Stabilization depletion

At high polymer concentrations, one may also have what is known as depletion stabilization. The polymer-depleted regions between the particles can only be created by demixing the polymer chains and solvent. In good solvents the demixing process is thermodynamically unfavorable, and under such conditions one can have depletion stabilization. 

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 stabilization of dispersed species induced by the interaction (steric stabilization) of adsorbed polymer chains. Example adsorbed proteins stabilize the emulsified oil (fat) droplets in milk by steric stabilization. Also termed depletion stabilization. See also Protection. 

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. 

Figure 10.18 is a schematic representation of depletion stabilization in which the polymer is prevented from the zone of close approach between two particles. As a result of this low polymer concentration between the particles due to size exclusion, there is a lower osmotic pressure, which results in (1) an attractive force for greater than theta solvents and (2) a repulsive force for less than theta solvents. Theta solvents will be discussed in the section on the thermodynamics of polymer solutions, but first a discussion of pol3naaer properties. 

Several theories have been put forward to account for the distributicm of polymer segments in the depletion zone. The theories of Feigin and Napper [48] and Scheutjens and Fleer [49] are qualitatively different from the theory of Asakura and Oosawa and de Cannes and coworkers [50,51] in that they predict not only depletion flocculation but also depletion stabilization. Depletion stabilization has not to date been verified experimentally although depletion fiocculation has been verified experimentally for several systems [52,53]. The effect of an adsorbed poljnner layer [54] and ordered solvent layers [55] on depletion flocculation is also under theoretical attack. The depletion stabilization interaction energy cannot simply be added to the other interaction energy terms to give the total interaction energy. 

FIGURE 11.11 Different interaction with homopolymer molecules. Cases (a) and (b) the molecule adsorbs on the particle and results flocculation, at low concentration and steric stabilization at high concentrations respectively. In cases (c) and (d) does not adsorb and results depletion flocculation, at low concentration, and depletion stabilization at high concentration respectively. 

The mixing of surfactant and polymer in the porous medium occurs due to both dispersion and the excluded volume effect for the flow of polymer molecules in porous media, which in turn could lead to the phase separation. Figure 16 illustrates the schematic explanation of the surfactant-polymer incompatibility and concomittant phase separation. We propose that around each micelle there is a region of solvent that is excluded to polymer molecules. However, when these micelles approach each other, there is overlapping of this excluded region. Therefore, if all micelles separate out then the excluded region diminishes due to the overlap of the shell and more solvent becomes available for the polymer molecules. This effect is very similar to the polymer depletion stabilization (55). Therefore, this is similar to osmotic effect where the polymer molecule tends to maximize the solvent for all possible configurations. ... 

There are at present two different mechanisms whereby polymer chains can impart colloid stability steric stabilization and depletion stabilization. 

Depletion stabilization. This differs from steric stabilization in that stability is imparted not by attached polymer but rather by macromolecules that are free in solution (see Fig. 1.6). Investigations of this phenomenon, which was first studied experimentally in 1975 (Li-in-on et ai, 1975) and theoretically in 1980 (Feigin and Napper, 1980b), are still in their infancy. 

In addition to electrosteric stabilization, it is possible to have combinations of depletion stabilization with both steric and/or electrostatic stabilization. The combination of depletion and steric stabilization is quite common at high concentrations of free polymer in the dispersion medium. 

A simpler approach to steric and depletion stabilization is to use the predictions of the random flight chain at the same reduced distance, say, HoKr y. The chain dimensions are used here as an arbitrary reduction parameter. The validity of this simple procedure can only be assessed after an exact theory has been elaborated. Unfortunately, such an... 

In the foregoing chapters the effects of attached polymer chains on colloid stability have been set forth. We now turn to consider the effects of macromolecules that are not attached to the particles but rather are free in solution. Surprisingly, such free polymer is still able to affect colloid stability, being capable of generating both stability and flocculation. Stability that is imparted by free polymer is termed depletion stabilization. Aggregation that is induced by free polymer is called depletion flocculation. The latter will be discussed in this Chapter, consideration of depletion stabilization being postponed until the next Chapter. 

