Steric stabilization bridging flocculation

Nabzar et al. [18] showed in flocculation investigations with noncharged polyacrylamide that by using anisomeric particles as kaolin several regions of stabilization and destabilization (flocculation) may come one immediately after another in dependence on the polymer concentration added. In this case a stabilization of the kaolin particles is reached by steric stabilization, whereas flocculation occurs due to a bridging mechanism. 

Thus the ratio of the size of the particle to the root mean square end to end distance of the polymer is an important factor. With small particles, diameter ca.50 run, bridging occurs easily and since they are small, or similar in dimensions, to the polymers flocculation occurs easily moreover, the number concentration of particles is high and hence a large number of bridges are formed. With large particles, diameter > 1 ym, the polymer molecule is much smaller than the particle and hence it forms an adsorbed layer which often confers a degree of steric stability (48). 

The stability of a dispersion can also be enhanced (protection, by steric stabilization) or reduced (sensitization, by bridging flocculation) by the addition of material that adsorbs onto particle surfaces. Figure 5.13 provides an illustration. Protective agents can act in several ways ... 

Figure 5.13 Illustrations of bridging flocculation (left) and steric stabilization (right) due to adsorbed polymer molecules, and depletion flocculation and depletion stabilization due to nonad-sorbed polymer molecules. From Nguyen and Schulze [53], Copyright 2004, Dekker.

The principles of colloid stability, including DLVO theory, disjoining pressure, the Marangoni effect, surface viscosity, and steric stabilization, can be usefully applied to many food systems [291,293], Walstra [291] provides some examples of DLVO calculations, steric stabilization and bridging flocculation for food colloid systems. 

Small quantities of polysaccharides can flocculate a dispersed phase through bridging (Ward-Smith, et al., 1994), whereby one attached molecule with other adsorption sites along it may attach itself to another or more surfaces, acting as the bridge this phenomenon is called bridging flocculation. A bridge may instead cause steric stabilization of the dispersed phase. In the view of van Oss (1991), steric stabilization is predominantly a polar repulsion between macromolecules that is influenced not by Brownian activity, but by osmosis. 

Adsorbing polymer b p bridging flocculation steric stabilization... 

No dependence of the critical concentration of flocculation on the molecular weight of NaPSS is shown in Figure 20. This is also a typical behavior for the suspension with no (essential) bridging flocculation. The flat configuration of the adsorbed polyions in stabilized suspensions shows no existence of a steric stabilization at plateau adsorption as well. 

At very low polymer concentrations (say several ppm), the predominant effect, if an effect is indeed observed, is bridging flocculation. This can be observed not only with electrostatically stabilized particles but also with certain sterically stabilized particles (Evans and Napper, 1973d). It has been shown that even quite low molecular weight poly(oxyethylene) can... 

This diagram is able to explain some puzzling observations disclosed by Cowell and Vincent (1982). They report clear differences between the stability behaviour of latex dispersions depending upon whether the particles are naked or pre-coated by terminally anchored chains. The addition of free polymer to the sterically stabilized particles resulted in the transition sequence stability- instability stability. This is precisely what would be predicted from Fig. 17.20 if bridging flocculation is absent (as it must be if the free polymer and stabilizing moieties are identical in chemical composition). 

Figure 3.38 Illustration of effects due to difference in scale between polymer and particle (a) network formation (b) bridging flocculation (c) steric stabilization...

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). 

For effective stabilization the droplet surfaces should be fully covered by the adsorbed surfactant or polymer, otherwise uncovered regions of adjacent particles or droplets may come into contact with each other, or bridging flocculation between them may occur. Further, the stabilizing surfactant or polymer should be strongly adsorbed (firmly anchored) to the surfaces. Molecular structure and solvation, adsorption layer thickness and hydrodynamic volume, and temperature also determines the effectiveness of steric stabilization [75-79]. One way to predict whether steric stabilization is likely for a given dispersion is to estimate the protrusion distance of the surfactant or polymer chains [80]. 

Adsorbing Polymers Bridging Flocculation AND Steric Stabilization... 

Steric stabilization by adsorbed homopolymers suffers from the conflicting requirements that the liquid be a poor solvent to ensure strong adsorption but a good solvent to impart a strong repulsion when the adsorbed polymer chains overlap. At low polymer concentrations, an individual polymer chain can become simultaneously adsorbed on two (or more) surfaces, resulting in an attractive interaction known as bridging flocculation (see Sect. 4.6.4). 

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