Emulsion electrostatic forces

Electrostatic forces, acting when the electric double layers of two drops overlap, play an important role. As mentioned above, oil drops are often negatively charged because anions dissolve in oil somewhat better than cations. Thus, the addition of salt increases the negative charge of the oil drops (thus their electrostatic repulsion). At the same time it reduces the Debye length and weakens the electrostatic force. For this reason, emulsion stability can exhibit a maximum depending on the salt concentration. 

In spite of the droplets being destabilized electrostatically, no evidence of droplet coalescence is seen. By the same token, an electrostatically stabilized emulsion might still coalesce and separate, sediment, or cream if other destabilizing forces overbalance the electrostatic component. Creaming refers to concentration of the dispersed phase without completely separating the oil and water phases. 

The DLVO theory, which was developed independently by Derjaguin and Landau and by Verwey and Overbeek to analyze quantitatively the influence of electrostatic forces on the stability of lyophobic colloidal particles, has been adapted to describe the influence of similar forces on the flocculation and stability of simple model emulsions stabilized by ionic emulsifiers. The charge on the surface of emulsion droplets arises from ionization of the hydrophilic part of the adsorbed surfactant and gives rise to electrical double layers. Theoretical equations, which were originally developed to deal with monodispersed inorganic solids of diameters less than 1 pm, have to be extensively modified when applied to even the simplest of emulsions, because the adsorbed emulsifier is of finite thickness and droplets, unlike solids, can deform and coalesce. Washington has pointed out that in lipid emulsions, an additional repulsive force not considered by the theory due to the solvent at close distances is also important. 

When the electrostatic stabilization of the emulsion is considered, the electrolytes (monovalent and divalent) added to the mixture are the major destabilizing species. The zeta potential of the emulsion particles is a function of the concentration and type of electrolytes present. Two types of emulsion particle-electrolyte (ions) interaction are proposed non-specific and specific adsorption.f H non-specific adsorption the ions are bound to the emulsion particle only by electrical double-layer interactions with the charged surface. As the electrolyte concentration is increased, the zeta potential asymptotes to zero. As the electrostatic repulsion decreases, a point can be found where the attractive van der Waals force is equal to the repulsive electrostatic force and flocculation of the emulsion occurs (Fig. 9A). This point is called the critical flocculation concentration (CFC). 

In an inverse latex, these electrostatic forces are quite difFerent. For water-in-oil emulsions, Albers and Overbeek (J, 2, 3) found that, with ionic emulsifiers, flocculation was promoted by gravity because of the diffuse nature of the electric double layer relative to that of oil-in-water systems and, with nonionic emulsifiers,... 

The forces such as electrical double layer, forces between emulsion droplets, hydrodynamic inertial forces, entropic (Diffusional) forces and the dispersion forces which act on the droplets or between the droplets separated at tens or hundreds of nanometers. Sedimentation and flocculation processes involve the forces such as the centrifugal force, applied electrostatic force and gravitational force. Before discussing the emulsion stability in terms of these forces, we would like to explain the thermodynamics of emulsion stabilization. 

Two approaching emulsion droplets may be resisted by electrostatic forces. Electrostatic forces consist of Coulombic repulsion between two like charged objects and attractive van der Waals forces. These two forces are accounted for by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. A third force. Born repulsion, occurs at very small separation distances when electron clouds overlap [1,6,20,21], In emulsion systems an electrical double-layer may form around the disperse phase droplets. While electrical double-layer repulsion is certainly important in o/w emulsions, it does not play a large role in the stabilization of w/o emulsions due to the low dielectric constant of oil [55,56],... 

Distortion of the particles themselves by electrostatic forces. The effect is not seen in latexes, but may be significant in emulsions of oil in water. 

Latexes prepared in inverse emulsions are less stable than conventional latexes, due to different electrostatic forces which are operative in both systems. Moreover, the very large difference in density between polymer particles... 

Heterocoagulation is the mutual adhesion of particles of a dissimilar nature upon collision, as a result of their individual Brownian motion. Brownian motion is a stochastic, or random, movement of colloidal particles suspended in a fluid (or gas) as a result of the internal thermal energy of the system, and thus of collisions with the solvent (or gas) molecules, as pointed out independently by Einstein and Smoluchowski. Derjaguin pointed out that the term heteroadagulation should be used for adhesion of small particles that move through Brownian motion onto much larger objects, whose Brownian motion can be neglected, such as fibers [1]. For example, Jachowicz and Berthiaume [2] reported the deposition of cationic, anionic, and neutral silicon oil droplets in the form of oil-in-water emulsions on native or cationically modified human hair fibers, driven by electrostatic forces. 

Also polydimethylsiloxane (PDMS) emulsions and micro-emulsions are well known as conditioners in textile finishing imparting softness, dimensional stability and wrinkle and stretch recovery. When combined with quats and esterquats they improve the water absorbency of softened cotton fabric. Long-chain PDMS with amino- or amido-functional side groups anchor the silicone to the fibre by way of attractive electrostatic forces. Interfibre friction is reduced in this way, producing a distinct and substantive softener effect. ... 

To determine the charge, the electrophoretic mobility, i.e., the mobiHly of the charged particles in the electric held, is determined. From this the so-called potential can be calculated, which is a parameter for the stabilization by means of electrostatic forces. The tnec/uinira/ stability of the emulsifying film is also of considerable importance. On the one hand, it must be sufficiently rigid to stabilize the interface, but on the other hand must also be sufficiently flexible that collision of emulsion droplets does not lead to rupture of the emulsifying film and to resulting coalescence. 

The chapter by Urdahl, Wayth, Fordedal, Williams, and Bailey begins by discussing droplet break-up processes under both laminar and turbulent flow conditions and in electrostatic fields. The authors then discuss the droplet coalescence process under normal Brownian motion, under gravity sedimentation, and in laminar shear, including turbulent collisions as well as collisions due to electrostatic forces. The remainder of the chapter is devoted to electrostatic-induced separation of the water-in-oil emulsions and emerging technologies. 

Studies involving the use of organically modified clay particles in heterophase polymerization are rather scarce. Indeed, we are aware of only two reports that combine the emulsion or suspension polymerization approaches and ion-ex-change reaction. In one of these reports, AI BA is immobiHzed in the clay interlayer region to yield exfoliation of MMT in the PMMA matrix through suspension polymerization [135]. In another relevant study, it was demonstrated that exfoliated structures could be obtained by post-addition of an aqueous dispersion of layered silicates (either MMT or laponite) into a polymethyl methacrylate latex suspension produced in the presence of suitable cationic compounds (cationic initiator, monomer or surfactant) [136]. Since the latex particles were cationic and the clay platelets anionic, strong electrostatic forces were developed at the polymer/clay interface. 

The electrostatic stabilization mechanism is well documented for simple OfW emulsions (Myers, 1998a). Multiple emulsion droplets are much larger in size, and therefore the repulsive electrostatic forces are less pronounced (Figure 5.5). 

Figure 5.5 Multiple emulsion repulsive electrostatic forces.

Ionic surfactants, as their name suggests, have an ionisable hydrophilic group. When appropriately charged surfactants are adsorbed in large numbers to the interface of emulsion droplets they increase the surface charge on them. Since the charge is the same sign on all the droplets, electrostatic forces are developed that oppose (and thus reduce the rate of) droplet encoimter. 

Emulsions may be flocculated by the addition of polymers. Excluding those cases where the addition of a polymer affects the van der Waals or electrostatic forces directly (e.g. the addition of polyelectrolytes), the process of polymer-induced flocculation may proceed by two mechanisms, bridging or depletion. These are depicted schematically in Figure 4.3. 

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