Emulsion electrostatic effects

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. ... 

V. Electric charge of the membrane. Electrostatic attraction or repulsion by the membrane enviromnent influences significantly the effectiveness of prooxidants and antioxidants. The rate of oxidation is much higher in emulsions prepared with ionic emulsifiers (SDS, potassium pahnitate) than with nonionic emulsifiers (Span 20, ethenoxylated tetradecanol. Tween 20) (Table 10.2). Oxidation is accelerated in the emulsions stabilized by anionic emulsifiers, such as SDS. In these emulsions, electrostatic attraction occurs between the negatively charged oil-water interface, and the positively charged metal ions present either as trace impurities or added. Metals in the water phase become hydrated and more reactive with polar hydroperoxides and water-soluble radicals (e.g. OH, OOH) at the oil-water interface. 

The adequate choice of emulsifiers is of primary importance for the success of ME. A chosen emulsifier should rapidly adsorb to the newly formed oil-water interface to reduce the interfacial tension to an optimum level. However, the emulsifier molecules should not adsorb to the membrane surface by electrostatic interactions because it can cause the alteration of membrane polarity from hydrophilic to hydrophobic and pore blocking. The effect of dynamic interfacial tension of emulsifiers on direct ME has been investigated by several authors (Schroder et al., 1998 Van der Graaf et al., 2004 Rayner et al., 2005). As a rule, the faster the emulsifier molecules adsorb at newly formed interfaces, the smaller the mean droplet size of the resultant emulsion. The effect of emulsifier charge on droplet formation in direct ME has been investigated by Nakashima et al. (1993) for SPG membranes and by Kobayashi et al. (2003) for silicon microchannels. Both studies concluded that to avoid electrostatic attractions between the emulsifier molecules and the membrane wall, the functional groups of the chosen emulsifiers must not carry the charge opposite... 

Thus, in the relatively simple case of oil in water emulsions, where a surface active agent such as a soap is used as the emulsifying agent, it is known that the soap adsorbed on the surface of the oil particles decreases the interfacial tension, thus stabilizing the emulsion. The adsorbed soap ions also give a net electrostatic charge to the dispersed oil droplets, serving to repel other oil droplets, with the net effect that flocculation is hindered (and stability is increased). It is even possible to measure the amount of adsorbed soap ions and to calculate the values of the surface potential. 

Surfactants such as sulfated fatty alcohols may be hydrated to a higher extent than the fatty alcohols alone and thus stabilize o/w emulsions. The eombination of an anionic and a nonionic srrrfactant has proved to be partieularly effeetive, sinee the electrostatic repulsion forces between the ionie surfaetant moleeules at the interface are reduced by the incorporation of nonionic molecules, thus improving the emulsion stability. The combination of cetyl/stearyl sulfate (Lanette E) and eetyl/ stearyl alcohol (Lanette 0) to yield an emulsifying eetyl/stearyl aleohol (Lanette N) is an example of this approach. The polar properties of this srrrfactant mixtrrre are dominant, and o/w creams are formed. In contrast to w/o systems, the stabilizing effect of the surfactant mixtirre is not mainly due to adsorption at the interfaee. Instead, the mixed surfactants are highly hydrated and fonn a lamellar network, whieh is... 

Demulsification with electrostatic fields appears to be the most effective and economic way for breaking of W/0 emulsion in ELM processes 190, 91]. Electrostatic coalescence is a technique widely used to separate dispersed aqueous droplets from nonconducting oils. Since this type of technique is strictly a physical process, it is most suitable for breaking emulsion liquid membranes to recover the oil membrane phase for reuse. 

Electrostatic and non-electrostatic biopolymer complexes can also be used as effective steric stabilizers of double (multiple) emulsions. In this type of emulsion, the droplets of one liquid are dispersed within larger droplets of a second immiscible liquid (the dispersion medium for the smaller droplets of the first liquid). In practice, it is found that the so-called direct water-in-oil-in-water (W/O/W) double emulsions are more common than inverse oil-in-water-in-oil (O/W/O) emulsions (Grigoriev and Miller, 2009). In a specific example, some W/O/W double emulsions with polyglycerol polyricinoleate (PGPR) as the primary emulsifier and WPI-polysaccharide complexes as the secondary emulsifying agent were found to be efficient storage carriers for sustained release of entrapped vitamin Bi (Benichou et al., 2002). 

