Depletion flocculation stabilization

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 ... 

DAR Jones. Depletion flocculation of sterically stabilized particles. PhD thesis, Bristol,... 

Vincent, B., Edwards, J., Emmett, S., Jones, A. (1986). Depletion flocculation in dispersions of sterically-stabilized particles ( soft spheres ). Colloids and Surfaces, 18, 261-281. 

Radford, S.J., Dickinson, E. (2004). Depletion flocculation of caseinate-stabilized emulsions what is the optimum size of the non-adsorbed protein nano-particles Colloids and Surfaces A Physicochemical and Engineering Aspects, 238, 71-81. ... 

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). 

There seems to be a sort of analogy here with the arrested phase separation of a protein-stabilized depletion-flocculated emulsion containing a thermodynamically incompatible hydrocolloid like xanthan gum (Moschakis et al., 2005 Dickinson, 2006b). 

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.

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. 

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. 

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 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. 

Rawson, S. Ryan, K. Vincent, B. Depletion flocculation in sterically stabilized aqueous systems using poly electrolytes. Colloids and Surfaces 1988, 34, 89-93. 

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. 

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]. 

Syneresis. This chapter began with consideration of the depletion layer effect. This phenomenon can be seen in coatings that contain large latices (>300 nm) not highly stabilized by surface-attached (hydroxy-ethyl)cellulose fragments (16), and is in part the problem observed in the last sections of Chapter 27. The phenomenon is not necessarily restricted to HEC-thickened formulations and depletion flocculation. In our studies, syneresis is observed in thickened aqueous solutions and in dispersed systems containing the model trimer associative thickener (Scheme II) it can be overcome by addition of conventional surfactants. Syneresis in HMHEC-thickened solutions is discussed in Chapter 19 in the absence of a dispersed phase. Syneresis is discussed in the following chapter where additives that substantially enhance low shear viscosities are added to inhibit syneresis. 

The Effects of Free Polymer on Colloid Stability Theories of Depletion Flocculation and Depletion Phase Separation... 

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. 

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. 

Electrostatically stabilized dispersions. Perhaps the most conclusive experiment that has been performed to-date to demonstrate the existence of depletion flocculation has been disclosed by Sperry et al. (1981). Other workers [e.g. de Hek and Vrij (1979) and Vincent and coworkers] studied dispersions whose particles were enveloped in a steric layer. Accordingly, the presence of the steric layer could have been mandatory for the occurrence of flocculation. Indeed, both Vincent et al. (1978) and Vrij (1976) have proposed that the steric layers are either wholly or partially responsible for the observed phenomena. What Sperry et n/. (1981) have succeeded in doing is to generate flocculation in a latex system from which steric layers were seemingly absent. 

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. 

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. 

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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