Microgel and Viscosity

Gel phase in invisible and the sol remains clear. Within the microgel regions the refractive index is essentially the same as that of the surrounding fluid sol. The silica concentration is the same and thus there is no difference in density so that the gel phase does not settle out. 

This discussion refers to homogeneous gels formed by uniform solidification of a sol. Such gels are formed below about pH 6 whether or not a salt such as Na SO, is present, and above pH 6 only if the salt concentration (sodium) is not too high. In hot solution containing more than about 0.3 N salt, silica is precipitated rather than gelled. 

Thus if a silica sol contains substantial amounts of salts and is neutral or alkaline, a homogeneous gel is not obtained instead, the silica usually appears as a white precipitate, or in some cases as a white opaque gel owing to partial precipitation before gel formation. Similar effects arc observed when small amounts of coagulants 

The volume of microgel phase can be estimated from viscosity on the assumption that the microgel regions are spherical in shape and of approximately uniform size. The volume fraction of microgel phase (which is also the fraction of silica immobilized in gel) can be calculated from the Mooney equation (128a)  

the use of a hydrophobic cation base (CHjl N OH instead of Na OH at pH 5.5 reduced the activation energy to about 8.0 kcal mole . When this cation is adsorbed on silica there is hydrophobic bonding between the silica particles and gel, thus holding the surfaces together for longer times upon collision, and promoting silo, ane bond formation. 

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Fig. 7 Particle diameter of the microgels and viscosity of the suspension of styrene-acrylate copolymer microgels in melamine resin/xylene solution as a function of the melamine concentration.

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