Depletion flocculation, causes

At moderate to high polymer concentrations, the free polymer chains in the solution may begin to exercise an influence. One such effect is the so-called depletion flocculation caused by the exclusion of polymer chains in the region between two particles when the latter are very close to each other (i.e., at surface-to-surface distances less than or equal to approximately the radius of gyration of the polymer chains). The depletion effect is an osmotic effect and is discussed further in Section 13.6. 

Another interesting phenomenon is that of depletion flocculation. This can be observed with dispersions (e.g. lattices) which contain inert additives, such as free polymer, non-ionic surfactant or even small (e.g. silica) particles. As the latex particles approach one another, the gaps between them become too small to accommodate the above additives, but the kinetic energy of the particles may be sufficient to enable them to be expelled from the gap i.e. a de-mix occurs, for which AG is positive. When this de-mix has been achieved, an osmotic situation exists in which the remaining pure dispersion medium will tend to flow out from the gap between the particles in order to dilute the bulk dispersion medium, thus causing the particles to flocculate. 

When a liquid dispersion contains non-adsorbing polymers there will be a layer of liquid surrounding each dispersed species that is depleted in polymer, compared with the concentration in bulk, solution. This causes an increase in osmotic pressure in the system compared with what it would be were the dispersed species absent. If the dispersed species move dose to each other then the volume of solvent depleted is reduced, reducing the overall osmotic pressure, which provides a driving force for flocculation. Xanthan gum, added in low concentrations, can cause depletion flocculation [291]. 

Depletion flocculation arises when a large unadsorbed, flocculating cosolute molecule does not fit properly into a small interparticle volume at the interface and the cosolute molecule accompanied by solvent is consequently expelled from the interface. As a result, the interparticle distance is shortened, causing an approach to x , and flocculation. Depletion stabilization is possible if the particle-cosolute attraction is greater than the particle-particle or cosolute-cosolute attraction. 

Increased depletion attraction. The presence of nonadsorbing colloidal particles, such as biopolymers or surfactant micelles, in the continuous phase of an emulsion causes an increase in the attractive force between the droplets due to an osmotic effect associated with the exclusion of colloidal particles from a narrow region surrounding each droplet. This attractive force increases as the concentration of colloidal particles increases, until eventually, it may become large enough to overcome the repulsive interactions between the droplets and cause them to flocculate (68-72). This type of droplet aggregation is usually referred to as depletion flocculation (17, 18). 

A special case to consider is the existence of what is termed depletion flocculation. This term originated from the observation that the addition of a small amount of nonadsorbing polymer will cause flocculation in a system. The reason for this effect is that, as the particles approach each other, the mobile chains of nonadsorbing polymer are squeezed out from between the particles. As the particles approach to very close distances, almost pure solvent exists between the particles, and at a given separation, the osmotic pressure that results from this pure solvent drives it out into the bulk solution and thereby causes flocculation. 

The presence of additives, such as high-molecular-weight polymers that may cause bridging or depletion flocculation. 

State (i) represents a phenomenon that is referred to as depletion flocculation and is caused by the addition of a free nonadsorbing polymer [35]. In this case, the polymer coils cannot approach the particles to a distance A (this is determined by the radius of gyration of free polymer R f), as the reduction in entropy on close approach of the polymer coils is not compensated by an adsorption energy. Thus, the suspension particles will be surrounded by a depletion zone with thickness A. Above a CFV of the free polymer, the polymer coils are squeezed out from between the particles and the depletion zones begin to interact At this point the interstices between the particles are free from polymer coils, such that an osmotic pressure is exerted outside the particle surface (the osmotic pressure outside is higher than in between the particles), and this results in a weak flocculation [35]. A schematic representation of depletion flocculation is shown in Figure 9.11. 

Starch-induced flocculation of 1% (w/w) kaolin suspensions and the adsorption of NaCMC were investigated by Jamstrom et al., where the kaolin was partly pretreated with sodium polyacrylate of low molecular weight (PAA) [22], In all flocculation experiments the kaolin suspension as well as the starch solution were adjusted to pH 7.5. The flocculation behavior of the PAA-treated suspension indicated that depletion flocculation is a very likely flocculation mechanism. When a hydrophobically modified starch was added, no flocculation occurred, and no adsorption of the modified starch on the kaolin particles could be detected. It is reasonable that water is a less good solvent for the hydrophobically modified starch and thus may cause the impossibility of depletion flocculation. On the other hand, a dispersing... 

In practice, depletion flocculation in a dilute dispersion causes the particles to sediment rapidly. Increasing the polymer concentration at first leads to increasing sedimentation (rate), but at still higher concentrations, sedimentation slows down or will be altogether absent. This occurs if the polymer concentration c is larger than the chain overlap concentration c see Section 6.4.2. At such conditions, the particles are immobilized in a... 

Also other species may cause depletion interaction. A case in point is surfactant micelles, for example, if an emulsion has been made with an unnecessary large concentration of surfactant, so that micelles remain after emulsification, this may cause aggregation of the droplets. In foods, however, the surfactant concentrations needed for depletion flocculation to occur are generally unacceptable. 

Assume that to a dispersion of spherical particles in water a given amount of a linear dextran (see Table 6.1) is added to increase the viscosity. However, the addition of the polymer also tends to cause depletion flocculation, which is undesirable. What would be the best value of the molar mass M of the polymer—large, small, or indifferent—if flocculation is to be prevented, while the viscosity becomes as high as possible ... 

