Depletion flocculation colloid stability

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. ... 

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

Two diverse views of non-specific adhesion processes form the bases for contemporary theories introduced to rationalize observations of colloidal stability and flocculation in solutions of macromolecules (see 16-18 for general reviews). The first view is based on adsorption and cross-bridging of the macromolecules between surfaces. Theories derived from this concept indicate a strong initial dependence on concentration of macromolecules there is a rapid rise in surface adsorption for infinitesimal volume fractions (32) followed by a plateau with gradual attenuation of surface-surface attraction because of excluded volume effects in the gap at larger volume fractions (19-20). The interaction of the macroinolecule with the surface is assumed to be a snort range attraction proportional to area of direct contact. The second - completely disparate - view of non-specific adhesion is based on the concept that there is an exclusion or depletion of macromolecules in the vicinity of the surface, i.e. no adsorption to the surfaces. Here, theory shows that attraction is caused by interaction of tne (depleted) concentration profiles associated with each surface which leads to a depreciated macrornolecular concentration at the center of the gap. The concentration... 

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 life time of the transient gel is determined by the strength of the depletion interaction and the colloid concentration and plays a role in many practical systems. For example in salad dressing, which is an oil-in-water emulsion, the depletion flocculation of the oil droplets induced by the addition of a polysaccharide such as xanthan leads to the formation of a particle network [112, 113], The yield stress of this network (in the sense of food science) stabilizes the... 

It was found fairly recently that free (non-adsorbing) polymer can affect colloid stability (4-11). Flocculation caused by free polymers is called depletion flocculation (5,6). The first theory for the depletion flocculation was the theory proposed by Asakura and Oosawa (10,11) of Nagoya University in Japan. According to them, when two particles approach each other in a polymer solution to a distance of separation that is less than the diameter of polymer molecules, polymer may be extruded from the inter-particle space. This leads to a polymer-depleted-free zone between two particles. An osmotic force is then exerted from the polymer solution outside the particles and this results in flocculation. 

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. 

Feigin RI, Napper DH. 1980. Depletion stabilization and depletion flocculation. J Colloids Interface Sci 75 525-541. 

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],... 

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. 

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. 

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]. 

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