Flocculation bridging

Bridging flocculation can sometimes be prevented or destroyed by adding surfactant in sufficient quantity to adsorb and cover all of the interfaces in a system, thus displacing the bridging molecules from at least one of each pair of interfaces. 

The formation of floes due to bridging flocculation has a dramatic effect on sedimentation rates, sediment volumes and on the ease of filtration. Effective flocculation may occur over a narrow range of polymer concentration because too little polymer will not permit floe formation, while too much polymer adsorption will eliminate the fraction of free particle surface needed for the bridging action (i.e. the polymer molecules will adsorb onto single particles in preference to bridging several particles). It has been proposed that the optimum degree of bridging flocculation may occur when particle surfaces are half covered with adsorbed polymer. 

3) This is a mechanism of r regation or flocculation in which long.c hain polymers adsorb onto particle surfaces leaving loops and ends extending out into solution. If these loops and ends contact and adsorb onto another particle, then a so-called bridge is formed. 

If a solution of a high-molecular-weight polymer is added to a dilute dispersion, bridging flocculation may occur. In this it is 

If we make the gross approximation of assuming that the solution is ideal, i.e. dp = / Tdlnc , then 

An alternative way of looking at this phenomenon is to say that the gap between the particles prevents polymer from entering the 

At this point we see qualitatively that the mechanisms of steric stabilisation and depletion flocculation are closely related. In the former instance the concentration of polymer segments in the space between the particles increases as the particles come together, leading to a repulsion caused by the osmotic flow of solvent into this space in the latter case the concentration between the particles is lower than that in the bulk, and diffusion of solvent out of the interparticle space results in an attraction. 

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At lower surface coverage, however, tire possibility exists that one polymer chain may attach itself to two particles. If tire adsorjDtion is strong enough, tliis results in an aggregation of tire particles, known as bridging flocculation [33,46, and 47],... 

The well-known DLVO theory of coUoid stabiUty (10) attributes the state of flocculation to the balance between the van der Waals attractive forces and the repulsive electric double-layer forces at the Hquid—soHd interface. The potential at the double layer, called the zeta potential, is measured indirectly by electrophoretic mobiUty or streaming potential. The bridging flocculation by which polymer molecules are adsorbed on more than one particle results from charge effects, van der Waals forces, or hydrogen bonding (see Colloids). 

The presence of insufficient but very large polymers can also reduce the stability. When the particles attain a separation such that the polymer layers on an adjacent particles may bridge between the particles, a favourable interaction occurs and a loss of stability ensues. This is termed bridging flocculation. 

Transmission electron microscopy also gave evidence for bridging flocculation at partial coverage. Figure 3 shows electron micrographs of the bare particles and the particles covered partially with adsorbed Vinol 350. The partially covered particles are interconnected with fibrillar links, which are not observed in the bare-particle sample. 

These results confirm the existence of weak or labile floes at partial PVA coverage, particularly with the high-molecular-weight fully-hydrolyzed Vinol 325 and Vinol 350. In contrast, the partially-hydrolyzed Vinol 523, which is comparable in molecular weight to the Vinol 325, gave an adsorption isotherm with little scatter, indicating the absence of flocculation. Partially hydrolyzed PVA shows specific interactions with polystyrene surfaces (mentioned below), and the absence of flocculation in this case is consistent with the theory proposed by Clark and Lai (14) for bridging flocculation. 

An essential prerequirement for the aggregation is the presence of different epitopes on the antigenes so that their functionality is greater than one. Reversible bridging flocculation of poly(lysine) with acrylic microgels has also been reported [376]. 

High shear forces are prevelant in the approach flow system to the paper machine (i.e. as the fibre suspension approaches the point of deposition on the wire), and these have a large impact upon the efficiency of retention aids (Figure 7.8). A study of the effect of shear can often be helpful in establishing the mechanism of retention. Bridging flocculation is irreversibly sensitive to shear (i.e. when the shear forces are removed the suspension does not reflocculate) whereas charge neutralisation is reversibly sensitive to shear. 

Because dry strength additives are often polyelectrolytes, they are able to behave as retention aids. This leads to flocculation of the fibres and to mass distribution (formation) variations in the sheet which can reduce strength. The flocculation can be destroyed irreversibly by shear, which suggests that bridging flocculation is involved. 

McClements, 2006 Anal et al., 2008). Different combinations of proteins and polysaccharides (e.g., P-lactoglobulin + pectin, carrageenan or alginate casein + pectin) have been investigated within the context of multilayer emulsion stabilization (Guzey and McClements, 2006). It seems that the main technical challenge associated with the utilization such complex formation for layer-by-layer emulsion stabilization is the avoidance of bridging flocculation (McClements, 2005, 2006). 

Experiments on interactions of polysaccharides with casein micelles show similar trends to those with casein-coated droplets. For example, Maroziene and de Kruif (2000) demonstrated the pH-reversible adsorption of pectin molecules onto casein micelles at pH = 5.3, with bridging flocculation of casein micelles observed at low polysaccharide concentrations. In turn, Tromp et al. (2004) have found that complexes of casein micelles with adsorbed high-methoxy pectin (DE = 72.2%) form a self-supporting network which can provide colloidal stability in acidified milk drinks. It was inferred that non-adsorbed pectin in the serum was linked to this network owing to the absence of mobility of all the pectin in the micellar casein dispersion. Hence it seems that the presence of non-adsorbed pectin is not needed to maintain stability of an acid milk drink system. It was stated by Tromp et al. (2004) that the adsorption of pectin was irreversible in practical terms, i.e., the polysaccharide did not desorb under the influence of thermal motion. 

Especially troublesome is bridging flocculation. It is therefore much more convenient to prepare emulsions with protein and polysaccharide components both present together in the aqueous medium before homogenization (Dickinson et al., 1998 Garti et al., 1999 Dickinson, 2008a). Moreover, in a direct comparison between the two techniques (Jourdain et al., 2008), it has been demonstrated that the experimentally more straightforward mixed emulsion approach can actually produce a better level of stability than the bilayer approach. 

In the case of very low polymer concentrations, bridging flocculation may occur as a polymer chain forms bridges by adsorbing on more than one particle (see also Fig. 13.12f). 

Fig. 3.2 Schematic representation for the mechanism of polymer-bridging flocculation. (From ref. [8])...

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. 

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