Emulsion repulsive forces between droplets

Energy Barrier. An energy barrier results in repulsive forces between droplets when they approach each other. The half-life of an emulsion changes drastically when energy barriers of different height are introduced in the system. Increasing the barrier to 20 kT gives a typical half-life of a few years which is sufficient for most appHcations. 

Emulsifiers stabilize emulsions in various ways. They reduce interfacial tension and may form an interfacial film that prevents coalescence of droplets. In addition, ionic emulsifiers provide charged groups on the surface of the emulsion droplets and thus increase repulsive forces between droplets. Emulsifiers can also form liquid crystalline microstructures such as micelles at the interface of emulsion droplets. These are formed only at emulsifier concentrations larger than the critical micelle-forming concentration. These microstructures have a stabilizing effect. 

By using MCT, Leal-Calderon et al. [ 10] measured the total repulsive force between tiny colloidal droplets stabilized with sodium dodecyl sulfate (SDS) (Fig. 2.5). The measurements were performed for emulsions with three different concentrations... 

Flocculation may occur in emulsions through a variety of different processes, described below, that either increase the attractive forces or decrease the repulsive forces between the droplets. 

To stabilize emulsions, a surfactant, which increases the repulsive force between oil droplets, is used. Nonionic surfactants are the preferred type because they are effective in brines, are generally cheaper, and often form less viscous emulsions than do ionic surfactants. In addition, their emulsions are easier to break, and they do not introduce inorganic residues that might lead to refinery problems. They are chemically stable at oil reservoir temperatures and are noncorrosive and nontoxic. The surfactant type and concentration required for a particular situation can be determined by conducting laboratory tests. A typical concentration of 0.1 lb of surfactant per barrel of oil is used for emulsions containing about 50-70% oil (2). 

When a relatively water-insoluble vinyl monomer, such as styrene, is emulsi ed in water with the aid of an anionic surfactant and adequate agitation, three phases result (see Fig. 6.14) (1) an aqueous phase in which small amounts of both monomer and surfactant are dissolved (i.e., they exist in molecular dispersed state) (2) emulsi ed monomer droplets which are supercolloidal in size (> 10,(X)0 A), stabihty being imparted by the reduction of surface tension and the presence of repulsive forces between the droplets since a negative charge overcoats each monomer droplet (3) submicroscopic (colloidal) micelles which are samrated with monomer. This three-phase emulsion represents the initial state for emulsion polymerization (Fig. 6.14). 

Another repulsive force between the lipid bilayers in water is the hydration force which is a short range force with exponential falloff. This is related to repulsion between dipoles and induced dipoles. It is quite obvious that the hydration force will tend to inhibit coalescence of emulsion droplets with a multilayer structure as schematically shown in Fig. 5.12. 

Fig. 18. The repulsion force from adsorbed particles is greater than the van der Waals force between flocculated emulsion droplets under certain...

Case I (see Fig. 2.17) corresponds to the situation such that the emulsion is initially stabilized with SDS at 8 10 mold (CMC). The repulsive force as a function of distance between the ferrofluid droplets, stabilized with SDS alone is referred as 0% PVA. Then, PVA-Vac is introduced at different concentrations varying from 0.002 to 0.5 wt%. After each addition, the emulsion is incubated for 48 h to reach equilibrium. It can be seen that the force profiles remain almost the same as in the case of 0% PVA. As the surfactant concentration is equal to CMC, the expected decay length is 3.4 nm. The experimental value of the decay length obtained from the force profile, 2.9 nm (solid line), is in good agreement with the predicted value. Thus, if the emulsion is preadsorbed with surfactant molecules, the introduction of polymer does not influence the force profile significantly. 

Finally, some studies have been performed on the addition of salt to the aqueous phase of oil-in-water HIPEs [109]. For systems stabilised by ionic surfactants, increasing salt concentration reduces the double-layer repulsion between droplets however, stability is more or less maintained, probably due to steric and polarisation repulsions. Above a sufficiently high salt concentration, emulsions become unstable due to salting-out of the surfactant into the oil-phase. For nonionic surfactants, the situation is similar, except that there are no initial double-layer forces. In addition, Babak [115] found that increasing the electrolyte concentration reduced the barrier to coagulation between emulsion droplets, and therefore increased coalescence. Generally, therefore, stability of o/w HIPEs is not enhanced by salt addition. 

Colloidal interactions between emulsion droplets play a primary role in determining emulsion rheology. If attractions predominate over repulsive forces, flocculation can occur, which leads to an increase in the effective volume fraction of the dispersed phase and thus increases viscosity (McCle-ments, 1999). Clustering of milk fat globules due to cold agglutination increases the effective volume fraction of the milk fat globules, thereby increasing viscosity (Prentice, 1992). 

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). 

Stable emulsions often form during industrial processing. On the microscopic scale, the reasons that the droplets remain dispersed fall into two broad categories (1) physical barriers to coalescence and (2) electrical repulsion between droplets. An example of a physical barrier is the presence of finely divided solids at the oil-water interface. Of primary concern, however, is the consideration of electrical forces because their influence is significant at relatively longer distances. Electrical repulsive forces arise... 

Creaming or sedimentation occurs when the dispersed droplets or floccules separate under the influence of gravity to form a layer of more concentrated emulsion, the cream. Generally a creamed emulsion can be restored to its original state by gentle agitation. This process, which inevitably occurs in any dilute emulsion if there is a density difference between the phases as a consequence of Stokes law, should not be confused with flocculation which is due to particle interactions resulting from the balance of attractive and repulsive forces. Most oils are less dense than... 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

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