Flocculation and subsequent stabilization of sols can also be caused by polymers that do not adsorb on the particle surface. In this case the mechanism of polymer action is different from the one described above and is related to the state of conformation of polymer molecules and change in the free energy of the system upon the transfer of polymer coil from gap between the particles into the solution bulk (a so-called depletion flocculation) [69,70]. 

The presence of polymers, either adsorbed on colloidal particles or free in solution, can lead to other interesting effects. For example, if a high-molecular-weight polymer is present at low concentration, remote segments of a polymer chain may be adsorbed on separate particles, causing them to be drawn together (bridging flocculation). The presence of an excess of non-adsorbed polymer can also result in flocculation (depletion flocculation). These and other special cases will be discussed in Chapter 9. 

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. 

THEORIES CMF DEPLETION STABILIZATION AND FLOCCULATION 379 have subsequently been corroborated by Cowell et al. (1978) and by Dodd (1980). The expression relatively high concentrations of free polymer in this context refers to concentrations greater than those needed to cause depletion flocculation. The absolute values of the free polymer concentration need not, however, be very large e.g. with polymers of molecular weight of over, say, 100000, volume fractions of free polymer of only a few per cent may he sufficient to prevent the onset of flocculation. A diagram illustrating the dependence of the reciprocal stability ratio (1/WO on the volume fraction of free polymer, as reported by Li-in-on et al. (1975) and Cowell et al. (1978), was presented in Fig. 16.4 and will not be discussed further. 

For naked particles, entry into the sequence of events shown in Fig. 17.20 occurs earlier because the particles are not coated. Addition of polymer causes steric stabilization to occur prior to depletion flocculation at much higher polymer concentrations. It is scarcely surprising that in this instance Cowell and Vincent reported the sequence instability stabilityinstability. The puzzling difference in behaviour is accordingly explained. 

Once an associative thickener is completely desorbed from the latex, it behaves like a non-adsorbing conventional thickener and can flocculate the latex by the depletion mechanism (see Section 13.3.1.1). Flocculation phase diagrams can be constructed showing the state of flocculation or deflocculation of the latex at a specified solids content, over a range of thickener and surfactant or cosolvent concentrations [97]. Because of their lower molar mass and less volumefilling backbones, the threshold concentration for desorbed HEUR thickeners to cause depletion flocculation is much higher than that for high molar mass cellulose ethers. 

Depletion interactions are interactions between two surfaces (particles) in the presence of firee, i.e., non-adsorbed, macromolecules, micelles, or very fine particles. Asakura and Oosawa (1954) first pointed out that if the distance between two surfaces h is smaller than the diameter of solute molecules Jm, this region will contain pure solvent depletion zone, cf. Fig. 3.14, left). Thus, an attractive force corresponding to the osmotic pressure of the bulk solution is acting on the two surfaces. Agglomeration caused by this effect is called depletion flocculation. In a second paper, the authors calculated the potential energy of this interaction for... 

Figure 1.5 Schematic representation of the situation giving rise to depletion flocculation. Polymer molecules are excluded from the space between particles causing an osmotic pressure differential between the excluded region and the continuous phase and giving rise to a net attractive depletion force.

On standing, concentrated suspensions reach various states (structures) that are determined by (1) Magnitude and balance of the various interaction forces, electrostatic repulsion, steric repulsion and van der Waals attraction. (2) Particle size and shape distribution. (3) Density difference between disperse phase and medium, which determines the sedimentation characteristics. (4) Conditions and prehistory of the suspension, e.g. agitation, which determines the structure of the floes formed (chain aggregates, compact clusters, etc.). (5) Presence of additives, e.g. high molecular weight polymers that may cause bridging or depletion flocculation. 

It was found fairly recently that free (non-adsorbing) polymer can affect colloid stability (4-11). Flocculation caused by free polymers is called depletion flocculation (5,6). The first theory for the depletion flocculation was the theory proposed by Asakura and Oosawa (10,11) of Nagoya University in Japan. According to them, when two particles approach each other in a polymer solution to a distance of separation that is less than the diameter of polymer molecules, polymer may be extruded from the inter-particle space. This leads to a polymer-depleted-free zone between two particles. An osmotic force is then exerted from the polymer solution outside the particles and this results in flocculation. 

It is likely that the flocculation of Ti02 dispersion observed in the high surfactant concentration is caused by the depletion effect of free surfactant molecules in this solution. A number of theories derived to explain the mechanism of depletion flocculation were reviewed by Napper in his textbook (6). 

FIGURE 15.13. Depletion flocculation occurs when nonadsorbing polymer dissolved in the continuous phase is excluded from regions between the droplets. This sets up an osmotic pressure difference that causes flocculation in a secondary minimum of the potential energy. Depletion flocculation is reversible with shear, but it reforms quickly when the shear is removed. Polymer molecules are shown much larger than actual scale. 

FIGURE 15.14. The potential energy curve for a depletion flocculated ionically stabilized emulsion shows that the sum of the van der Waals attraction and the osmotic attraction cause a secondary minimum (e.g.. Fig. 15.10). 

For those flocculation processes which are initiated by polymers, the depletion flocculation mechanism is also being discussed in addition to the mechanisms based on polymer adsorption. In the depletion mechanism, the osmotic pressure causes the displacement of polymer molecules from between neighboring particles as soon as these are closer than a particular minimum distance [8—10]. Theoretically, this mechanism is quite feasible, although it has as yet not been experimentally verified, in particular for real systems. 

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