Depletion flocculation, emulsion stability

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 15.14. The potential energy curve for a depletion flocculated ionically stabilized emulsion shows that the sum of the van der Waals attraction and the osmotic attraction cause a secondary minimum (e.g.. Fig. 15.10). 

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 ... 

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). 

Although polysaccharide molecules often act as stabilizers for the emulsions, it is also possible for them to act as destabilizers, by the mechanism of depletion flocculation. 

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... 

Stability and stability loss for the emulsion without electrolyte should be influenced by depletion flocculation by the anionic micelles [21]. As expected, creaming velocity increases by increasing the amount of added salt and increasing the storage temperature and duration. It should be mentioned that, due to the decreasing droplet concen-... 

While high-molecular-weight polysaccharides are commonly added commercially to improve the stability of emulsions through rheological control, their presence at low concentrations has in many instances been demonstrated to have actually the opposite effect because of the phenomenon of depletion flocculation. An attractive interdroplet attraction occurs when two approaching droplet surfaces... 

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 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. 

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. 

Water soluble polymers can influence the stability of emulsions to gravitational separation in a variety of ways. Non-adsorbed polymers may either increase or decrease stability depending on their effective size and concentration in solution. Increasing the concentration of a polymer in solution causes an increase in continuous phase viscosity (and may even lead to gelation), which should slow down droplet movement. On the other hand, the presence of non-adsorbed polymer also increases the magnitude of the depletion attraction between droplets, which may cause flocculation and therefore accelerate droplet movement. The presence of an adsorbed polymer may also influence stability to gravitational separation in a number of ways. For example, the size of the droplets produced... 

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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