Flocculation of particles

It was found that [5-7] the rate of flocculation of particles produced by the bridging action of polymer is the slower process and, consequently, the rate-determining step. The primary adsorption of polymer is fairly rapid, but the slow attainment of the adsorption equilibrium under agitation arises at least in part from the breakdown of floes offering new surfaces for adsorption. Thus, the bridging step is slow because a polymer adsorbed on one particle must find another particle having a free surface available to complete the bridge. 

Flocculation of particles and capture of oligomers to a point of constant particle population... 

The major unresolved questions for these systems are the coalescence and flocculation of particles under mechanical agitation and the parameters which influence copolymer composition drift. 

Adsorption on Kaolinite. As for polyacrylamides, adsorption of XCPS on kaolinite is conducted as a function of S/L and the results extrapolated to S/L=0. However, the S/L dependence of XCPS adsorption on kaolinite is considerably less than that for HPAM. This is due to the flat conformation of the adsorbed molecules of semirigid xanthan (25) compared to the more extended conformation of flexible HPAM (27). The absence of loops and tails in the adsorbed XCPS layer thus diminishes the probability of flocculation of particles by polymer bridging. The slight dependence in adsorption on S/L may therefore be attributed to coagulation of particles induced by Ca. ... 

Figure 13.4 shows the titration curve of a palladium nitrate solution [Pd] = 5 g/L by soda [NaOH] = 0.1N. Three regions can be clearly distinguished, corresponding to a true solution (starting point until addition of about 14 mL of soda), to the formation of colloidal PdO particles (from 14 to 24 mL added), and to the flocculation of particles with segregation of liquid and solid phases (above 25 mL). 

It is postulated that the main thermodynamic driving force for particle adsorption at the liquid-liquid interface is the osmotic repulsion between the colloidal particles and hydrophilic starch polymer molecules. This leads to an effective depletion flocculation of particles at the boundaries of the starch-rich regions. At the same time, the gelatin has a strong tendency to adsorb at the hydrophobic surface of the polystyrene particles, thereby conferring upon them some degree of thermodynamic... 

Verify (a) that Equation (40) is a solution to Equation (37), (b) that Equation (41) reduces to Equation (21) if = 0, and (c) that Equation (46) leads to a rate constant larger than kr if 4> = 0 for r > A and = — oo for r < A, where A > 2R,. Show that the experimental kr value cited in Problem 11 is consistent with a value of A equaling 5.4 Rs for an aqueous colloid at 20°C (r = 0.01 P). Discuss the relevancy of this last result to the rapid flocculation of particles between which van der Waals attraction exists. 

The stability of electrostatically charged sols has been studied extensively and is now reasonably well understood. More recently the stabilising action of adsorbed or chemically anchored non-ionic polymers has received much attention. There has been however little systematic work on polyelectrolyte stabilisers apart from a number of investigations of the flocculation of particles bearing adsorbed biopolymers, usually proteins, by simple salts ( 2). These have shown that polyelectrolyte covered particles can be more stable with respect to the addition of salt than simple charged systems, and the extra stability has been ascribed to the polymeric nature of the surface layer. The precise mechanism by which polyelectrolytes stabilise dispersions in the presence of high concentrations of salt has however remained unclear. 

It is noteworthy that the maximum of the particle number curve was observed in our experiments also at such concentrations of emulsifier at which the polymerizing system is not depleted of emulsifier vJSlat )[cmc) which was the case for the Experiment 2 (Figure b). Therefore, flocculation of particles is not the consequence of emulsifier lack in the system, as was suggested by Brooks ( ), but is due to the limited capability of its adsorption. 

Wu, W., Giese, R.L, and Van Oss, C.J. Stability vs. flocculation of particle suspension in water-correlation with the extended DLVO approach for aqueous system, compared with DLVO theory, Colloidd Surfaces B Biointerfaces, 14, 47,1999. 

It may finally be noted that depletion flocculation of particles by a soluble polymer is closely related to segregative phase separation of two soluble polymers—e.g., a protein and a polysaccharide—as described in Section 6.5.2. It is therefore not surprising that emulsion droplets, covered with a protein layer, can readily show depletion flocculation due to the addition of a nonadsorbing polysaccharide. 

The hooks on the surface of seeds from the Burdock plant enable them to become attached to the fur of passing animals. Observation of this phenomenon provided the inspiration for the invention of Velcro [104], which is based on interlocking hooks and loops. The hook and loop phenomenon is also reminiscent of the interweaving of adsorbed polymer molecules causing flocculation of particles in suspension (Section 5.6.3). 

Stability encompasses various compound properties and the first to be discussed is the stability to flocculation or aggregation. Lack of stability alludes to the sedimentation or flocculation of particles or droplets. This condition is either a required or an undesired aspect, depending on the application. 

Sometimes, flocculation of particles is useful by increasing the rate of sedimentation. With typical brine precipitates, the flocculants most frequently used are acrylic... 

It is commonly observed that gentle stirring promotes flocculation of particles which have been destablized and which may have commenced to aggregate by Brownian motion (see Part I of this chapter). This is due to the velocity gradients which are induced in the liquid causing relative motion and therefore collisions between the particles which are present. Such flocculation caused by fluid motion is called orthokinetic , to differentiate it from that caused by Brownian motion, called perikinetic . 

Neelsen et al. [140] investigated the effect of NaCl on the emulsion polymerization of vinyl chloride in the presence and absence of emulsifier (SDS) or inhibitor (p-benzoquinone (BQ) or chloranile (CA)). Under reaction conditions 2 [BQ] = [CA], the same induction periods and the conversion curves were obtained. The addition of NaCI (a coagulating agent) caused the decrease of the rate of polymerization and the increase of the particle size. The addition of NaCl led to an increase in the induction period. Here, the same induction period and the same shape of the conversion curves were obtained and [BQ] = [CA]. The different behavior of BQ in the presence of NaCl was explained by the reaction of O with BQ which leads to the chlorination of BQ. O radicals are generated by the reaction of peroxodisulfate with NaO (a labelled salt). The addition of NaO increased flocculation of particles which led to the formation of larger particles and lower rates of prflymerization. The dependence of the rate rtf polymerization on the NaO concentration was expressed by the following equation. 

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