Method, condensation

These methods allow good control of particle size and shape. Moreover, they are well-suited to the production of solid/liquid dispersions such as mineral sols and latexes. 

There are many methods for inducing nucleation and producing the desired evolution of the precursor solution. For example, one technique consists in modifying solubility in the medium, by varying the temperature or by introducing another solvent. The concentration of a precursor can thus be taken beyond its saturation level. 

However, the most interesting techniques are those which involve chemical reactions between precursor molecules. Indeed, when the reaction kinetics can be suitably controlled, extremely reliable processes may be developed to produce colloidal systems with a well-determined state of division of the material. Some processes may even lead to colloids in which all the particles have exactly the same size. 

The general principles for the synthesis of colloids of uniform particle size were established in the 1950s by the work of La Mer and Dinegar, who used colloidal sulfur sols. The basic reaction generating the colloidal sol was the decomposition of sodium thiosulfate in an acidic medium  

The variation in concentration as sulfur particles form is shown in Fig. 3.18. 

Approaching the formation of colloids from the other end of the size range involves one of several growth mechanisms. Such processes are commonly employed for the production of dispersions and aerosols, and less commonly in the production of emulsions. Typical examples of important condensation processes include fog formation (both water and chemical), silver halide emulsions (really dispersions) for use in photographic products, crystallization processes, colloidal silica, latex polymers, etc. 

In emulsion polymerization, a monomer or mixture of monomers is emulsified in a liquid phase (usually water) in the presence (except under very special 

FIGURE 10.3. In the process of emulsion polymerization, the incipient latex particle begins as a free-radical-initiated dimer or oligomer in solution (a). As polymerization proceeds, the growing chain precipitates and continues to grow, fed by new monomer taken from the reservoir of emulsified material (b). Potymerization continues until all 

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Another condensation method was investigated by Carothers and co-workers and reported in 1930. They reacted decamethylene dibromide with sodium in a Wurtz-type reaction but found it difficult to obtain polymers with molecular weights above 1300. 

Similar reactions can also be written for the alkoxysilanes but in commercial practice the chlorosilanes are favoured. These materials may be prepared by many routes, of which four appear to be of commercial value, the Grignard process, the direct process, the olefin addition method and the sodium condensation method. 

Tire condensation method (Section II,A,1) applied for 1,2,4-thiadia-zolium salts and phenylsubstituted imidazoles leads to the thia-tetraazaful-valenes 91 and 92 (70TL481). Moreover, 2,4,5-tris(ethylsulfanyl)imida-zolium chloride was converted to the unsymmetrical heterofulvalene 93 by a self-condensation reaction in the presence of potassium carbonate and elemental iodine (85PS223) (Scheme 37). 

Triazines can be obtained either by condensation methods using the fluorinated anhydride (89IZV928) or by trimerization of nitriles, e.g., per-... 

Fig. 16. Example of a A s.p. = f(t) relation, manifesting surface potential changes in a nickel-hydrogen system as a function of time and amount of hydrogen introduced onto a surface of a nickel film deposited at liquid nitrogen temperature hydrogen-nickel film interactions were studied by Tompkins-Eberhagen static condenser method at liquid nitrogen temperature. After Dus (60). Each dose of H2 — 2.5 X 10 molecules.

The Mukaiyama condensation method was also utilized in the synthesis of several antibiotics and related bioactive substances. These syntheses include apramycin, using 4-azido-2,3,6-tri-(P-benzyl-4-deoxy-) -D-glucopyranosyl fluoride (prepared from the corresponding a-D-glucopyranosyl chloride by the AgF method ) avermectin B , using protected sugar derivatives of oleandrosyl fluoride (a-L-Ole-F) and 0-(a-L-01e)-(1 4)-a-L-0le-F (both... 

Nucleosides containing deoxyfluoroglycopyranosyl residues were also prepared. 1-(6-Deoxy-6-fluoro-/ -D-gluco- and -galacto-pyranosyl)thymine (858 and 860) were obtained from l-/ -D-gluco- and -galacto-pyranosyl-thymine by the usual displacement reaction, or by the condensation method. 

Removal of the 2 -sulfonyloxy group of 859 in a basic medium, followed by reaction with metal halides (LiBr and Nal) or hydrogen halides (HCl-1,4-dioxane, HBr-acetone, or0.1% HFin l,4-dioxane-AlF3)gave, byway of the 2,2 -anhydro intermediate 861, the 2 -halo derivatives 862-865. The 2 -deoxy analog 866 and l-(6-deoxy-6-fluoro- ff-D-mannopyranosyl)thy-mine were also prepared from 864 (R = H) and 861 (R = H), respectively. l-(4-Deoxy-4-fluoro-y -D-glucopyranosyl)thymine was obtained by the condensation method. A different kind of nucleoside, 5-(5-deoxy-5-fluoro-2,3-0-isopropylidene-a-D-ribofuranosyl)-l,3-dimethyluracil has also been prepared. ... 

