Network exchange reactions

Scheme 2 Ligand-exchange reaction between EHO-OPPE and [Pt(PhCH=CH2)3] leading to EHO-OPPE-Pt networks...

In the previous consideration, we regarded the solvent as infinite. However, for the case under consideration this is not a fruitful approach, since we are studying an ion exchange reaction and the results of this reaction depend essentially on the ratio of the volume of the network V to the full volume of system Vf V/Vf. The lower this ratio, the larger the number of counter ions substituted by surfactant molecules. In the case of the thermodynamic limit, V/Vf =>0 and practically all counter ions should be out of the network, i.e. the equilibrium ion exchange reaction is artificially shifted to one of the limits. So, in this case it is necessary to study the swelling of the network in the system with finite ratio V/Vf. 

In spite of the fact that the concentration of surfactants in the outer solution is assumed to be smaller than the critical micelle concentration, inside the network, micelles are supposed to be formed. The reason for this assumption is, first of all, intensive adsorption of surfactants on the network as a result of the ion exchange reaction. Moreover, in Refs. [38, 39], it was shown that critical concentration of micelles formation c c" within a polyelectrolyte network is much less than that in the solution of surfactant c° . Indeed, when a micelle is formed in solution immobilization of counter ions of surfactant molecules takes place, because these counter ions tend to neutralize the charge of micelles (see Fig. 13), whereas there is no immobilization of counter ions when the micelles are formed in the network the charge of micelles is neutralized by initially immobilized network charges which do not contribute to the translational entropy (Fig. 13). 

The ion couling reaction of bifunctional l c with tri- and tetrafunctional carboxylates was carried out to produce first the ion-exchange, pseudo-network, products crosslinked through the Coulombic interaction, which was found to remain soluble in a good solvent like THF. This unique feature of pseudo-network products allows one to observe the products by solution H NMR spectroscopy. The spectra of the ion-exchange products of l c with tri- and tetrafunctional carboxylates demonstrated the nearly quantitative ion-exchange reaction to occur for both systems. 

The peculiar viscoelastic properties of wheat dough are the result of the presence of a three-dimensional network of gluten proteins. The network is formed by thiol-disulfide exchange reactions among gluten proteins. Peptide disulfides can interfere in a thiol-disulfide exchange system by reacting with a protein (PR)-thiol to liberate a peptide (R)-thiol and form a mixed disulfide, as follows ... 

Thus, Nature has integrated thiol/disulfide exchange reactions in the regulation of its metabolic and antioxidant networks. The potentially cytotoxic effects of protein S-thiolation will remain controversial until the relationship between the systems of glutathione reductase, thioredoxin, glutaredoxin and thioltransferase are better understood. 

FIGURE 13.6 Network for feed (a) strip, (b) competitive cation-exchange reactions, and local diffusion processes (c) C2A, (d) C2B, and, (e) CH species. (From Wodzki, R., Szczepanska, G., and Szczepanski, P., Sep. Purif. Technol, 36, 1, 2004. With permission.)... 

Figure 5.5 Network for feed (A), strip (B) competitive cation-exchange reactions, and local diffusion processes C2A (C), C B (D), and CH (E) species. From Ref. [27] with permission.

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