Gel-formation

Gels are disperse systems of at least two components in which the disperse phase in the dispersant forms a cohesive network. They are characterized by the lack of fluidity and elastic de-formability. Gels are placed between solutions, in which repulsive forces between molecules and the disperse phase predominate, and precipitates, where strong intermolecular interactions predominate. We differentiate between two types of gel, the polymeric networks and the aggregated dispersions, although intermediate forms are found as well. 

Exanples of polymeric networks are the gels formed by gelatin (cf. 12.3.2.3.1) and polysaccharides such as agarose (cf 4.4.4.1.2) and 

In a gel network, the multifunctional monomer acts as a branch unit. These branch units are connected to chains, which are linear segments of difunctional units that lead either to another branch unit or to an unreacted end  

The branching coefficient oc is defined as the probability that a given functional group on a branch unit is connected to another branch unit by a chain (rather than to an unreacted end). 

Further analysis is based on two assumptions (1) All functional groups are equally reactive. This might not always be true. For example, the secondary (middle) hydroxyl group in glycerine is probably not as reactive as the primary (end) hydroxyls. Furthermore (2), intramolecular condensations (i.e., reactions between A and B groups on the same molecule) do not occur. In other words. 

Define p = fraction of the total A groups in the branch units Ay 

The probabilities of finding each of the numbered types of bonds in the molecule shown on the right above are tabulated below  

Aluminosilicate gels are prepared by the mixing of reactive sources of alumina and silica. These include fumed silica, silica sols and alkoxides of silica, and aluminium, aluminates, aluminium salts and alkoxides of aluminium. Other synthetic routes have been explored that use structured aluminosilicate precursors, such as zeolites themselves, to supply aluminate and silicate species to 

Alkali and alkaline earth metal cations (such as Li, Na, K, Rb, Ca , Sr and Ba ) have been used to facilitate zeolite syntheses. Some characteristic syntheses in which inorganic cations control zeolite crystallisation are given in Table 5.1. They are commonly used in the soluble hydroxide form to ensure pH values of 12-13 in the reaction mix, which are ideal for crystallisation to occur. For cations such as calcium, the lower solubility in alkaline solution requires higher reaction temperatures and longer crystallisation times. Once in solution, the inorganic cations exist as hydrated structure-making species, which are surrounded by extensively H-bonded shells of water molecules. 

Important zeolite types synthesised with inorganic cations as structure directing agents using verified synthesis procedures  

Organics Cations added Gel composition (content formally as oxides) Zeolite (topology) Composition 

MTEA methyl- triethylammonium Na 10 Na2O 20 MTEA BrrAlzOjrlOO Si02 2000 H2O (v) ZSM-12 (MTW) N ao.sCMTE A) 1 3 Alo.8Si27.2O56 

The weight-average chain length, x , can be obtained by inserting the number distribution. Equation 8.3 into Equation 6.23 and integrating at constant p  

FIGURE 8.3 Time course of reaction of multifunctional A to difunctional B monomer. First small polymers are formed that are not crosslinked, but as enough A and B groups react, a tight network forms. 

FIGURE 8.4 A growing polymer chain with two branch points indicating the position of the trifunctional monomers that have been added. 

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The abihty of algiaates to form edible gels by reaction with calcium salts is an important property. Calcium sources are usually calcium carbonate, sulfate, chloride, phosphate, or tartrate (20). The rate of gel formation as well as the quaUty and texture of the resultant gel can be controlled by the solubihty and availabiUty of the calcium source. 

Additional hydrolysis to promote polymerisation and cross-linking leading to a three-dimensional matrix and gel formation. 

Silicate Grouts. Sodium silicate [1344-09-8] h.3.s been most commonly used in the United States. Its properties include specific gravity, 1.40 viscosity, 206 mPa-s(=cP) at 20°C Si02 Na20 = 3.22. Reaction of sodium silicate solutions with acids, polyvalent cations, such organic compounds as formamide, or their mixtures, can lead to gel formation at rates, which depend on the quantity of acid or other reagent(s) used. 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes include compHcated phase changes, viscosity effects, gel formation, and Hquefication of Hquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

Despite all these safeguards to extend the service life of the antifreeze, fluid replacement is requited periodically. Typically, fluids are replaced because of irreversible damage caused by one of four conditions contamination, gel formation because of glycol/siUcate reaction, extensive glycol degradation caused by overheating or excessive oxygen exposure, or inhibitor depletion. 

Excellent coUoidal stability but gel formation may interfere with tack. 

Isobutyronitrile (2-methylpropionitrile, isopropyl cyanide) [78-82-0] M 69.1, b 103.6 , d 0.7650, n 1.378. Shaken with cone HCl (to remove isonitriles), then with water and aq NaHC03. After a preliminary drying with silica gel or Linde type 4A molecular sieves, it is shaken or stirred with CaH2 until hydrogen evolution ceases, then decanted and distd from P2O5 (not more than 5g/L, to minimize gel formation). Finally it is refluxed with, and slowly distd from CaH2 (5g/L), taking precautions to exclude moisture. 

Table 5 indicates that RuCl2(PPh3)3 has been frequently used for selective hydrogenation of C=C in NBR [48-52]. This is commercially available and is also easy to synthesize. In most of the patented processes, low-molecular weight ketone solvents are used to avoid the gel formation. The activity of the catalyst can be enhanced by the use of certain additives, such as trieth-ylamine [59], isopropanol [52], and ammonium hexaflu-orophosphate [50] in the reaction system. This might be... 

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