Silica particles particle bridging

Particles grow by further polymerization of discrete colloidal particles by nucleation. The particle growth follows either Oswald s ripening (in which, smaller, unstable particles dissolve and precipitate on the surfaces of the relatively larger, insoluble, and stable particles) or condensation of intermediate size particles. Particle bridging plays an important role in this as well as the next stage of silica formation. 

Figure 1c shows the spectrum of aerosil that has been slurried in water and then dried at 100 C. This treatment initiates gel formation, so that the sample is no longer a chain of silica particles held together by electrostatic forces, but a porous network held together by siloxane linkages. The most obvious features in this spectrum are an increase in the water adsorption features at 3400 cm"" and 1632 cm". In addition, a band at 976 cm" is evident, that was much less obvious in the spectra of the other two silica samples. This feature is due to siloxane bridges formed during gel formation (10,12). 

Figure 3.2 Modifying silica particles to include ethylene bridges gives the particles exceptional chemical stability. Si, grey O, red H, black C, orange. The images highlight the difference between silica (top) and ethylene-bridged silica (bottom) particles.

Figure 25-6 Schematic structure of silica particle. [From R. E. Majors. LCGC May 1997. p. SB.] In particles that are <2 m in diameter, some of the Si—O Si bridges inside the silica are replaced by Si CH2—CH2—Si bridges, which provide mechanical rigidity to withstand high pressure.

Ancle of repose % Silica Particle shape Surface texture Change of state temp °F Hazardous (explos., toxic, etc) Tendency to bridge Moisture contcat % free... 

Due to their disperse character and small particle size, silicas are used as flow aids, i.e. they are used to improve the flow behaviour of other materials. The adsorption of the fine silica particles on other type powdered compounds reduces interparticle interactions. Particle adhesion, electrostatic adhesion, Van Der Waals forces and liquid bridge formation is reduced or avoided.33 This allows free-flowing behaviour of strongly interacting or irregularly shaped powdered materials. 

On the other hand, there is evidence that the pH may affect the reactivity of the hydroxyl groups, in that OH" ions have been found to catalyse the condensation of two hydroxyl groups on different silica particles to water and an oxygen bridge. In analogy, an alternative description of the initial reaction between a hydrolysed metal species and the silica surface would be... 

Coacervation, a fourth type of aggregation, in which the silica particles are surrounded by an adsorbed layer of material that makes the particles less hydrophilic, but does not form bridges between particles. The particles aggregate as a concentrated liquid phase immiscible with the aqueous phase. 

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