Dispersions of hydrophilic fumed silica are investigated in a range of polar organic media. The silica forms stable, low-viscosity sols exhibiting shear thickening behavior in a host of liquids, including ethylene glycol and its oligomers and short-chain alcohols, such as n-propanol. In contrast, the silica flocculates into colloidal gels in other liquids, such as glycols with methyl end-caps and longer-chain alcohols. We suggest that there is a causal relationship between the hydrogen-bonding ability of the liquid and the colloidal microstructure observed. In strongly hydrogen-bonding liquids, a solvation layer is envisioned to form on the silica surface through hydrogen bonding between liquid molecules and surface silanol groups (Si-OH). This gives rise to short-range, non-DLVO repulsions ("solvation forces") which stabilize the silica particles. In contrast, in the case of liquids with limited hydrogen-bonding ability, silanols on adjacent silica particles are envisioned to interact directly by hydrogen bonding. This leads to particle flocculation and ultimately to gelation. Our study further fuels the debate regarding the existence of solvation forces in dispersions.

Microscopic dynamics of the interfacial layer and macroscopic mechanical properties of [3-(acryloxy)propyl]trimethoxysilane (APMS) at a polymer-silica interface were studied using NMR and three-point bending tests. Wide-line deuterium NMR studies of deuterium-enriched [3-(acryloxy)propyl]trimethoxysilane-d (APMS-d), showed that when chemically bonded to silica, the acrylic group of APMS moved rapidly at the interface with air, more slowly when coated with a poly(methyl methacrylate) (PMMA) overlayer, and slowest when copolymerized with methyl methacrylate. A two-component line shape was found for the coated composite and these were assigned to relative immobile and mobile surface groups. From three-point bend tests, the PMMA/glass-fiber composite, made from glass laminates treated with APMS, had almost two times the flexural strength of a composite made from glass laminates not treated with APMS. Electron micrographs of the fracture surfaces revealed that the untreated fibers were smooth, while those from the APMS-treated glass were rougher, indicating the presence of polymer at the interface.

The adsorption of 3-acryloxypropyltrimethoxysilane (APMS) onto silica and structures formed from the adsorption were studied using gravimetric analysis and NMR. APMS molecules were hydrolyzed at room temperature and then condensed at 110 °C with other APMS molecules or on a silica surface. After heat treatment at 110 °C, the alkyl groups of APMS from the bulk species or the immobilized species on the silica surface were investigated using 13C cross-polarization, magic-angle spinning (CP-MAS) NMR. 29Si MAS and CP-MAS NMR techniques showed that most of the APMS silanol groups were condensed to 27% T2 and 73% T3 species during heat treatment at 110 °C. The condensed APMS was then pyrolized at 600 °C. Most of the APMS alkyl groups had been replaced with oxygen during pyrolysis resulting in 21% Q3 and 79% Q4 species. The differences in the masses of APMS following the heat treatment steps (at 110 °C and 600 °C) made it possible to confirm the nature of the chemical species present. From the gravimetric measurements, there were 26% T2 and 74% T3 species after 110 °C heat treatment and 18% Q3 and 82% Q4 species after pyrolysis. The results from NMR and gravimetric analysis were in good agreement. A Langmuir isotherm was applied to estimate the saturated layer coverage of APMS, which was determined using the mass profile of APMS adsorbed on a high-surface-area fumed silica; it showed the saturated layer coverage to be about 0.6 mmol/100 m2.