Interaction between phases, multiple emulsions

Some progress towards an understanding of these systems is possible by considering the influence of the dispersal of water in the oil droplets on the interactions between the multiple drops and by consideration of the influence of the size of the water droplets on their internal stability and on the possibility of coalescence with the external phase. It is premature to consider all this in detail as the application of colloid stability theory to simpler emulsions has not been particularly successful. For type A systems the approach of Void [156] may perhaps be used if the oil layer is thought of as the homogeneous adsorbed layer. Alternatively, the effect of the internal phase on the size of the oil droplet is perhaps worth considering. The approach of Void [156] has been adopted by Florence and Whitehill [150,151] and, in the first instance, interactions tetween identical droplets were considered. In the simplest case two type A droplets may be considered. 

A technique based on the formation of a multiple emulsion with an external aqueous phase was developed for the encapsulation of water-soluble drugs in order to replace the external oil phase. Possible unwanted interactions between the oil and the emulsified wax such as swelling or dissolution of the wax, clean-up requirements of the final product, and recovery of the oil phase could be eliminated. In analogy to the encapsulation of water-soluble drugs within polymeric microparticles by a w/o/w-solvent evaporation method, a molten wax phase was used instead of an organic polmer solution. A heated aqueous solution of pseudoephedrine HCl was emulsified into the molten carnauba wax, followed by the emulsification of this w/o-emulsion into a heated external aqueous phase. The temperature of the internal and external aqueous phases had to be kept above the melting temperature of the wax in order to avoid premature... 

Figure 7. Three possible interactions (.attractive and repulsive) between phases in multiple emulsions are shown in the upper diagram, while below those arrangements of the aqueous (W), surfactant (s and Sg) and oil phases are shown which must be taken into account in calculation of attractive and repulsive forces in these interactions. ...

The release rate of methotrexate encapsulated in the internal phase of W/OAV emulsions, stabilized by an interfadal interaction between albumin and sorbitan monooleate, was measured as function of two formulation variables— the oil phase and the secondary emulsifier composition. The release rate was significantly affected by the nature of the oil phase and surfactants with high HLB values (Omotosho et al., 1989). The influence of the oil phase of the W/ OAV emulsions on the oral absorption of 5-fluorouracil in the rat was determined by measuring liver and lymphatic accumulations of the drug. The multiple emulsion system showed potential as a lymphotropic carrier to the mesenteric lymph nodes following oral administration (Omotosho et al., 1990). 

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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