Cation-coated silica sol

The industrial development of silica sol manufacturing methods is reviewed. Primary attention is focused on the preparation of monodispersed sols from water glass by the ion-exchange method. Details are given for variations of manufacturing process and for the characteristics of both the processes and sols obtained. Furthermore, the following surface modifications of particles are demonstrated silica sols stabilized with ammonia, amine, and quaternary ammonium hydroxide aluminum-modified or cation-coated silica sol and lithium silicate. Finally, future trends in silica sol manufacturing are discussed from the viewpoint of not only raw materials and improvement of the procedures but also the function of the silica sols and their particle shape. 

Figure 7. Flow chart of production of cation-coated silica sol.

Scheme 3A-E. Mechanism of inorganic coating of organogelators. Schematic representation for the creation of paper-like roU silica by sol-gel polymerization of TEOS in the organogel state of 4 A mixtiue of gelator and TEOS B gelation C sol-gel polymerization of TEOS and adsorption onto the cationic gelator D before calcination E paper-like roll multilayer structure of the sihca formed after calcination. Reprinted with permission from [49]. Copyright 2000 American Chemical Society...

Even cationic species such as the (hydrated) hydrogen ion (H ) can be lured into the hydrophobic silicone films from water medium as long as they contain appropriate additives to allow penetration of the analyte. For instance, Nivens et al. [140,141] have reported fabrication of silicone/ sol-gel films using a hydroxy-terminated poly(dimethyl siloxane (PDMS-OH), tetraethoxysilane (TEOS) and 3-ami-nopropyltriethoxysilane (APTES) mixture in ethanol-water. Tetramethylammonium hydroxide (TMAOH) is added to the mixture to entrap the fluorescent pH indicator dye 8-hydroxypyrene-l,3,6-trisulfonic acid trisodium salt (HPTS) [102], after base-catalyzed polycondensation of the silanes and the PDMS-OH. Silica optical fibers are stripped from their jacket at the distal end and dip-coated with the indicator gel before the end of gelation (24 h). The optical fluorosensor is able to measure between pH 6.0 and 8.5 for 6 months if stored in buffer medium. 

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