Phase volumes multiple emulsions

This kind of classification is not always appropriate. For example, O/W/O denotes a multiple emulsion containing oil droplets dispersed in aqueous droplets that are in turn dispersed in a continuous oil phase. The type of emulsion that is formed depends upon a number of factors. If the ratio of phase volumes is very large or very small, then the phase having the smaller volume is frequently the dispersed phase. If the ratio is closer to 1, then... 

The current state OF THE ART of various aspects of macro- and microemulsions is reflected in this volume. The symposium upon which this volume is based was organized in six sessions emphasizing major areas of research. Major topics discussed include a review of macro- and microemulsions, enhanced oil recovery, reactions in microemulsions, multiple emulsions, viscoelastic properties of surfactant solutions, liquid crystalline phases in emulsions and thin films, photochemical reactions, and kinetics of microemulsions. 

The presence of liquid crystal structures at both the w-o and o-w interfaces in multiple emulsions has been investigated by Kavaliunas and Frank (31). Microscopic examination of w/o/w emulsions between crossed polarizers revealed the presence of liquid crystal phases at both inner (w-o) and outer (o-w) interfaces in a w/o/w system composed of water, p-xylene and nonylphenol diethylene glycol ether. Liquid crystalline phases were also detected in o/w/o emulsions at both interfaces. The presence of these liquid crystal structures was found to improve the stability of the emulsions markedly. Matsumoto (32, 33) have concluded that the oil layers in w/o/w systems are likely to be composed of or contain,at least in proximity to the aqueous phase,multilamellar layers of the lipophilic surfactant used in the formulation this is postulated in part to explain the rate of volume flux of water through the oily layer. 

A multiple emulsion provides a slightly different kind of volume fraction complication since the dispersed phase is itself an emulsion. A variation on the Mooney equation for double emulsions is... 

MA Kassem, SM Safwai, MA Aiiia. M El-Mahdy. Influence of the phase volume ratio of multiple emulsions on the ocular activity of prednisolone. STP Pharma 5 309-315 0 995). 

Multiple emulsions have a complex morphology and various important parameters for their prepara tion and characterization have been described (39, 47). Examples are the characteristics of the W/0 glo bules in W/O/W systems, such as their size and volume fraction, W/0 ratio inside the W/O globules, and aver age number and size of water droplets inside the W/O globules. The time dependence of Arose parameters are closely related to the stability of multiple emulsions and their morphology. Other important features are transport properties of substances encapsulated into discrete droplets and the permeability of the layer separating Are internal from the external continuous phase. As was shown above, the NMR PFG method is a sensitive tool to study structure and complex dynamic phenomena, and therefore it is a promising technique in the study of multiple emiAsions. 

The other two branches of the standard inversion line are essentially vertical, and are located typically at 30% water on the negative SAD side of optimum formulation, and at 70% water on the positive side. When the water content is low, the emulsion is always W/0, regardless of the formulation. Similarly, when the oil content is low, an 0/W can be expected, whatever the formulation. In these extreme WOR regions, the phase which is present in larger volume becomes the external phase of the emulsion. It may be said that the composition dominates. However, a closer look at the conductivity value indicates the presence of multiple emulsions in the B" and zones, i.e., where the composition effects dominate over the normal formulation trend. These B" and regions have been called abnormal in opposition to the other ones which are labeled normal because they follow the Bancroft rule and the wedge theory (172). 

Garcia-Fuentes et al. described a modified method based on a w/o/w multiple emulsion technique to encapsulate hydrophilic macromolecules in the SLN matrix. The aqueous drug solution is dispersed by e.g. probe sonication in a 1 10 volume ratio in the water-immiscible organic solvent (e.g. dichloromethane) containing the matrix lipid and lecithin as stabilizer. To this w/o emulsion different volumes of an aqueous phase containing e.g. poloxamer as emulsifier is added and double w/o/w multiple emulsions are prepared by further sonication. Upon evaporation of the organic solvent, the lipid nanoparticles are formed. 

One of the main instabilities of multiple emulsions is the osmotic flow of water from the internal to the external phase or vice versa. This leads to shrinkage or swelling of the internal water droplets respectively. This process assumes the oil layer to act as a semipermeable membrane (permeable to water but not to solute). The volume flow of water,, may be equated with the change of droplet volume with time dv/dt. 

Multiple emulsions of WilOlWi are prepared following a two-step emulsification process that allows the control of the concentration of both the primary (WilO) and the multiple emulsions. Concentrated emulsions are obtained by swelling the dispersed water phase. In the following discussion, ( )d refers to the volume fraction of the droplets relative to the oil phase, ( g is the volume fraction of the globule phase, and ( ) that of the droplet phase, relative to the total sample volume. We thus have the relationship ( ) = ( )g < d-... 