Therefore, two contributory factors may provide an explanation for more effective electrostatic / steric stabilization of the so-called mixed emulsions in comparison with the sequentially assembled biopolymer interfaces of the bilayer emulsions firstly, a greater hydrophilicity of the adsorbed protein-polysaccharide complexes, caused by the larger net negative charge, and, secondly, a more bulky architecture of the normal complexes as compared to the interface complexes. 

For ionic surfactants another effect often dominates and usually salt tends to stabilize emulsions. Reason without salt the distance between surfactants in the interface is large because the molecules electrostatically repel each other. This prevents a high surface excess. The addition of salt reduces this lateral repulsion and more surfactant molecules can adsorb at the interface. Then, according to the Gibbs adsorption isotherm Eq. (3.52), the surface tension is reduced and the emulsion is stabilized. 

One of the central questions in the stability of foams is why are liquid films between two adjacent bubbles stable, at least for some time In fact, a film of a pure liquid is not stable at all and will rupture immediately. Formally this can be attributed to the van der Waals attraction between the two gas phases across the liquid. As for emulsions, surfactant has to be added to stabilize a liquid film. The surfactant adsorbs to the two surfaces and reduces the surface tension. The main effect, however, is that the surfactant has to cause a repulsive force between the two parallel gas-liquid interfaces. Different interactions can stabilize foam films [570], For example, if we take an ionic surfactant, the electrostatic double-layer repulsion will have a stabilizing effect. 

For electrostatically or sterically interacting drops, emulsion viscosity will be higher when droplets are smaller. The viscosity will also be higher when the droplet sizes are relatively homogeneous, that is, when the drop size distribution is narrow rather than wide. The nature of the emulsifier can influence not just emulsion stability but also the size distribution, mean droplet size, and therefore the viscosity. To describe the effect of emulsifiers on emulsion viscosity Sherman [215] has suggested a modification of the Richardson Equation to the following form ... 

Sauce bearnaise, for example, is an O/W emulsion that is mainly stabilized by egg-yolk protein in an aqueous phase of low pH. Perram et al. [830] describe how this system is primarily stabilized by electrostatic repulsive forces, and show how DLVO theory can be used to describe the effects of pH, surface charge, ionic strength, and temperature, on the stability of this emulsion. 

Steric stabilization differs from electrostatic stabilization in not being a function of a net force, but of the thickness of an adsorbed layer. When < >, equals 5-10%, stabilizing and destabilizing forces extend beyond the length of the electrostatic, interparticle barrier (Cabane et al., 1989). At this distance, attraction and repulsion are inconsequential, and electrolytes therefore have little effect. Bergenstahl (1988) proposed that the steric stabilization of emulsions by gums in the presence of a surfactant involves adsorption of the gum on the surfactant to form a combined structure constituted by a primary surfactant layer covered by an adsorbed polymer layer. 

The third mechanism by which proteins affect the stability of emulsions is rheological. This mechanism derives fundamentally from electrostatic and steric effects. The importance of viscosity has been described earlier. The viscosity of a caseinate solution is, inter alia, an indicator of the degree of bound water absorbed by the hydrophilic groups, as well as the water trapped inside the molecular aggregates (Korolczuk, 1982). The viscosity parameters (K, apparent viscosity at zero shear stress n, the power law factor and o-y, the yield stress) of sodium caseinate have been studied and found to be affected by concentration (Hermansson, 1975), precipitation and solution pH of caseinate (Hayes and Muller, 1961 Korolczuk, 1982), de-naturation (Hayes and Muller, 1961 Canton and Mulvihill, 1982), sodium chloride (Hermansson, 1975 Creamer, 1985), calcium chloride (Hayes and Muller, 1961) and temperature (Korolczuk, 1982). 

It has been reported in an ocular pharmacokinetic study of cyclosporin A incorporated in deoxycholic acid-based anionic and stearylamine-based cationic emulsions in rabbit that, when compared to anionic emulsion, the cationic emulsion showed a significant drug reservoir effect of more than 8 h in corneal and conjunctival tissues of the rabbit eye following topical application [106], Since cornea and conjunctiva are of anionic nature at physiological pH [107], the cationic emulsion would interact with these tissues electrostatically to implicate the observed cyclosporin A reservoir effect. This hypothesis is supported, in principle, by an ex vivo study which showed that cationic emulsion carrier exhibited better wettability properties on rabbit cornea than either saline or anionic emulsion carrier [108],... 

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