In the dynamic condenser, or the vibrating plate or vibrating condenser method (Fig. 5), also called Kelvin, Zisman, or Kelvin-Zisman probe, the capacity of the condenser created by the investigated surface and the plate (vib. plate) is continuously modulated by periodical vibration (GEN.) of the plate. The ac output is then amplified and fed back to the condenser to obtain null-balance operation (E,V). " " ... 

In contrast to the ionizing electrode method, the dynamic condenser method is based on a well-understood theory and fulfills the condition of thermodynamic equilibrium. Its practical precision is limited by noise, stray capacitances, and variation of surface potential of the air-electrode surface, i.e., the vibrating plate. At present, the precision of the dynamic condenser method may be limited severely by the nature of the surfaces of the electrode and investigated system. In common use are adsorption-... 

Figure 5. A block schematic diagram of the dynamic condenser method for voltaic measurements.

A disadvantage of the glutaraldehyde condensation method is that dimers of the hapten and polymers of carrier protein may also form. To overcome this problem, the reaction time is limited to 2-3 h, or an excess of an amine-containing compound, e.g., lysine or cysteamine hydrochloride, is added. A two-step approach also minimizes dimerization. ... 

The principle of the vibrating condenser method, originally proposed by Thomson, is depicted in Fig. 3.6. Space A between the metal phases ocy a ... 

Fig. 3.6 Thomson s vibrating condenser method. A, space filled with an inert gas at low pressure P, potentiometer G, null instrument...

Recently Sergeev et al. 90 91> have developed a low temperature condensation method for the formation of inclusion compounds of thiourea with reactive and volatile guests, avoiding the use of solvents. The two guests in the joint inclusion compound of thiourea with 1,3-cyclopentadiene and maleic anhydride underwent Diels-Alder addition at 170 K. These two substances do not react at this low temperature unless they are present in the thiourea complex the usual endo isomer of the product is formed. Apart from copolymerisation reactions this appears to be the first use of the thiourea canal to study reactions between different materials. 

The synthetic methods of symmetrically substituted Pcs are similar to those of the parent unsubstituted Pcs. However, unsymmetrically substituted Pcs are usually prepared by statistical condensation methods, subphthalocyanine expulsion methods or cross-condensation methods. 

The acid condensation methods do not distinguish between monosaccharides and polysaccharides, as the various classes of carbohydrates each have different absorption maxima, which results in different molar absorptions at any chosen wavelength. Furthermore, when treated with concentrated sulfuric acid, some three- and four-carbon compounds will condense into structures which will produce colours with those reagents. When the object of the analysis is to obtain some estimate of the total amount of carbohydrate or carbohydratelike material present, the inclusiveness of these methods is useful. However, when the object is to distinguish between the easily metabolised simple sugars and the complex storage and structural materials, these methods give no information at all. 

Oxidative processes (route B) represent another common route to triazolopyridines (compounds described in Schemes 54 and 55). These preparations all start from aldehyde hydrazones and use different oxidative reagents for the cyclization (Table 2). Generally, those conditions are milder than condensation methods. Moreover, the oxidizing reagents are compatible with other moieties, even the sugar-derived polyol 209. In the case of compound 208, the hydrazone (major diastereomer) was obtained by tautomerization of the corresponding enhydrazine, the... 

Ullrafine particles (UFPs) of metal and semiconductor nitrides have been synthesized by two major techniques one is the reactive gas condensation method, and the other is the chemical vapor condensation method. The former is modified from the so-called gas condensation method (or gas-evaporation method) (13), and a surrounding gas such as N2 or NII2 is used in the evaporation chamber instead of inert gases. Plasma generation has been widely adopted in order to enhance the nitridation in the particle formation process. The latter is based on the decomposition and the subsequent chemical reaction of metal chloride, carbonate, hydride, and organics used as raw materials in an appropriate reactive gas under an energetic environment formed mainly by thermal healing, radiofrequency (RF) plasma, and laser beam. Synthesis techniques are listed for every heal source for the reactive gas condensation method and for the chemical vapor condensation method in Tables 8.1.1 and 8.1.2, respectively. 

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