Colloidal dispersions depletion flocculation

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

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

The theory of Asakura a Asakura and Oosawa (1954) were the first to recognize that depletion effects could give rise to the flocculation of colloidal dispersions. They considered the specific example of two parallel flat plates immersed in solutions of rigid molecules, either spherical or rod shaped. When the distance between the plates is smaller than the diameter of the solute molecules, assumed for the purposes at hand to be spherical, none of these molecules can enter the domain between the plates. This region is then composed entirely of solvent. The solution outside the plates retains its bulk concentration of solute and so it exerts an inward force, arising from its osmotic pressure, on the plates. 

Fig. 4.1 Types of instability that occur after mixing a colloidal dispersion with a polymer solution. When the polymer chains do not adsorb depletion leads to partitioning of colloids and polymers over different phases (2). In case of adsorption (and low polymer concentrations) bridging between different particles can induce flocculation (1)...

The predictions of different quantitative criteria for stability-instability transitions were investigated [461], having in mind that the oscillatory forces exhibit both maxima, which play the role of barriers to coagulation, and minima that could produce flocculation or coalescence in colloidal dispersions (emulsions, foams, suspensions). The interplay of the oscillatory force with the van der Waals surface force was taken into account. Two different kinetic criteria were considered, which give similar and physically reasonable results about the stability-instability transitions. Diagrams were constructed, which show the values of the micelle volume fraction, for which the oscillatory barriers can prevent the particles from coming into close contact, or for which a strong flocculation in the depletion minimum or a weak flocculation in the first oscillatory minimum could be observed [461]. 

Another unique phenomenon involving colloidal dispersions stabilized by low molecular weight, weakly adsorbed polymer chains is the depletion flocculation mechanism [41], as shown in Figure 2.12. When an isolated pair of the particles approach each other, the weakly adsorbed polymer chains are squeezed out of the overlap volume due to the greatly reduced space available for these polymer chains. This then results in the imbalance of the local osmotic pressure that is, the concentration of the adsorbed polymer is lower than that in the continuous bulk phase. Thus, water molecules are forced to diffuse out of the overlap region to counterbalance the osmotic pressure effect. The net effect is that the particles are pulled together and flocculation takes place. 

Oscillatory structural forces appear in thin films of pure solvent between two smooth solid surfaces and in thin liquid films containing colloidal particles including macromolecules and surfactant micelles (Israelachvili 1992). In the first case, the oscillatory forces are called the solvation forces and they are important for the short-range interactions between solid particles and dispersions. In the second case, the structural forces affect the stability of foam and emulsion films as well as the flocculation processes in various colloids. At lower particle concentrations, the structural forces degenerate into the so-called depletion attraction, which is found to destabilize various dispersions. 

So- far only non-adsorbing polymers have been considered, which flocculate by a depletion mechanism. Other molecules, which are slightly hydrophobic, may be able to be adsorbed at the oil-water interface. To prepare an emulsion a surfactant is also needed, but the surfactant is slowly displaced at the interface by the polysaccharide molecules. Adsorbing polymers are well known in colloid science, and their effect on the dispersion stability depends on the surface coverage of the polymer. At high concentrations, the drop surfaces are completely covered and... 

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