Multiple emulsions electrolyte effects

Invivition adsorption is encountered in porous solid panicles that are wet by the continuous phase. The latter diffuses into the pores, thereby increasing 0,. Osmotic diffusion occurs in multiple emulsions, in which there is an electrolytic unbalance between the innermost internal phase and the continuous phase. Thi.s condition induces migration of liquid from the region of high osmotic pressure to low osmotic pressure. Transference of continuous phase into the droplets pro-duce.s an increase of 0. whereas transference from the droplets to the continuous phase produces a decrease of ( ),. Both effects account for an increase or decrease of viscosity, respectively. Invivition and osmotic diffusion are of great importance in concentrated systems in which a relatively small increase of leads to large increases of viscosity and on the complexity of the rheological behavior. 

Two main criteria are essential for the preparation of stable multiple emulsions (i) Two emulsifiers with low and high HLB numbers. Emulsifier 1 should prevent coalescence of the internal water droplets, preferably producing a viscoelastic film which also reduces water transport. The secondary emulsifier should also produce an effective steric barrier at the 0/W interface to prevent any coalescence of the multiple emulsion droplet, (ii) Optimum osmotic balance This is essential to reduce water transport. This is achieved by addition of electrolytes or nonelectrolytes. The osmotic balance in the external phase should be slightly lower than that of the internal phase to compensate for curvature effects. 

The first chapter by J. Jiao and D. J. Burgess discusses the thermodynamic instability of multiple emulsions as a result of the excess of free energy caused by the formation of the emulsion droplets. In multiple emulsions consisting of three distinct liquid phases, counteracting the effect of the Laplace pressure by electrolyte addition to the inner dispersed aqueous phase will increase the destabilization of the system owing to osmotic pressure. In addition the authors discuss the effects of both osmotic and Laplace pressure as well as the interfacial rheological properties of these complex systems and their stability. 

The composition of the primary emulsion dispersed phase may have a significant effect on the overall stability of a system, especially when interactions between the components and surfactant are possible, or when the components themselves may be somewhat surface-active. In most instances of multiple-emulsion formulation, the internal primary and external secondary phases will be similar in that each will be aqueous or an oil, but the nature of addenda included in each will differ. In particular, there may be significant differences in the level and nature of organic additives and electrolytes present that could alter the stability of the total system. 

Electrolytes in particular can exhibit significant effects on the stability of emulsions prepared with one or more ionic surfactants. There are multiple potential effects, including (1) changes in the role of the surfactant at the various interfaces as a result of changes in their electrical properties, (2) changes in the namre of the interfacial films due to the presence of specific ionic interactions between surfactant and electrolyte, and (3) alterations in the transport properties of the intervening phase due to differences in the osmotic pressure between the two phases. 

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