When disperse phase of the coarse emulsion wets the membrane wall and suitable surfactants are dissolved in both liquid phases, the process results in a phase inversion namely a coarse OAV emulsion is inverted into a fine W/O emulsion (Figure 6.1c), and vice versa (Suzuki et al, 1999). The main advantage of this method is that a fine emulsion can be easily prepared from a low concentration coarse emulsion at high rates. For polytetrafluoroethylene (PTFE) membrane filters with a mean pore size of 1 im, the maximum dispersed phase volume fraction in phase-inverted emulsions was 0.9 and 0.84 for O/W and W/O emulsions, respectively (Suzuki et al., 1999). Flow-induced phase-inversion (FIPI) phenomenon was observed earlier by Akay (1998) who used a multiple expansion-contraction static mixer (MECSM) consisting of a series of short capillaries with flow dividers. Hino et al. (2000) and Kawashima et al. (1991) inverted a W/O/W emulsion made up of liquid paraffin. Span 80 (a hydrophobic surfactant), and Tween 20 (a hydrophilic surfactant) into a W/ O emulsion by extrusion through polycarbonate membranes with a mean pore sizes of 3 and 8 im. Inside the membrane pores, surfactant molecules are oriented with their hydrophobic groups toward the wall surface and with hydro-phihc groups toward the solubilized water molecules as a result of a lamellar structure formed inside the pores. The structure ruptured at the pore outlets. 

Liquid-to-liquid emulsification is a critical step in the multiple emulsion microencapsulation process (W/OAV or O/W/O). It was found that the size of these droplets decreases with increasing homogenization intensity and duration. The emulsion droplet size depends, as expected, on viscosity, total volume size, and the volume ratio of the continuous phase to the dispersed phase in the rotor/stator design being investigated. All these physical parameters influence the structure of the microspheres obtained by this technique. 

Cosurfactant facilitates droplet size reduction in the fME range. The multiple emulsion was diluted with equal volume of ice-cold external phase (<4°C), stirred for 15 minutes, and immediately transferred to a refrigerator. The pH of the external phase was increased to about 8.0 to 8.9 and then restored to 7.4 with addition of an acid. The multiple emulsion was formed with a relatively high percentage of the secondary surfactant further a cosurfactant had to be added to reduce the size of multiple emulsion. The formulation and process variables Uke amount of surfactants, phase volume, and time period of sonication for both emulsification steps were optimized by way of the appropriate evaluation parameters, mainly solidification, droplet size, yield, viscosity, and/or stability. 

Emulsions are metastable colloids made out of two immiscible fluids, one being dispersed in the other, in the presence of surface active agents. The droplet volume fiaction may vary from zero to almost one dense emulsions are sometimes called biliquid foams since their structure is very similar to the cellular structure of air-liquid foams for which the continuous phase is very minor. From dilute to highly concentrated, emulsions exhibit very different internal dynamics and mechanical properties. When diluted, droplets are agitated by Brownian motion and behave as viscous Newtonian fluids, whereas when more concentrated, namely above the random close packing volume fraction which is 64% for monodisperse droplets, the internal dynamics are severely restricted and they behave as viscoelastic solids. Simple direct emulsions are composed of oil droplets dispersed in water while inverse emulsions are composed of water droplets dispersed in an oil continuous phase. In fact, emulsions are in principle made out of two immiscible phases for which the interfacial tension is therefore non-zero, and may involve other hydrophilic-like or lipophilic-like fluids in the presence of suitable surface active species, each phase being possibly comprised of numerous components. Sometimes, simple emulsions may also contain smaller droplets of the continuous phase dispersed within each droplet of the dispersed phase. Such systems are called double emulsions or multiple emulsions. ... 

For systems as potentially complex as multiple emulsions, it is very important that a clear and consistent system of nomenclature be employed. For a W/O/W system, for example, in which the final continuous phase is aqueous, the primary emulsion will be a W/O emulsion, which is then emulsified into the final aqueous phase. The surfactant or emulsifier system used to prepare the primary emulsion is referred to as the primary surfactant, and the volume fraction of the primary dispersed phase is the internal aqueous phase of the final multiple emulsion. Subscripts are used to further avoid ambiguities as to components or their locations in the system. For example, in a W/O/W system the aqueous phase of the primary emulsion would be denoted as Wi and the primary emulsion as Wj/O. After the primary emulsion has been further dispersed in the second aqueous phase W2, the complete system is denoted W1/O/W2. In the case of a O/W/O multiple emulsion, the notation is O1/W/O2. Additional refinements to fit even more complex systems, including the order of multiple emulsions, have been suggested. 

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