Salts, concentration IPECs

Upon the formation of IPECs, the concentration of a low molecular weight salt (a , b" ") in the system increases. The increasing salt concentration shifts equilibrium (1) to the left, that is, it favors the dissociation of interpolymer salt bonds. This phenomenon is observed at high concentrations of the polymeric components and/or upon addition of low molecular weight salts to mixtures of oppositely charged polyelectrolytes and has been reported in numerous works [8, 24—26]. It is widely used when IPECs are applied. 

Critical pH (pH, Fig. 10.3) and (Fig. 10.4) values depend on the composition of original soluble IPEC the higher is q> the lower are pH (if HPE is a poly(carboxylic acid) and /c. However, qualitatively the dependences represented in Figs. 10.3 and 10.4 are similar at all q> < cp. It is essential that the phase separation observed under a change of pH or salt concentration is quite reversible. 

Extrinsic factors (environment) such as the medium conditions also play a large part in the complexation process, especially pH and ionic strength (salt and polyelectrolyte concentrations). Also of prime importance is the way that the complexation itself is conducted, i.e., mixing parameters such as the stoichiometry of the components, the addition rate, and order of addition of the components (kinetic versus thermodynamic). Even if this process is fast and kinetically cOTitroUed (in water without added salt), i.e., far from the thermodynamic equilibrium, it can be followed by a slower stage in which the chains redistribute to a IPEC conformation closer to equilibrium [59]. 

Thus, the kinetics of polyion exchange reactions can be governed by varying the concentrations of a low molecular weight salt in the system. If the ionic strength of solutions is low, then macromolecular dissociation of IPECs to their polymeric components does not happen under such conditions. The rate of polyion exchange... 

Whether IPEC are soluble [2, 4-6] or of limited solubility in aqueous media is determined by a ratio of ionizable groups, z, and a relative length of oppositely charged polyions as well as by a water-phase composition (nature and concentration of a simple salt, pH, presence and concentration of organic additives etc.). IPEC formed at z = 1 are always insoluble at 0 1, but have limited... 

It is also shown that in case of polyion substitution reactions not only kinetics but also a position of equilibria is extremely sensitive to a structure and a length of polyion counterparts as well as to a nature and concentration of small counterions [8, 9, 13]. In particular, sulfonate- and sulfate-containing polyanions usually replace carboxylate-containing polyanions in corresponding IPEC with various polycations [9]. The rate of polyion substitution reaction between IPEC (PMA /Q- 4VP) and poly(potassium vinyl sulfate) (PVSK) drastically depends on the concentration of a simple salt [9] ... 

Fig. 10.4. The dependence of relative turbidity of PIC(PMANa Q-P4VP) solution on the concentration of the low molecular salt. (IPEC composition, concentration and characteristics of the components are equal to those in Fig. 10.3) pH = 7.5, 20 °C. (Clarification of the solution at [NaCl] > 0.4 M is caused by IPEC dissociation to the original polyelectrolyte components)...

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