Nonadsorbing Polymers Depletion Flocculation

Polymer molecules dissolved in the medium surrounding the particles may influence colloidal stability. A nonadsorbing polymer molecule with radius R cannot come within a distance R of the particle surface. For a highly flexible, coily polymer with a radius of gyration, R, this might be possible but the coil must then be deformed 

FIGURE 16.6 Polymer depleted layers around particles. 

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. 

When the mole fraction of polymer in the medium approaches unity, that is in a polymer melt, osmotic effects are absent. Colloidal particles dispersed in a polymer melt may be stable because aggregation causes deformation with a decreased conformational entropy of the polymer molecules. 

FIGURE 16.7 (a) Attraction between particles due to overlapping depletion regions and 

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Nonadsorbing polymer Depletion stabilization V Depletion flocculation... 

The flocculation of dispersed species induced by nonadsorbing polymer molecules due to depletion forces. When solutes such as polymer molecules do not, for some reason, enter the gap between adjacent surfaces an attractive force is created between the surfaces. This depletion force arises out of the solute s ability to influence osmotic pressure in bulk but not in the gap between the surfaces. 

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... 

As discussed below, a dispersion that has been somehow stabilized can also be made to flocculate by adding to the suspension a nonadsorbing polymer, which induces depletion flocculation (Asakura and Qosawa 1954, 1958 Vrij 1976 Fleer and Scheutjens 1982 Li-... 

A similar, and even more dramatic, viscosity enhancement was observed by Buscall et al. (1993) for dispersions of 157-nm acrylate particles in white spirit (a mixture of high-boiling hydrocarbons). These particles were stabilized by an adsorbed polymer layer, and then they were depletion-flocculated by addition of a nonadsorbing polyisobutylene polymer. Figure 7-9 shows curves of the relative viscosity versus shear stress for several concentrations of polymer at a particle volume fraction of 0 = 0.40. Note that a polymer concentration of 0.1 % by weight is too low to produce flocculation, and the viscosity is only modestly elevated from that of the solvent. For weight percentages of 0.4-1.0%, however, there is a 3-6 decade increase in the zero-shear viscosity ... 

Figure 7.12 Shear-stress dependence of the relative viscosity for dispersions in water of charged polystyrene particles of radius a = 115 nm with nonadsorbing Dextran T-500 polymer (synthesized from glucose) added as a depletion flocculant. The polymer molecular weight is 298,(HX), and its radius of gyration Rg is 15.8 nm. Volume fractions and polymer concentrations are

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 is produced by addition of a free nonadsorbing polymer [7]. In this case, the polymer coils caimot approach the particles to a distance A (this is determined by the radius of gyration of free polymer, Rq), as the reduction in entropy on close approach of the polymer coils is not compensated by an adsorption energy. The suspcakesension particles or emulsion droplets will be surrounded by a depletion zone with thickness A. Above a critical volume fraction of the free polymer, the polymer coils wiU be squeezed out from between the particles... 

Depletion flocculation As discussed above, the addition of a free nonadsorbing polymer can produce weak flocculation above a CFV of the free polymer, [Pg.157]

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. 

As summarized in the next sections, the micropipet technique has been used to measure adhesive interbilayer interactions based on these attractions that are enhanced by depletion flocculation produced by nonadsorbent polymer, and are attenuated by short-range hydration repulsion, thermal undulations and electrostatic double-layer repulsion energies [14,15,22,25]. 

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. 

Practically, the addition of a nonadsorbing polymer to a dispersion can induce flocculation of dispersed particles due to the depletion attraction. This was first observed by Cowell, Lin-In-On, and Vincent [1434]. When large amounts of poly (ethylene oxide) are added to an aqueous dispersion of hydrophilized polystyrene latex particles, the particles start to flocculate. For an organic dispersion, namely, hydrophobized silica particles in cyclohexane, de Hek and Vrij [1435] observed depletion-induced flocculation when dissolved polystyrene was added. Other combinations of particles and polymers followed [1436]. Phase diagrams for different particle-solvent-polymer systems were successfully drawn using the depletion potential of Asakura as interaction potential between dispersed spheres [1437] and for dissolved polymers using statistical mechanics [1438]. 

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