Polymerization above pH

When the concentration of silica is low, generally under 1 %, and under certain conditions of pH and low salt concentration, the primary particles form small spherical aggregates or secondary colloidal particles. In aqueous solution this is much more likely to occur at ordinary temperature than in hot solution where growth of primary particles with decrease in their numbers reduce the probability of aggregation. 

Such a mechanism must have been involved in the formation of 200 nm spheres in a solution of pure silica sol prepared by hydrolyzing SiCU and removing HCl by electrodialysis, as reported by Radezewski and Richter (128b). The purified clear sol contained about 0.5% SiOj and the pH was 6.8. Similarly, uniform porous spherical silica particles up to 1 micron in diameter are formed by the aggregation of primary particles less than 5 nm in size formed by the hydrolysis of ethyl silicate in a water-alcohol-ammonia system as developed by Stdber and Funk (128c). 

All this is true provided the electrolyte concentration is low, as when silicic acid is prepared by hydrolyzing an ester or when the sodium or potassium cation is removed from a silicate solution to give pH 6-10.  

Only above pH 7 can silica particles of suitable size be prepared and concentrated for industrial use, as discussed in Chapter 4. - 

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This anomalous pH behavior results from the presence of polyborates, which dissociate into B(OH)2 and B(OH) as the solutions are diluted. Below pH of about 9 the solution pH increases on dilution the inverse is tme above pH 9. This is probably because of the combined effects of a shift in the equihbrium concentration of polymeric and monomeric species and their relative acidities. At a Na20 B202 mol ratio equal to 0.41 at pH 8.91, or K20 B202 mol ratio equal to 0.405 at pH 9 the pH is independent of concentration. This ratio and the pH associated with it have been termed the isohydric point of borate solutions (62). 

Polymeric phospholipids based on dioctadecyldimethylammonium methacrylate were formed by photopolymerization to give polymer-encased vesicles which retained phase behavior. The polymerized vesicles were more stable than non-polymerized vesicles, and permeability experiments showed that vesicles polymerized above the phase transition temperature have lower permeability than the nonpolymerized ones [447-449]. Kono et al. [450,451] employed a polypeptide based on lysine, 2 aminoisobutyric acid and leucine as the sensitive polymer. In the latter reference the polypeptide adhered to the vesicular lipid bilayer membrane at high pH by assuming an amphiphilic helical conformation, while at low pH the structure was disturbed resulting in release of the encapsulated substances. 

The most common support is highly pure, spherical, microporous particles of silica that are permeable to solvent and have a surface area of several hundred square meters per gram (Figure 25-5). Most silica should not be used above pH 8, because it dissolves in base. Special grades of silica are stable up to pH 9 or 10. For separation of basic compounds at pH 8-12, polymeric supports such as polystyrene (Figure 26-1) can be used. Stationary phase is covalently attached to the polymer. 

One of the major disadvantages of bonded-phase columns is the instability of the silica support at high and low pH. Generally, mobile phases should be used within the range pH 2-8 because, below pH 2, hydrolysis of the bonded functional groups occurs, resulting in decreased retention. Above pH 8, silica dissociates and the silica support starts to dissolve. Polymeric supports, on the other hand, can be operated over the range pH 2-12, and often provide a selectivity different from that of the silica-based columns however, the polymeric phases tend to be less efficient than silica-based columns. 

Ion suppression is not often applied to strong acids or strong bases because of the extremes in pH that would be required for retention. Suppression by the addition of buffers is restricted to the range pH 3-8, because silica-based stationary phases are unstable in solutions of either high or low pH (i.e., above pH 8 or below pH 2). These restrictions do not apply to polymeric supports, and polymer-based stationary phases can be used in the separation of a wider range of solutes using the ion-suppression technique. 

Chitosan and its derivatives are the most widely used cationic polymeric excipients. Chitosan consists of (31—>4 D-glucosamine units and is derived by the deacetylation of chitin from insects, crustaceans and fungi. It interacts ionically with the anionic substructures of sialic acid residues on the mucus layer. Chitosans are rapidly hydrated in a low pH environment like the gastric fluid and do not swell above pH levels of 6.5, exhibiting no more mucoadhesion. 

Al3+ is amphoteric and will dissolve in acidic and alkaline solutions. The [A1(H20)6]3+ ion is formed at low pH but undergoes increasing protolysis as the pH increases above four, and polymeric species such as [A11304(0H)24(H 20)12]7+ can be identified. The very insoluble Al(OH)3 is formed at neutral pH but redissolves above pH 10 ... 

Poly-9-(f -methacryloyloxyethyl)adenine (polyMAOA, 25 a), poly-l-(fi-methacryloyl-oxyethyl)uracil (polyMAOU, 27 a), -thymine (poly-MAOT, 29 a), poly-9-(f -acryloyl-oxyethyl)adenine (polyAOA, 25b), poly-1 -(fi-acryloyloxyethyl)uracil (polyAOU, 27b) and -thymine (polyAOT, 29 b) were prepared by free-radical polymerization of their corresponding monomers26,27). PolyMAOA and polyAOA are soluble in DMSO, ethylene glycol and acidic aqueous solution (below pH 3), while the polymers having uracil and thymine moieties are soluble in DMSO, DMF and alkaline aqueous solution (above pH 10). 

When adequate precautions (largely addition of low concentrations of EDTA) are taken to exclude extraneous reactions, the rate decreases above pH 12 in general agreement with the extent of formation of HOO . With these precautions, polymerization of acrylonitrile is not observed (despite an earlier contrary report). There is thus no reason to invoke radical paths. Two investigations agree that the acid-catalysed reaction includes the process... 

Because gel times decrease steadily between pH 2 and 6, it is generally assumed that polymerization above the IEP occurs by a bimolecular nucleophilic condensation mechanism (SN2-Si) involving the attack of hydrolyzed, anionic species on neutral species (2) ... 

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