Medium internal phase emulsions

The methods presented so far involve the use of polymers, either dissolved in a solvent or in a solid state. An alternative approach makes use of a water-in-oU emulsion, wherein the monomer is dissolved in the organic continuous phase. The monomer is then allowed to polymerize and cross-link around aqueous droplets. Depending on the physical conditions of the emulsion, adjacent emulsion droplets can be interconnected. This allows the removal of the aqueous phase via vacuum drying. Internal phase emulsions are classified as medium internal phase emulsions (MIPEs), with the fraction of the aqueous phase in the range 40—74%, or as high internal phase emulsions (HIPEs), with a volume fraction >74%. Scaffolds prepared via HIPEs had... 

The preparation of satisfactory disperse systems consists of three main steps preparing the internal phase in the proper size range, dispersing the internal phase in the dispersion medium, and, finally, stabilizing the resultant product. These three steps may be done sequentially, but in many cases (e.g., emulsions), they are usually done simultaneously. 

An emulsion is defined as a dispersion of two immiscible liquids, one of which is finely subdivided and uniformly distributed as droplets (the dispersed phase) throughout the other (the continuous phase). A third component (or multiple additional components), the emulsifying agent(s), is necessary to help stabilize the emulsion. The emulsifying agent(s) coats the droplets and prevents droplet coalescence by either reducing the interfacial tension or by creating a physical repulsion between the droplets. The dispersed phase is occasionally also defined as the internal phase the continuous phase is occasionally also defined as the external phase or dispersion medium. Virtually all emulsions are inherently physically unstable. 

In w/o/w double emulsion solvent extraction technique, the solvent can be ranoved from the microparticles through extraction of the solvent present in the internal phase. This can be achieved by the addition of w/o/w emulsion to a third solution, which is a nonsolvent of the polymer but miscible with both water and the organic solvent. Under these conditions, the solvent contained in the polymer droplets is extracted into the aqueous medium. 

Polymer latexes are heterogeneous systems which consist of two phases, namely, a dispersion medium and a disperse phase. For example, conventional emulsion polymerization systems comprise a continuous aqueous phase and a dispersed oil (polymer) phase. Inverse emulsion systems comprise the continuous oil phase and the dispersed aqueous phase. The dispersion medium is known as the continuous phase or the external phase. As shown, it is aqueous in nature or organic in the case of inverse systems. The disperse phase of a latex is known as the discrete phase, the internal phase and the dispersed monomer or polymer. The polymer latex comprises... 

Another important use of the PHS-PEO-PHS block copolymer is the formation of a viscoelastic film around water droplets [11, 12] this results from the dense packing of the molecule at the W/O interface, which leads to an appreciable interfacial viscosity. The viscoelastic film prevents transport of water from the internal water droplets in the multiple emulsion drop to the external aqueous medium, and this ensures the long-term physical stability of the multiple emulsion when using polymeric surfactants. The viscoelastic film can also reduce the transport of any a.i. in the internal water droplets to the external phase. This is desirable in many cases when protection of the ingredient in the internal aqueous droplets is required and release is provided on application of the multiple emulsion. 

The direct analysis of the organic phase of an emulsion polymerisation is possible. Quantification involves using the bending mode peak of water, which makes up the bulk of the reaction medium, as an internal standard. The process is demonstrated for MMA but is generally applicable to emulsion polymerisations. It does not require the introduction of an extraneous internal standard component into the reaction mixture (319). 

The last part of the chapter dealt with the preparation of stable water-in-oil-in-water (w/o/w) multiple emulsions that are suitable for application in cosmetics. The main criterion for producing stable w/o/w systems is to use two polymeric surfactants one with a low HLB number for preparation of the primary w/o emulsion and one with a high HLB number for preparation of the final w/o/w multiple emulsion. The primary emulsifier should produce a viscoelastic film fhat prevents leakage from the internal water droplets to the outside continuous phase. It will also ensure high stability (minimum coalescence) of the internal water droplets. The secondary emulsifier should also provide an effective barrier to prevent flocculation and coalescence of the multiple emulsion droplets on storage. It is also essential to balance the osmotic pressure of the internal aqueous droplets and the outside continuous medium. 

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