Osmotic depletion flocculation

A non-adsorbing polymer in solution can also destabilise a dispersion through a mechanism called depletion flocculation. When polymer molecules do not interact favourably with the particle surfaces from an enthal-pic perspective, they are repelled from the surface regions due to entropic reasons. A depletion zone around the particles is created which has a lower average polymer concentration than the bulk solution. The osmotic... 

The first observation of depletion flocculation by surfactant micelles was reported by Aronson [3]. Bibette et al. [4] have studied the behavior of silicone-in-water emulsions stabilized by sodium dodecyl sulfate (SDS). They have exploited the attractive depletion interaction to size fractionate a crude polydisperse emulsion [5]. Because the surfactant volume fraction necessary to induce flocculation is always lower than 5%, the micelle osmotic pressure can be taken to be the ideal-gas value ... 

Once an emulsion has been formed, its stability with respect to depletion flocculation is determined primarily by the nature of thermodynamically unfavourable interactions (Ay > 0) between the biopolymers which influences the osmotic pressure in the aqueous phase according to equation (3.9) (see also equation (3.19)). That is, the value of A, influences the depth of the minimum in the depletion potential, AGdep (see equation (3.41) and Figure 3.6). 

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... 

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]. 

Because of restrictions on the number of possible configurations, non-adsorbing polymers tend to stay out of a region near the surfaces of the particles, known as the depletion layer. As two particles approach, the polymers in the solution are repelled from the gap between the surfaces of the particles. In effect the polymer concentration in the gap is decreased and is increased in the solution. As a result, an osmotic pressure difference is created which tends to push the particles together. The resulting attractive force is the reason for depletion flocculation. In contrast to this, depletion stabilisation has been mentioned above. 

It is well known that the presence of an excess of nonadsorbed polymer can result in flocculation of colloidal particles by the so-called depletion flocculation mechanism [13], In the clay-PEO system, the excess PEO molecules in the supernatant fluid would exert an osmotic pressure on the gel, and the effect should be similar to that of applying an external pressure to the gel indeed, because the... 

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. 

Depletion flocculation occurs when two particles approach each other to within a distance that is smaller than the particle size so that no other particle can fit into the space between them. The osmotic pressures between the particles and in the rest of the dispersion are not balanced, and this pressure difference pushes the two particles toward each other. This leads to a further increase in viscosity and stability. 

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. 

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. 

The effect of water-soluble polymers on the properties of dilute lamellar dispersions was studied and the primary effect was found to be due to the exclusion of polymer from the water layers. This exclusion results in an osmotic compression of the water layers and a reduction in the vesicle diameter, phase volume, and dispersion viscosity. Depletion flocculation leads to fusion and morphological changes in the dispersion. Polyelectrolytes screen the electrostatic repulsion between bilayers and vesicles and this effect is superimposed on the osmotic compression. There was no apparent direct interaction between the polymmrs and the bilayers observed. 

The range over which depletion attraction operates equals 2R. In particular, for highly swollen polymers, R may reach values of some tens of nanometer and, hence, the depletion forces may be effective over separation distances between particles that exceed the range of dispersion and double layer forces (cf Section 16.1). On the other hand, the osmotic forces are relatively weak. Depletion flocculation occurs when the molar polymer concentration is sufficiently high, which is more readily achieved by using polymers of a relatively low degree of polymerization. 

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.

Another method of reducing creaming or sedimentation is to induce weak flocculation in the emulsion system. This may be achieved by controlling some parameters of the system, such as electrolyte concentration, adsorbed layer thickness and droplet size. These weakly flocculated emulsions are discussed in the next section. Alternatively, weak flocculation may be produced by addition of a free (non-adsorbing) polymer. Above a critical concentration of the added polymer, polymer-polymer interaction becomes favourable as a result of polymer coil overlap and the polymer chains are squeezed out from between the droplets. This results in a polymer-free zone between the droplets, and weak attraction occurs as a result of the higher osmotic pressure of the polymer solution outside the droplets. This phenomenon is usually referred to as depletion flocculation [59] and can be applied for structuring emulsions and hence reduction of creaming or sedimentation. 

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. 

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). 

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