Droplets colloid stability

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

Experiments on interactions of polysaccharides with casein micelles show similar trends to those with casein-coated droplets. For example, Maroziene and de Kruif (2000) demonstrated the pH-reversible adsorption of pectin molecules onto casein micelles at pH = 5.3, with bridging flocculation of casein micelles observed at low polysaccharide concentrations. In turn, Tromp et al. (2004) have found that complexes of casein micelles with adsorbed high-methoxy pectin (DE = 72.2%) form a self-supporting network which can provide colloidal stability in acidified milk drinks. It was inferred that non-adsorbed pectin in the serum was linked to this network owing to the absence of mobility of all the pectin in the micellar casein dispersion. Hence it seems that the presence of non-adsorbed pectin is not needed to maintain stability of an acid milk drink system. It was stated by Tromp et al. (2004) that the adsorption of pectin was irreversible in practical terms, i.e., the polysaccharide did not desorb under the influence of thermal motion. 

Two kinds of barriers are important for two-phase emulsions the electric double layer and steric repulsion from adsorbed polymers. An ionic surfactant adsorbed at the interface of an oil droplet in water orients the polar group toward the water. The counterions of the surfactant form a diffuse cloud reaching out into the continuous phase, the electric double layer. When the counterions start ovedapping at the approach of two droplets, a repulsion force is experienced. The repulsion from the electric double layer is famous because it played a decisive role in the theory for colloidal stability that is called DLVO, after its originators Derjaguin, Landau, Vervey, and Overbeek (14,15). The theory provided substantial progress in the understanding of colloidal stability, and its treatment dominated the colloid science literature for several decades. 

The colloidal stability of polymer dispersion prepared by the emulsion copolymerization of R-(EO)n-MA was observed to increase with increasing EO number in the macromonomer [42, 96]. Thus C12-(EO)9-MA did not produce stable polymer latexes, i.e., the coagulum was observed during polymerization. This monomer, however, was efficient in the emulsion copolymerization with BzMA (see below). The C12-(EO)20-MA, however, appears to have the most suitable hydrophilic-hydrophobic balance to make stable emulsions. The relative reactivity of macromonomer slightly decreases with increasing EO number in macromonomer. The most hydrophilic macromonomer with co-methyl terminal, Cr(EO)39-MA, could not disperse the monomer so that the styrene droplets coexisted during polymerization. The maximum rate of polymerization was observed at low conversions and decreased with increasing conversion. The decrease in the rate may be attributed to the decrease of monomer content in the particles (Table 2). In the Cr(EO)39-MA/St system the macromonomer is soluble in water and styrene is located in the monomer droplets. Under such conditions the polymerization in St monomer droplets may contribute to the increase in r2 values. 

In the simplest example of colloid stability, suspension partides would be stabilized entirely by the repulsive forces created when two charged surfaces approach each other and their electric double layers overlap. The repulsive energy VR for spherical particles, or rigid droplets, is given approximately as ... 

Many food colloids are stabilized from proteins from milk or eggs [817]. Milk and cream, for example, are stabilized by milk proteins, such as casein micelles, which form a membrane around the oil (fat) droplets [817]. Mayonnaise, hollandaise, and bearnaise, for example, are O/W emulsions mainly stabilized by egg-yolk protein, which is a mixture of lipids (including lecithin), proteins, and lipoproteins [811,817]. The protein-covered oil (fat) droplets are stabilized by a combination of electrostatic and steric stabilization [817]. Alcohols may also be added, such as glycerol, propylene glycol, sorbitol, or sucrose sometimes these are modified by esterification or by... 

In colloid science the term colloid stability means that a specified process that causes the colloid to become a macrophase, such as aggregation, does not proceed at a significant rate. Colloid stability is different from thermodynamic stability (see Ref. [978]). The term colloid stability must be used with reference to a specific and clearly defined process, for example, a colloidally metastable emulsion may signify a system in which the droplets do not participate in aggregation, coalescence, or creaming at a significant rate. See also Kinetic Stability, Thermodynamic Stability. 

Colloidal stability is usually controlled by the type and amount of the employed surfactant. In miniemulsions, the fusion-fission rate equilibrium during sonication and therefore the size of the droplets directly after primary equilibration depends on the amount of surfactant. For sodium dodecylsulfate (SDS) and styrene at 20% dispersed phase, it spans a range from 180 nm (0.3% SDS relative to styrene) down to 32 nm (50 rel.% SDS) (Fig. 4a). Again, it is anticipated that rapidly polymerized latexes also characterize the parental miniemulsion. As... 

With cetyl alcohol, there is the complication that the polarity of the molecule may cause it to reside at the surface of the droplet, imparting additional colloidal stability. Here, the surfactant and costabilizer form an ordered structure at the monomer-water interface, which acts as a barrier to coalescence and mass transfer. Support for this theory lies in the method of preparation of the emulsion as well as experimental interfacial tension measurements [79]. It is well known that preparation of a stable emulsion with fatty alcohol costabilizers requires pre-emulsification of the surfactants within the aqueous phase prior to monomer addition. By mixing the fatty alcohol costabilizer in the water prior to monomer addition, it is believed that an ordered structure forms from the two surfactants. Upon addition of the monomer (oil) phase, the monomer diffuses through the aqueous phase to swell these ordered structures. For long chain alkanes that are strictly oil-soluble, homogenization of the oil phase is required to produce a stable emulsion. Although both costabilizers produce re-... 

In the same work, it is also supposed that colloidal stability, rather than monomer ripening, plays an effective role in determining the final droplet size. Such a conclusion was supported by two different experimental results. First, it was noticed that droplet size increases right after the emulsification process stops, and a stable situation is typically achieved after just a few hours. However, if surfactant is added immediately after, this growth in size does not occur. Second, it is shown that there is a clear correlation between final droplet size and amount of oil phase used in the recipe. In particular, when the oil fraction in the system increases, droplet size also increases. 

These results can be effectively explained by supposing that colloidal stability plays a major role in determining miniemulsion stability. In fact, it is clear that addition of surfactant stops the droplet growth, which is explained by the enhanced colloidal stability. Moreover, in more concentrated systems, where the rate of droplet coalescence is larger, one obtains larger droplets, as... 

When speculating about the colloidal stability of a monomer droplet dispersion in water, one could use the Deryaguin-Landau-Verwey-Overbeek theory, also known as DLVO theory, to analyze the stability of the system. This has been done in Fig. 10a, where we show the effect of different surface potentials upon the rate of coagulation, fi, defined in the case of two droplets of the same size as ... 

Some progress toward an understanding of these systems is also possible by considering the influence of the presence of water within the oil drops on the interaction between the oil drops and by consideration of the influence of the size of the internal water droplets on their internal stability and on the possibility of coalescence with the external aqueous phase. It is premature to consider all this in detail as the application of colloid stability theory to simpler emulsions has not been particularly successful (37). For type A w/o/w emulsions, the approach of Void (38) may perhaps be used if the oil layer is thought of as the homogeneous adsorbed layer. 

The colloidal stability (or diffusional degradation) of monomer droplets can be further understood in terms of the Morton equation [48], in which the partial molar free energy of mixing of polymer in the monomer phase (spherical droplets) is expressed as ... 

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