Alkaline condition, colloid

Lewis, D.G. Schwertmann, U. (1980) The effect of [OH] on the goethite produced from ferrihydrite under alkaline conditions. J. Colloid Interface Sd. 78 543—553 Lewis, D.G. (1992) Transformations induced in... 

Preparing a 1% casein solution is somewhat tricky because dried casein preparations are typically only sparingly soluble in neutral aqueous systems. This method, which uses alkaline conditions to prepare a casein sol, should work for most purposes. The casein substrate solutions used for peptidase assays are often more accurately described as substrate suspensions, since these preparations are typically not true solutions—the casein component being of a colloidal nature. 

Polysihcic acid made of 1.5-4 nm diameter discrete particles may partially or completely polymerize into chains and three dimensional networks. Once a polysihcic acid solution has been exposed to alkaline conditions, it is rapidly converted to colloidal silica particles larger than 4-5 nm diameter. 

New fibers are being developed which dissolve in the slightly alkaline pH of the lung. These fibers may eventually replace the more common alumino-silicate fibers especially in use where human contact with breathable fibers is likely. Alumino-silicate fibers do not have the same small size as asbestos fibers, but the mere fact that they are fibers, could become airborne, and the body has no rejection mechanism, has prompted some countries to view them with suspicion. Having fibers which provide the same thermal properties but which would dissolved in the lung would preempt the possible problem. However, fibers which dissolve in alkaline conditions cannot be used with alkali stabilized colloidal silicas. They need a binder which is stable but acidic in pH as described above. Such products were first developed at DuPont and are now available through W.R. Grace who acquired the Ludox colloidal silica business from DuPont... 

Recently a microwave technique for efhcient deacetylation of chitin nanowhiskers to a chitosan nanoscaffold has also been reported [65]. A chitosan nanoscaffold in the form of a colloidal solution was obtained from the deacetylation of chitin whiskers under alkaline conditions by using a microwave technique in only 1/7 of the treatment time of the conventional method (Fig. 3.8). Fourier-... 

The extract contains, in addition to the sucrose, further solved or colloidally dispersed components, which in the juice purification stage must be - as far as possible - precipitated or destroyed under alkaline conditions by adding calcium oxide. This is carried out by a multi-stage, continuous treatment with, first, calcium oxide and, second, carbon dioxide. Both processing aids are produced in a separate calcining process in parallel. The excess calcium oxide is precipitated in so-called carbonation vessels with carbon dioxide from the calcining process. All precipitates and deposits are separated in a multi-step filtration. The sludge... 

Valuable new data were developed, but the authors did not consider all the implications of the fact that at least under the alkaline conditions studied, the polymerization must have consisted of the nucleation and growth of dense amorphous particles of colloidal silica which were in solubility equilibrium with monomer. 

Many processes have been proposed and are employed for producing colloidal silica from low-cost sodium silicate solutions. Sols have also been made from hydrolyzable compounds such as ethyl silicate or silicon tetrachloride. To produce sols that are stable at a reasonably high concentration, it is necessary to grow the particles to a certain size under alkaline conditions where the particles remain negatively charged so they will not flocculate or gel. The particles. should also be nonporous. 

One difference between gels made from stabilized colloidal silica and gels from silicic acid is that the commercial colloids are made in hot, slightly alkaline conditions that remove water and micropores from the interior of the particles. On the other hand, the particles made by polymerization of silicic acid, usually below pH 7 and at less than 80 C, and often in the presence of sodium salts, may be microporous. This point has been discussed by Vysotskii et al. (221), who suggest that the pores may be accessible to water but not large molecules. 

Figure 10.12 TEM and SEM images with particle size distribution of Si nanomateiials in acidic and alkaline conditions. Reproduced from Zhao et al. [114], Colloids Surf. A 2009,346,229, with permission from Elsevier Science. 

Most zeolites are synthesized from proportional mixtures of sodium aluminate and sodium silicate and, in some cases, colloidal silica. The procedure is different to that for the preparation of silica alumina catalysts because zeolites form under alkaline conditions as the silica/alumina co-gel crystallizes in the presence of hydroxyl ions. The zeolite type formed depends on the proportion of silicate and aluminate in the solution and the reaction temperature and pressure. The time taken for zeolite crystals to form can range from a few hours to several days. Seeds or templates are often added to induce formation of the appropriate crystalline product. Knowhow is very important, and precise details for a specific preparation are not always published. 

For example, dispersions of colloidal silica at alkaline conditions are stabilized by adding certain salts although the electrostatic double-layer repulsion decreases... 

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