Multiple emulsions diffusion

Slow release rates and remarkable long shelf-life (months) were obtained compared to typical multiple emulsions stabilized by two short surfactants (SMO and polyoxyethylene (20) sorbitan monolaurate). Finally, the long lifetime of the emulsions allowed study via diffusing wave spectroscopy (DWS) of the interactions between the droplets and the globule surface [37],... 

S. Magdassi and N. Garti Release of Electrolytes in Multiple Emulsions Coalescence and Breakdown or Diffusion Through Oil Phase Collois Surf. 12, 367 (1984). 

Mechanisms of drug release from multiple emulsion systems include diffusion of the dmg molecules from the internal droplets (1), from the medium of the external droplets (2), or by mass transfer due to the coalescence of the internal droplets (3), as shown in Fig. 7.16(b)... 

In some disciplines, certain multiple emulsions have been termed liquid membrane systems, as the liquid film which separates the other liquid phases acts as a thin semi-permeable film through which solute must diffuse moving from one phase to another. There are, therefore many potential practical applications of multiple emulsions. 

Optimum osmotic balance of the internal and external aqueous phases. If the osmotic pressure of the internal aqueous droplets is higher than the external aqueous phase, water will flow to the internal droplets and result in swelling of the multiple emulsion drops with the ultimate production of a W/O emulsion. In contrasL if the osmotic pressure in the outside external phase is higher, water will diffuse in the opposite direction and the multiple emulsion will revert to an O/W emulsion. 

Wen and Papadopoulos used videomicroscopy to measure the rate of shrinkage of a single drop of pure water immersed in a surfactant-containing oil phase, which itself was in contact with an aqueous solution of NaCl. They found that the solubilization rate of water was controlled by phenomena at the drop interface, suggesting that transport in multiple emulsions is limited by interfacial effects, not diffusion. 

This is another innovative emulsion technique proposed for the efficient encapsulation of hydrophilic drug compounds involves the formation of double emulsions (multiple emulsions). In this technique, an aqueous core solution (W,) is emulsified in a polymer-organic solvent solution (O) to form the primary W/0 emulsion, which is further emulsified in an external aqueous solution (Wn), giving rise to the double emulsion of Wi/O/Wu type. Evaporation or extraction of the organic solvent yields a solid microcapsule with an aqueous core. The organic phase in (O) acts as a barrier between the two aqueous compartments, W, and Wu, to prevent the diffusion of hydrophilic drug compounds out of the core toward the external aqueous solution. Figure 45.5 depicts the microencapsulation by the Wi/O/Wn emulsion technique. 

We end this section by summarizing the areas where we feel that the NMR diffusion method will prove important in future studies of emulsions and refer to a more detailed account presented in Chapter 10 of this book. As theories describing emulsion stability become more refined, there will be a need for data on droplet size distribution and also on total emulsion droplet area and how these quantities evolve with time. As outlined above, NMR is eapable of providing sueh data. Another important question pertains to the mi-crostrueture of the continuous phase, which can be studied both in the emulsion phase and also in the phase-separated systems which yield the emulsion. Finally, we note that one important class of emulsions, namely, multiple emulsions, is practically virgin territory with regard to NMR studies. In the characterization and understanding of important features of these systems NMR will most likely play an important role. 

We have shown that the NMR self-diffusion method is sensitive to the mean displacement of a molecule of interest on the time scale of the NMR experiment (A). This fact allows us to measure molecular transport inside the emulsion droplets, as in the case of determination of droplet sizes, and the exchange between the emulsion droplets, as in the case of highly concentrated emulsions. In more complex systems the NMR self-diffusion method is sensitive to the molecular exchange between the emulsion droplets and the continuous phase, as in the case of multiple emulsions. Many emulsion systems are currently used as carriers for drugs or other bioactive substances, such as pesticides. The selective measurement of the diffusivity of the individual components within flic emulsion system is therefore of theoretical and practical relevance. The NMR self-diffusion technique is an appropriate tool to study the drug release from emulsion droplets. This useful information may be obtained in a rapid and nondestructive way. 

A liquid membrane (as in a multiple emulsion), in which case chemical release is controlled by diffusion and dissolution. 

FIGURE 11.16. Multiple emulsion degradation can take place by several mechanisms. Important pathways include (a) secondary emulsion coalescence with little change in drop size in the PE, (ft) PE drop coalescence with httle change in secondary emulsion characteristics, and (c) loss of PE internal phase to the final external phase due to diffusion or solubilization. 

Multiple emulsions are the basis of so-called liquid membrane separation processes, where one solute is prefermtially transferred from, say, the outer to the inner aqueous phase of a water-in-oil-in-watCT emulsion. The preferred solute is transferred faster because it is more soluble in the oil phase than other species dissolved in the inner aqueous phase. Transport across the oil phase may occur by diffusion of solute molecules or solute-containing reverse micelles or microemulsion drops. Sometimes a carrier is added to the oil to increase solute solubility in the oil by formation of a solute/carrier complex. 

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. 

Fig. 13.26. Schematic representation of the possible breakdown pathways in W/O/W multiple emulsions (a) coalescence (b) to (e) expulsion of one or more internal aqueous droplets (f) and (g) less frequent expulsion (h) and (i) coalescence of water droplets before expulsion (j) and (k) diffusion of...

Magdassi and Carti [82] showed thaf the stability of multiple emulsions is also affected by the migration of electrolytes from the internal to the external phase. This migration is the result of the diffusion rather than of the droplets breakdown. The release... 

Magdassi S, Garti N. 1984. Release of electrolytes in multiple emulsions Coalescence and breakdown or diffusion through oil phase Colloids Surf 12 367-373. 

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