Patch flocculation

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. 

Figure 7.14. Sequence showing schematically (a) charge-stabilized particles, (b) particles flocculated by the patch flocculation mechanism, and (c) particles restabilized by steric repulsive (chain overlap) interactions at higher levels of polymer surface coverage...

The increase in the rheological parameters, ri0> G0 and 1(5 with reduction in surface coverage points towards an increase in particle interaction. This could be the result of either flocculation by polymer "bridging" (which is favourable at coverages <0.5) or as a result of coagulation due to the van der Waals attraction between the "bare" patches on the particles. In the absence of any quantitative relationship between interaction forces and rheology, it is clearly difficult... 

The two major theories of flocculation, the bridging model (1) and the electrostatic patch model (2, 3 ), provide the conceptual framework for the understanding of polymer-aided flocculation, but they do not directly address the kinetics of the process. Smellie and La Mer (4) incorporated the bridging concept into a kinetic model of flocculation. They proposed that the collision efficiency in the flocculation process should be a function of the fractional surface coverage, 0. Using a modified Smoluchowski equation, they wrote for the initial flocculation rate... 

Figure 7.5 Typical flocculation, mobility and adsorption data for flocculation by the patch charge neutralisation mechanism.

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. 

The particles should be completely covered by the dispersant (the amount of dispersant should correspond to the plateau value). Any bare patches may cause flocculation either by van der Waals attractions (between the bare patches) or by bridging flocculation (whereby a polymer molecule will become simultaneously adsorbed onto two or more particles). 

This flocculation is proposed to be due to strong adsorption at the solid-liquid interface (particle surface) due to the phase separation forming regions or patches rich in pyrrolidone. It is known... 

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. 

Charge neutralization (simple and/or patch) is often the mechanism for flocculation by low molecular-weight polyelectrolytes. 

Figure 11.2. With the scanning tunnelling microscope, dispersed polyaniline (PAni) can be shown to consist of primary particles that are no larger than 10 nanometres (millionths of a millimetre). In (a) they can be seen as light, yellow-coloured patches. Once the volume concentration exceeds a critical threshold, these flocculate and—as can be seen in the scanning electron micrograph (b)—form network-like strucmres. Each of the particles behaves like a metal measuring a few nanometres, i.e. it possesses freely mobile electrons. These can tunnel between the particles and thereby conduct electricity. [Reproduced from ref. 17b with kind permission from Gordon and Breach publishers.]...

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. 

From the above discussion, one can summarize the most important criteria for effective steric stabilization when using polymeric surfactants. First, there should be enough polymer to ensure complete coverage of the particle surface by the chains. This will prevent any attraction between the bare patches of the particles, while it also prevents any bridging flocculation (simultaneous adsorption of the chain on more than one particle). 

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).

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