Flocculation depletion

Although depletion is one of the main mechanisms of aggregation, very little effort has been made to develop PBMs for depletion flocculation. This could be an important area of research for application of aggregation models in the processing of agrochemicals and cosmetics. 

This work was supported by the National Science Foundation (NSF Grants INT-96-05197 and INT-01-17622) and the NSF Industry/University Cooperative Research Center (lUCRC) for Advanced Studies in Novel Surfactants at Columbia University (NSF Grant EEC-98-04618). The authors thank the management of Tata Research Development and Design Centre for the permission to publish this chapter. V.R. thanks Professor E. C. Subbarao, Professor Mathai Joseph, Professor P. C. Kapur and Dr Pradip for their advice and encouragement. 

3 Somasundaran, P. and Arbiter, N. (eds) (1979) Beneficiation of Mineral Fines, AIME, USA. 

4 Moudgil, B.M. and Somasundaran, P. (1994) Dispersion and Allegation Fundamentals and Applications, Engineering Foundation, USA. 

5 Somasundaran, P., Markovic, B., Krishnakumar, S. and Yu, X. (1997) Colloidal systems and interfaces stability of dispersions through polymer and surfactant adsorption, in Handbook of Surface and Colloid Chemistry (ed. K.S. Birdi), CRC Press, USA. 

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.  

If one adds a polymer that is not adsorbed or poorly adsorbed on the particles to a colloidal solution, there may occur another phenomenon, termed depletion flocculation. In depletion flocculation, as two particles approach, polymer chains that are weakly adsorbed, or simply are located between the particles, become squeezed out of the area of closest approach, leaving bare surfaces that are attracted in the normal way. However, there may arise an additional attractive force as a result of the removal of polymer from the intervening region (Fig. 10.12). 

As polymer is forced out of the area between the approaching particles, the local osmotic balance is displaced that is, the solution concentration between the particles is less than that in the bulk. Osmosis then forces solvent to flow from between the particles out into the solution. The net effect on the particles is that they are drawn together by the solvent flow (a type of hydrodynamic suction effect, if you will), resulting in a loss of stability and flocculation. 

The above picture of depletion flocculation is, of course, very schematic and simple-minded, but it should serve to illustrate the concepts involved. A more detailed discussion would involve the introduction of complex theories of polymer adsorption and solution phenomena that are beyond the scope of this book. 

The coagulation of emulsions and dispersions due to random Brownian motion has historically been the topic of most general interest to surface and colloid scientists because of the experimental accessibility of data (with sufficient 

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]. 

At sufficiently high volume fraction of the suspensions and high volume fraction of free polymer a 100% sediment volume is reached and this is effective in eliminating sedimentation and formation of dilatant sediments. 

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Jenkins P and Snowden M 1996 Depletion flocculation in colloidal dispersions Adv. Colloid Interface Sc/. 68 57-96... 

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... 

M. Heidenreich, R. Kimmich 1999, (Magnetic-resonance determination of the spatial dependence of the droplet size distribution in the cream layer of oil-in-water emulsions Evidence for the effects of depletion flocculation) Phys. Rev. E 59, 874. 

Vincent, B., Edwards, J., Emmett, S. and Jones, A. (1986) Depletion flocculation in dispersions of sterically-stabilised particles (soft spheres). Colloids Surf,... 

Hazardous Materials Table, 20 809 Dependent chemical reactions, 21 336-337 Dephlegmators, 3 54-56 10 616 Depleted uranium, 25 421 Depletion allowance, 9 539 Depletion flocculation, 10 122, 123 Depletion provisions, magnesium, 15 347 Depletion region, 14 838 23 35 width, 22 244 Depolarization, in cardiac... 

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,... 

In the case of biopolymer molecules residing in the space between colloidal particles or droplets, the force associated with the deep energy minimum at contact is often referred to as the depletion force because the intervening biopolymer species are depleted from the narrow gap between the pair of neighbouring particles. This attractive interparticle interaction underlies the phenomenon of reversible depletion flocculation in oil-in-water emulsions (see equation (3.41) in chapter 3). 

Dickinson, E., Golding, M. (1997). Depletion flocculation of emulsions containing unadsorbed sodium caseinate. Food Hydrocolloids, 11, 13-18. 

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). 

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. 

Chanamai, R. and McClements, D.J. 2001. Depletion flocculation of beverage emulsions by gum arabic and modified starch. J. Food Sci. 66 457-463. 

Manoj, P., Fillery-Travis, A.J., Watson, A.D., Hib-berd, D.J., and Robins, M.M. 1998. Characterization of depletion-flocculated polydisperse emulsion I. Creaming behavior. J. Colloid Interface Sci. 207 283-293. 

McClements, D.J. 2000. Comments on viscosity enhancement and depletion flocculation by polysaccharides. Food Hydrocolloids 14 173-177. 

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. 

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.

Dense Non-aqueous Phase Liquid Depletion Flocculation... 

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