Flocculation of emulsions

This can occur if the energy barrier is small or absent (for electrostatically stabilised emulsions) or when the stabilising chains reach poor solvency (for sterically stabilised emulsions, that is if / 0.5). For convenience, the flocculation of electrostatically and sterically stabilised emulsions will be discussed separately. 

As discussed above, the condition for kinetic stability is 25 kT since, when Gmax 5kT flocculation will occur. Two types of flocculation kinetics may be distinguished fast flocculation with no energy barrier and slow flocculation when an energy barrier exists. 

The fast flocculation kinetics was treated by Smoluchowsld [27], who considered the process to be represented by second-order kinetics and to be simply diffusion-controlled. The number of particles n at any time t may be related to the final number (at t = 0) by the following expression  

The half-life ( = (V2) nf] can be calculated at various or volume fraction (p, as given in Table 10.4. 

The slow flocculation kinetics was treated by Fuchs [28] who related the rate constant k to the Smoluchowsld rate by the stabiHty constant W  

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

Dickinson, E., Hunt, J.A., Horne, D.S. (1992). Calcium induced flocculation of emulsions containing adsorbed p-casein or phosvitin. FoodHydrocolloids, 6, 359-370. 

Evidence for the flocculation of emulsion droplets is commonly derived from a combination of rheological and creaming stability experiments. For instance, a marked increase in both the viscoelasticity of emulsions of moderately high oil volume fraction (40 vol%) and the rapid serum... 

Chen, J. and Dickinson, E. 1995a. Protein/surfactant interfacial interactions. Part 1. Flocculation of emulsions containing mixed protein + surfactant. Colloids Surf. A Physicochem. Engin. Aspects 100 225-265. 

Surfactants are used for stabilization of emulsions and suspensions against flocculation, Ostwald ripening, and coalescence. Flocculation of emulsions and suspensions may occur as a result of van der Waals attraction, unless a repulsive energy is created to prevent the close approach of droplets or particles. The van der Waals attraction Ga between two spherical droplets or particles with radius R and surface-to-surface separation h is given by the Hamaker equation,... 

The second contribution to the steric interaction arises from the loss of configurational entropy of the chains on significant overlap. This effect is referred to as entropic, volume restriction, or elastic interaction, Gei. The latter increases very sharply with a decrease in h when the latter is less than 8. A schematic representation of the variation of Gmix, Gei, G, and Gj =G X + Gei + Ga) is given in Fig. 10. The total energy-distance curve shows only one minimum, at h 25, the depth of which depends on 5, R, and A. At a given R and A, G decreases with an increase in 5. With small particles and thick adsorbed layers (5 > 5 nm), G, becomes very small (approaches thermodynamic stability. This shows the importance of steric stabilization in controlling the flocculation of emulsions and suspensions. 

Thus, a plot of r versus t gives a straight line, in the initial time of flocculation, and k can be calculated from the slope of the line. Flocculation of emulsions can also be assessed by direct droplet counting using optical microscopy (with image analysis), using the Coulter counter and Photosedimentation methods. 

Flocculation of emulsions resembles that of suspensions and the driving force is the van der Waals attraction, which becomes significant at short distances of separation. To prevent flocculation, one needs a strong repulsive force that operates at intermediate separations, thus preventing the close approach of the droplets. Two main repulsive forces can be distinguished Electrostatic and steric (discussed in detail in Chapters 6 and 7). 

It is the process in which emulsion drops aggregate, without rupture of the stabilizing layer at the interface. Flocculation of emulsions may occur under conditions when the van der Waals attractive energy exceeds the repulsive energy and can be weak or strong, depending on the strength of inter-drop forces. 

Flocculation of emulsions is the result of van der Waals attraction that is universal for all disperse systems. The van der Waals attraction was described before. It showed that is inversely proportional to the droplet-droplet distance of separation... 

The flocculation of emulsions can be assessed using turbidity measurements. For dilute emulsions (which may be obtained by carefully diluting the concentrate in the supernatant liquid), the rate of flocculation can be determined by measuring turbidity, T, as a function of time,... 

Surfactants are also essential for prevention of flocculation of emulsions. In the absence of an emulsifier, the droplets undergo rapid flocculation (diffusion controlled process), and this results to strong flocculation. The latter is due to the van der Waals attraction which increases rapidly with decrease of separation distance between the droplets. To... 

MP Aronson. Flocculation of emulsions by free surfactant in purified systems. Colloids Surf 58 195-202, 1991. 

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