Depletion flocculation steric stabilization

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. 

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.

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. 

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. 

The effects of steric barriers. The methodology whereby the depth of the minimum and the height of the maximum in the potential energy curves for spheres can be computed has been elaborated in the preceding sections. By assuming critical values for the minimum (—3 b7) and maximum (15A b7), it was possible to predict the onset of depletion flocculation (at V2 ) and depletion stabilization (at V2 ). 

Scheutjens and Fleer (1982) have developed a theory for depletion stabilization and depletion flocculation based upon their statistical thermodynamic approach to polymer adsorption and steric stabilization. This theory is cast in terms of the most primitive model for a polymer molecule, the random flight chain. This weakens the theory in so far as providing quaintitative predictions at the fundamental level for real systems is concerned. The theory does, however, offer qualitative results over a wide range of conditions, being especially powerful in establishing the various trends involved. 

It would appear that the effects of solvency on depletion flocculation and depletion phase separation are quite complex. The presence of steric layers means that depletion effects alone do not determine stability and proper allowance must be made for the effects of solvency on the steric interactions. 

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. 

FIGURE 10.12. In a sterically stabilized system cxjntaming low-molecular-weight or weakly adsorbed polymer (a), as two particles approach, the loosely bound polymer may desorb, leaving bare spots on the approaching surfaces, leading to an enhanced flocculation tendency (b). That process is referred to as depletion flocculation. ... 

In Sects. 1.2.1 and 1.2.2 we shall first qualitatively consider double layer and Van der Waals interactions, the two contributions to the DLVO potential (Sect. 1.2.3), and then discuss (polymeric) steric stabilization by end-attached polymer in Sect. 1.2.4. While not further discussed here we mention that adsorbing polymers, proteins or particles can also be used to protect colloids against flocculation. For protein adsorption, often used for instance in food emulsions, we refer to [28]. Using particles to stabilize colloids is referred to as Ramsden-Pickering stabilization [29]. Finally, the depletion interaction will be treated in Sect. 1.2.5. 

Bridging and depletion flocculation are two of the major mechanisms under the family called sensitisation , i.e. controlled flocculation of dispersions by addition of small quantities of materials which, if used in larger amounts, would act as stabilizing agents. Steric forces can also be attractive in some cases (at short separation) and then bridging can... 

Polymers are often used to stabilize colloidal systems by grafting them on the particle surfaces to provide steric repulsion [1,2], Polymers can also induce flocculation due to either depletion or bridging interactions [3],... 

Typical potential ener r diagrams that might be expected for simple combinations of depletion stabilization with steric or electrostatic stabilization are displayed in Fig. 17.14. The minimum in the potential enCTgy curve need not be below the zero energy level in order to induce weak flocculation, although it would be expected that this type of behaviour would be rare and depend upon the specific details of the system in question. 

In its second function, the additive must form some type of film or barrier (monomolecular, electrostatic, steric, or liquid crystalline) at the new L-L interface that will prevent or retard droplet flocculation and coalescence. The process of barrier formation or adsorption must be rapid relative to the rate of drop coalescence or a rather coarse emulsion will result. Also, with the formation of more interface, the adsorption of the emulsifier depletes its bulk concentration, so that attention must be paid to the quantity of the material employed relative to the final result desired, as well as its quality as an emulsifier. As will be seen below, the exact role of an emulsifier in emulsion formation can be quite complex, and is not always completely understood. In any case, its (or, in many cases, their) presence will be vital to successful emulsion formation and stability. 

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