Depletion flocculation emulsions

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). 

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. 

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 ... 

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). 

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. 

Hibberd, D.J., Holmes, A.K., Garrood, M., Fillery-Travis, A.J., Robins, M.M., Challis, R.E. 1997b. Ultrasonic monitoring of oil in water emulsions during depletion flocculation.. / Coll. Interface Sci. 193, 77-87. 

Ultrasonic techniques have also been used to study typical oll-ln-water emulsions as such and those undergoing either depletion flocculation or floe disruption. These studies are of industrial interest as flocculation of the droplets within an emulsion may often be the first stage in the deterioration of a product however, it can also be beneficial by Increasing the apparent viscosity of the product via the formation of a continuous network of droplets. 

Depletion flocculation has also been induced in oil-in-water emulsions by adding different concentrations of a non-adsorbing biopolymer (xanthan) to the aqueous phase. At low frequencies, the attenuation coefficient of the emulsions decreased with increasing... 

Increased depletion attraction. The presence of nonadsorbing colloidal particles, such as biopolymers or surfactant micelles, in the continuous phase of an emulsion causes an increase in the attractive force between the droplets due to an osmotic effect associated with the exclusion of colloidal particles from a narrow region surrounding each droplet. This attractive force increases as the concentration of colloidal particles increases, until eventually, it may become large enough to overcome the repulsive interactions between the droplets and cause them to flocculate (68-72). This type of droplet aggregation is usually referred to as depletion flocculation (17, 18). 

Depletion flocculation is produced by addition of a free nonadsorbing polymer [7]. In this case, the polymer coils caimot approach the particles to a distance A (this is determined by the radius of gyration of free polymer, Rq), as the reduction in entropy on close approach of the polymer coils is not compensated by an adsorption energy. The suspcakesension particles or emulsion droplets will be surrounded by a depletion zone with thickness A. Above a critical volume fraction of the free polymer, the polymer coils wiU be squeezed out from between the particles... 

Interesting effects are observed when a dispersion contains both larger and smaller particles the latter are usually polymer coils, spherical or cylindrical surfactant micelles, or microemulsion droplets. The presence of the smaller particles may induce clustering of the larger particles due to the depletion attraction (see Section 5.4.S.3.3, above) such effects are described in the works on surfactant-flocculated and polymer-flocculated emulsions. Other effects can be observed in dispersions representing mixtures of liquid and solid particles. Yuhua et al. ° have established that if the size of the solid particles is larger than three times the size of the emulsion drops, the emulsion can be treated as a continuous medium (of its own average viscosity), in which the solid particles are dispersed such treattnent is not possible when the solid particles are smaller. 

Also other species may cause depletion interaction. A case in point is surfactant micelles, for example, if an emulsion has been made with an unnecessary large concentration of surfactant, so that micelles remain after emulsification, this may cause aggregation of the droplets. In foods, however, the surfactant concentrations needed for depletion flocculation to occur are generally unacceptable. 

It may finally be noted that depletion flocculation of particles by a soluble polymer is closely related to segregative phase separation of two soluble polymers—e.g., a protein and a polysaccharide—as described in Section 6.5.2. It is therefore not surprising that emulsion droplets, covered with a protein layer, can readily show depletion flocculation due to the addition of a nonadsorbing polysaccharide. 

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-... 

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