Polymer charge patch

Fig. 3. Adsorbed cationic polymer forming charge patch on particle surface (32).

Analyses of phase boundaries reveal evidence for polymer saturation in the presence of excess protein. Phase boundaries also facilitate comparisions of the behavior of various proteins. The failure of net surface charge density as a universal parameter for protein-polyelectrolyte interaction is believed to be related to the existence of "charge patches" on the protein surface. The determination of a more realistic protein charge parameter possesses great importance, since the ionic interactions of proteins are exploited in a variety of applications, including protein purification via ion exchange liquid chromatography. 

Leong, Y.K., Interparticle forces arising from an adsorbed strong polyelectrolyte in colloidal dispersions Charged patch attraction. Colloid Polym. Sci., 277, 299, 1999. 

As mentioned earlier, polymers can exert a dramatic influence on colloidal stability. However, the description of particle interactions in the presence of polymers is complex, and even a qualitative estimation of conditions for attraction or repulsion has yet to be developed. Three theories have been proposed to explain flocculation of charged particles by oppositely charged polyelectrolytes bridging, simple charge neutralization, and charge patch neutralization. 

Simple Charge Neutralization and Charge Patch Neutralization Oppositely charged polyelectrolytes reduce the particle surface charge density such that particles may approach each other sufficiently closely so that the attractive van der Waals force becomes effective. Flocculation caused by this mechanism should not be sensitive to the molecular weight of the polymer. 

The adsorption of polymers to the surface of particles leads to the two fundamental mechanisms of flocculation by polyelectrolytes. These are commonly referred to as the electrostatic or charge patch mechanism and the bridging mechanism and may act individually or, in some instances, simultaneously. 

Figure 3.7 Schematic representation of the charge patch mechanism of flocculation (cationic polymer and negatively charged particles in the example shown).

In the cases of aggregation by charge patch neutralization or bridging, it is necessary to include steric or bridging forces. Bridging flocculation takes place when polymer chains adsorb on more than one particle and act as bridges. Invoking... 

Stock composition, kinetics of adsorption and hydrodynamic shear dictate the point at which a cationic polymer is added to a papermaking furnish in order to induce flocculation. Flocculation of cellulose fibers in turbulent flow proceeds very rapidly and is completed in less than two seconds.120-123 Flocks form due to charge interactions through a patch-type or a bridging-type mechanism. However, these flocks will be sensitive to shear force and deflocculation and reflocculation might occur. 

A characteristic difference between charge neutralization and patching is that the rate of coagulation for the former mechanism increases with electrolyte concentration. Once an optimal electrolyte concentration has been attained, however, the rate of flocculation by patching will decrease with electrolyte content due to the fact that the electrolyte cations will force the polymer from the particle surface. 

Relatively short-chained cationic polymers of average molar mass and high charge density are suitable for patch flocculation. Modified polyethylene imines, polyamines, and polyamide-amine-epichlorohydrin resins are in this category. 

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