Electrical Emulsion Stability

Those systems in which the emulsifier carries a charge would impart specific characteristics to the emulsion. A double layer will exist around the oil droplets in an O/W emulsion. If the emulsifier is negatively charged, then it will attract positive counterions while repelling negative charged ions in the water phase. The change in 

FIGURE 9.4 Four-component system Oil (O)-Water (W)-Emulsifier (E)-Cosurfactant (S) (ratio of 0 S versus S W). 

Total force = repulsion forces + attraction forces 

The nature of the total force thus determines whether 

This is a very simplified picture, but a more detailed analysis will be given elsewhere. The attraction force arises from van der Waals forces. The kinetic movement will finally determine whether the total force can maintain contact. 

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A reduction in the electrical charge is known to increase the flocculation and coalescence rates. Sufficient high zeta potential (> — 30 mV) ensures a stable emulsion by causing repulsion of adjacent droplets. The selection of suitable surfactants can help to optimize droplet surface charges and thus enhance emulsion stability. Lipid particles with either positive or negative surface charges are more stable and are cleared from the bloodstream more rapidly than those with neutral charge [192, 193]. 

The repulsive interaction of droplets possessing like electrical charge seems to contribute to emulsion stability. Electrical charges on droplets in emulsions can arise by ionization, absorption, or frictional electricity produced by the large shearing forces required for emulsion formation. 

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. 

Emulsion Capacity and Stability. A 0.5 g sample of the freeze-dried protein fraction was redissolved in a minimum of 0.3 M citrate-phosphate buffer at pH 7.0 and mixed thoroughly with 50 ml of 1 M NaCl for 1 min in a Sorvall Omnimixer at 1000 rpm in a one pint Mason jar set in a water bath (20°C). Crisco oil (50 ml) was added to the jar and an emulsion formed by mixing at 500 rpm with simultaneous addition of oil at the rate of 1 ml/min until the emulsion broke. The endpoint was determined by monitoring electrical resistance with an ohmeter. As the emulsion broke a sharp increase (l KS2 to 35- 0 KSi) was noted. Emulsion capacity was expressed as the total volume of oil required to reach the inversion point per mg protein. This method is similar to that used by Carpenter and Saffle (8) for sausage emulsions. To establish emulsion stability the same procedure was used except that 100 ml of oil was added and a stable emulsion formed by blending at 1000 rpm for 1 min. A 100 ml aliquot was transferred to a graduate cylinder and allowed to stand at room temperature. Observations were made of the volume of the oil, emulsion and water phases at 30, 60, 90 and 180 min. 

Very often, the microstructure and the macroscopic states of dispersions are determined by kinetic and thermodynamic considerations. While thermodynamics dictates what the equilibrium state will be, kinetics determine how fast that equilibrium state will be determined. While in thermodynamics the initial and final states must be determined, in kinetics the path and any energy barriers are important. The electrostatic and the electrical double-layer (the two charged portions of an inter cial region) play important roles in food emulsion stability. The Derjaguin-Landau-Verwey-Oveibeek (DLVO) theory of colloidal stability has been used to examine the factors affecting colloidal stability. 

It follows directly from the previous considerations of emulsion stability that if an emulsion is stabilized by electrical repulsive forces, then demulsifi-... 

Electrokinetics. Bottle tests and centrifugation may be somewhat crude, but they do offer a relative measure of emulsion stability that combines, to some extent, all of the factors that affect stability. Electrokinetic measurements are somewhat more elegant because they allow direct measurement of the degree of electrostatic stability in an emulsion system. The zeta potential, or relative magnitude of the electric charge on the surface, is... 

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. 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

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. 

The breakdown of the stable emulsions and subsequent separation to oil and water (demulsification) are important in nuclear, petroleum, and environmental technologies. The emulsion stability is primarily induced by the use of surfactants and is enhanced by reduced size and narrow size distribution of the emulsion droplets. Disruption to low interfacial activity (hence instability) can be achieved by using demulsification agents, which are, however, costly and environmentally undesirable, as they are irrecoverable. Demulsification can also be achieved by electric and/or centrifugal fields, or by chemical treatment of the emulsion. 

Fordedal H, Sjoblom J. Percolation behavior in W/O emulsions stabilized by interfacially active fractions from crude oils in high external electric fields. J Colloid Interface Sci 1996 181 589-594. 

H. Fordedal, 0. Midttun. J. Sjdblom, O. M. Kvlheim, Y. Schildberg, J. L. Vollei. A multivariate screening analysis of W/O emulsions in high external electric fields as studied by means of dielectric lime domain spectroscopy. I). Model emulsions. stabilized by interfacially active fractions from crude oils. J. Collord Interface Sci. 182 117-125, 1996. 

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