Herbert Barthel

Surface interactions of dimethylsiloxy group-modified fumed silica

Fumed silica is widely used in industry as a rheological additive. Produced by a flame process, fumed silica consists of aggregates of hydrophilic primary particles. The thickening effect is related to particle-particle interactions leading to agglomerates and particle networks. These particle-particle interactions depend heavily on the liquid medium. Hydrophilic silica, whose surface is covered by silanol groups, shows greatest thickening in low polarity systems owing to strong hydrogen bonds. In polar media, wetting of the hydrophilic surface weakens particle interactions and breaks down thickening. In contrast, silylated hydrophobic silica is an effective rheological additive in high polar media. This effect is related to hydrophobic interactions between silica particles. Additionally, silylation of the silica surface by dimethylsiloxy groups results in greater thickening than silylation by trimethylsiloxy groups. Gas adsorption data reveal particle-particle interactions arising from interpenetration of the polymer-like silylation layer.

Particle Sizes of Fumed Silica

Fumed silica is a synthetic amorphous silicon dioxide produced by burning silicon tetrachloride in an oxygen-hydrogen flame. Surface areas range from 50-400 m 2 /g. Using particle sizing techniques, fumed silica shows micron sized particles leading to surface areas markedly lower than expected. Fumed silica appears as a fluffy solid with bulk densities down to 0.03 g/cm 3 , being invariant over the wide range of surface areas. Attempts to relate the variation of the surface area directly to the performance of fumed silica in technical applications, such as its thickening efficiency in fluids, mainly fail and remain ambiguous.

Fumed Silica — Rheological Additive for Adhesives, Resins, and Paints

Fumed silica, a synthetic silicon dioxide, is a powerful rheological additive for resins and paints to introduce thixotropy or even a yield point. The rheological effectiveness of fumed silica is based on its ability to form percolating networks which immobilize large volumes of liquid. By a combination of advanced rheological experiments, spectroscopical investigations, and quantum chemical calculations it could be demonstrated that the formation and stability of the silica network is strongly influenced by particle-resin interactions. The results can be used to develop comprehensive models, which explain the rheological performance of different grades of fumed silica in different resins.

Influence of specific surface area of pyrogenic silicas on their heat of immersion in water and on their surface properties assessed using inverse gas chromatography.

The heat of immersion and the surface properties of pyrogenic silicas were examined using microcalorimetry and inverse gas chromatography at finite concentration (FC-IGC). The microcalorimetric measurements showed a regular decrease of the heat of immersion, in water, per area unit with an increase in the surface areas. The desorption isotherms were established using FC-IGC. It is observed that BET constant value goes through a minimum, whereas the shapes of distribution function of the adsorption energies of isopropanol are changing. All results could be interpreted using a model of formation of the pyrogenic silica in the flame, which explains the change of surface functionality and geometry occurring around 200 m2/g.

Evolution of the surface polar character of pyrogenic silicas, with their grafting ratios by dimethylchlorosilane, studied by microcalorimetry.

The interactions of water, hexamethyldisiloxane, and dodecane with pyrogenic silica samples, modified by a controlled partial silylation with dimethyldichlorosilane, were studied by microcalorimetry and wettability measurements. The samples, having a coverage ratio lower than dimethylsilyl (DMS) monolayer capacity ( approximately 2.6 DMS/nm(2)), show a regular and linear decrease of their heat of immersion into water with the coverage ratio and correlate with the increase of residual silanol groups. Two critical coverage ratios were evidenced at about 25 and 50% of the DMS monolayer capacity, the grafted silica remaining hydrophilic, below 25% being strongly hydrophobic beyond. The heat of immersion into hexamethyldisiloxane decreases until 50% of the DMS monolayer whereas that of dodecane remains independent of the grafting ratio. This study demonstrates that the water/residual free silica surface plays the main role in the stabilization of the W/O Pickerings emulsions.

Influence of poly(methyl methacrylate) impregnation ratio on the surface properties of fumed silica and on the glassy temperature of poly(methyl methacrylate) using inverse gas chromatographic analysis

The surface properties of poly(methyl methacrylate) (PMMA) impregnated fumed silicas, in a large range of impregnation ratios, were examined using inverse gas chromatography at infinite dilution. It was observed that the dispersive component gamma(s)d does not decrease monotonously with the impregnation ratio. Two critical coverage ratios were evidenced corresponding at first to the shielding of the most energetic sites and then to the achievement of total coverage of the silica surface. The influence of the coverage ratio on the glassy temperature (tg) of the adsorbed PMMA was also studied, which was evidenced down to a very low coverage ratio (1 monomer unit/nm2).

Fumed silica synthesis: from molecules, protoparticles and primary particles to aggregates and agglomerates

Basing on quantum chemical simulations (semi-empirical methode PM3) and vibration spectroscopy (Infrared (IR) and inelastic neutron scattering (INS)) and using other experimental data, a model of the formation of fumed silica particles is proposed, leading from initial molecules, via protoparticles and primary particles up to aggregates and agglomerates. Also, defintions of products in the flame process steps are suggested, from a chemical point of view. This process may be described as a sequence of states: Molecules => (molecular clusters = protoparticles) => primary particles => aggregates => agglomerates.

Fumed silica synthesis: influence of small molecules on the particle formation process

Burning silicon tetrachloride in an oxygen-hydrogen flame produces fumed silica. This process is known for at least 50 years [1-5], but some important details are still uncertain. We would like to study several starting steps of fumed silica synthesis on the way from molecules to products. To do this we have performed quantum-level simulations of protoparticle and primary particle formation, from silicon dioxide molecules. Additionally, we have simulated the behavior of silica clusters in the presence of small molecules like water and hydrochloric acid. The reaction of silicon dioxide molecules leads to a silica cluster, which is covered with chemicaly highly active sites of one-coordinated oxygen atoms and three-coordinated silicon atoms. These clusters interact together and produce silica bulk like quartz glass. Reaction with water terminates the silica particle surface and leads to a complicated structure of the particle surfaces. The hydroxyl shell protects the particle body against the increase in particle size, but leads to aggregate and agglomerate formation.

Intermolecular interactions of polydimethylsiloxane oligomers with hydroxylated and silylated fumed silica

To gain a better microscopic understanding of the intermolecular interactions between fumed silica particles and polydimethylsiloxane (PDMS) polymers, a quantum-chemical modelling in the framework of modified semiempirical AM I and PM3 methods has been performed for a series of superclusters simulating fragments of the real particle surface interacting with five-member PDMS oligomers. Impacts of chemical composition and structural configuration of both substrates and oligomers on the adsorption of the latter has been studied.

Trimethylchlorosilane modified silica surfaces: characterization by inverse gas chromatography using PDMS oligomers as probes

Inverse gas chromatography (IGC), at infinite dilution conditions, is a very sensitive method for the monitoring of changes in the surface properties of divided solids submitted to thermal or chemical treatments. For instance, the evaluation of the surface activity of fumed silica samples, prepared by controlled silylation, is a key for the tailoring of silicas used as fillers in the (polydimethylsiloxane) PDMS elastomer industry. The aim of the present work is to demonstrate how IGC measurements, using PDMS oligomers as specific molecular probes, are suitable to illustrate the strong influence of the degree of silylation upon the surface properties of a fumed silica.