiety also exhibits greater steric constraints than the n -propyl moiety of n PM. We have previously reported on how these differences in side-group hydrophobicity, electronegativity, and steric bulk influenced the sol–gel kinetics for an unstirred aqueous reaction system . These differences in reaction kinetics arising from silane precursor properties are considered the dominating factors in controlling the size and shape of the particles generated during the condensation processin particular, the steric hindrance associated with the bulky CF 3 side chain of 3F is expected to result in a slower condensation process compared to n PM, which may favor the formation of more nuclei and smaller overall particles.…”

Sol-Gel Chemistry: From Molecule to Functional Materials

“…The same authors described an optical approach to follow the kinetic of the polycondensation process of organosilanes [2]. Instead of 29 Si NMR spectroscopy, which is commonly used, they turned to original and quick methods such as in situ optical turbidity scanning and dynamic light scattering.…”