The theories of Feigin and Napper (1979) and Scheutjens and Fleer (1982) are qualitatively different from those of Asakura and Oosawa (1954 1958) (and subsequent elaborations thereof) and de Gennes and coworkers (Joanny et ai, 1979 de Gennes, 1981 1982) in that they predict not only depletion flocculation but also depletion stabilization. For this reason, presentation of the former two omnibus theories will be delayed until their predictions with regard to both depletion stabilization and depletion flocculation are elaborated. The de Gennes approach, which does not predict the occurrence of depletion stabilization, will be presented at this juncture. 

The fact that de Hek and Vrij observed phase separation rather than flocculation in their experiments sug ts that it would be unlikely that depletion stabilization could be observed in these systems, if only because the dispersed particles are stable after phase separation. There is, however, another possible explanation if phase separation occurs because the free... 

One possible explanation for the phase separation in both aqueous and nonaqueous systems is the very high occupancy of the space by the sterically stabilized particles. This would mean that the free polymer cannot diffuse into the dispersion media without a significant loss of configurational entropy. The exigencies created by such severe volume restrictions at high dispersed phase concentrations could be responsible for phase separation. The fact that the polymer chains cannot physically diffuse into the dispersion would prevent the chains from inducing either depletion flocculation or depletion stabilization. 

It is apparent that up to a critical volume fraction Vj of free polymer, the particles displayed long-term stability (1/1F=0). Any further increase in the free polymer concentration resulted in the onset of instability, which was manifested by an increase in l/W. In all cases, however, l/W reached a maximum value and then declined. Flocculation was thus not evident at high concentrations of free polymer. It usually occurred only over a finite range of free polymer concentrations. This lack of flocculation at higher concentrations of free polymer is the phenomenon of depletion stabilization mentioned in the prefatory remarks to this chapter. It will be considered in detail in the next chapter. 

Cowell et al. also found that the maximum in the instability (i.e. in X/W) corresponded to the free polymer being of a molecular weight comparable to that of the anchor polymer. Even imder these conditions, however, the dispersions only flocculated at a rate of about one-third of the hypothetical Smoluchowski diffusion controlled rate. This rate is significantly less (ca one-half) than that expected when allowance is made for hydrodynamic interactions between the approaching particles. This rate reduction, if real, may indicate the presence of a small repulsive energy barrier that must be surmounted during the flocculation process, as predicted by the theories of depletion stabilization. 

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. 

The molecular weight dependence of the critical polymer concentration for depletion stabilization... 

Little work has been published on depletion stabilization in nonaqueous dispersion media. Clarke and Vincent (1981a) have noted that it is possible to prevent silica particles stabilized by polystyrene in ethylbenzene from undergoing depletion flocculation by adding a high concentration of polystyrene (v2 =0 015 for a free polystyrene molecular weight ofca 2 x 10 ). This was the first reported observation of depletion stabilization in nonaqueous dispersion media. 

If the stability of particles at relatively high concentrations of free polymer arises from the presence of a repulsive maximum in the potential energy curve, then it follows that any stability that is imparted by free polymer must be of a kinetic type. Depletion stabilization thus corresponds to thermodynamic metastability. In this regard, it obviously resembles electrostatic stabilization rather than steric stabilization, which usually corresponds to thermodynamic stability. 

An alternative approach to depletion stabilization As mentioned previously, the presence of the maximum in the potential energy curve is essential for the occurrence of depletion stabilization. The origin of this maximum can be envisaged in an alternative, but entirely equivalent, fashion from that set forth in the penultimate section. 

Quantitative theories of depletion stabilization and depletion flocculation... 

It was mentioned in Section 15.2.2.2 that both Feigin and Napper (1980a,b) and Scheutjens and Fleer (1982) had developed portmanteau theories that comprehend both depletion stabilization and depletion flocculation. These... 

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