Precipitation of polyelectrolytes

The most investigated examples are to be formd in the precipitation of polyelectrolytes by metal ions. Here, networks are formed by the random crosslinking of linear polymer chains, and the theory requires some modification. The condition for the formation of an infinite network is that, on average, there must be more than two crosslinks per chain. Thus, the greater the length of a polymer chain the fewer crossUnks in the system as a whole are required. 

Ikegami, A. Imai, N. (1962). Precipitation of polyelectrolytes by salts. Journal of Polymer Science, 56, 133-52. 

The precipitation of polyelectrolytes by the addition of multivalent counterions may be explained in these terms. When there are no multivalent ions in solution there is a strong repulsive force between polyions and the osmotic pressure is large. The solubility of polyions is a result of these repulsive forces. 

Sodium alumiaate is used ia the treatment of iadustrial and municipal water suppHes and the use of sodium alumiaate is approved ia the clarification of drinking water. The FDA approves the use of sodium alumiaate ia steam generation systems where the steam contacts food. One early use of sodium alumiaate was ia lime softening processes, where it iacreases the precipitation of ions contributing to hardness and improves suspended soHds removal from the treated water (17). Sodium alumiaate reacts with siHca to leave very low residual concentrations of siHca ia hot process water softeners. Sodium alumiaate is often used with other chemicals such as alum, ferric salts, clays, and polyelectrolytes, as a coagulant aid (18,19). 

M. C. van der Leeden and co-workers, "Role of Polyelectrolytes ia Barium Sulphate Precipitation," Innovators Digest, RepofiNo. D221H, Technical University of Delft, The Netherlands, 1991. 

Water-soluble polymers and polyelectrolytes (e.g., polyethylene glycol, polyethylene imine polyacrylic acid) have been used success-hilly in protein precipitations, and there has been some success in affinity precipitations wherein appropriate ligands attached to polymers can couple with the target proteins to enhance their aggregation. Protein precipitation can also be achieved using pH adjustment, since proteins generally exhibit their lowest solubility at their isoelectric point. Temperature variations at constant salt concentration allow for frac tional precipitation of proteins. 

The first step in the precipitator is the addition of polyelectrolyte solution in the flash mix tank [T-98], surge tank [T-99], and then into the slow mix unit [T-100] containing a variable speed mixing paddle. The purpose of this unit is to coagulate and flocculate53 the metal hydroxide precipitates. 

Method Chemical precipitation of dissolved and complexed metals by reaction with lime and subsequent removal of the precipitated solids by gravity settling in a clarifier. Alum and polyelectrolyte are added for coagulation and flocculation. 

Destabilization of the dissolved compounds by addition of iron or aluminum salts and addition of polyelectrolytes to support agglomeration and formation of larger size precipitation. 

Olvera de la Cruz, M., Belloni, L., Delsanti, M., Dalbiez, J.P., Spalla, O. and Drifford, M. (1995) Precipitation of highly charged polyelectrolyte solutions in the presence of multivalent salts. J. Chem. Phys., 103, 5781-5791. 

Only if an inert electrolyte like NaCl is added, the strong electrostatic interactions are increasingly screened and the highly expanded polyelectrolyte coils start to shrink. Eventually the unperturbed dimensions are approached at high enough concentrations of inert salt [18-20]. In such cases, phase separation occurs if this inert salt level is surpassed [21-23]. The latter phenomena has been denoted as salting out of polyelectrolytes, or alternatively, as H-type precipitation, because the concentration of inert salt required to cause precipitation of the polyelectrolyte is high and independent of the polymer concentration [23]. 

The present review deals mainly with two examples of polyelectrolyte phase behavior as discussed above. As an example for an H-type precipitation, the solution properties of polyvinylpyridinium chains are monitored as function of added inert salt. Here, we focus on the determination of the effective charge density and of the solvent quality parameter which are supposed to play a central role for the understanding of polyelectrolyte solution without specific counterion interactions. The second system under investigation comprises the interaction of polyacrylic acid with alkaline earth cations which exhibit very specific interactions, thus representing an example for type L-precipitation. Here the coil dimensions close to the phase boundary are compared to those close to type H-precipitation with inert added salt. 

It should be noted, that the miscibility gaps discussed throughout the paper are not to be confused with classical coexistence curves, i.e., the miscibility gap does not refer to the phase separation into a salt-rich and a salt-poor phase, but rather to the solubility/precipitation of the polyelectrolyte at constant Cp=0.1%. 

From the experiments just outlined [74-76], a few points are worth being emphasized A powerful procedure was developed to gradually approach phase boundaries of polyelectrolyte precipitation. The approaches can be performed in a highly systematic manner and lead to states which are located extremely close to the precipitation threshold. Approaches could successfully be accompanied by LS experiments. The experiments demonstrated, that the polyelectrolyte chains shrank dramatically in size immediately before the phase boundaries were reached. A sudden increase of the scattering intensity indicated the phase boundaries. These developments give rise to the hope that intermediates may be revealed which have not become accessible in preceding investigations [78-81]. 

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