Submicrometer emulsion systems

Using Quasi-Elastic Light Scattering To Study Particle Size Distributions in Submicrometer Emulsion Systems... 

Quasi-elastic light scattering is an excellent technique for studying the formation and stability of submicrometer emulsions. Improvements in the methods of quasi-elastic light scattering data acquisition and analysis that enable full particle-size distribution studies of sub-micrometer emulsion systems are discussed. Using several oil/water emulsion systems as examples, we demonstrate the ability of these techniques to determine the effect of emulsifier concentration on the particle-size distribution produced by an inversion method of emulsification. Some of the benefits of obtaining the full distribution are also discussed. 

The topic of ocular bioavailability of indomethacin administered in a sub-micrometer emulsion was al.so examined by the research group led by M, J. Alon.so at the University of Santiago de Compo-stela, Spain (32). These workers tested several carriers in vivo in rabbits nanopaiticles, nanocapsules, microparticles, and a submicrometer emulsion. The latter, prepared by the method of Yu et al. (33) with poloxamer 188 and soybean lecithin and containing 0.1% w/v indomethacin, had particles of diameter 0.21 0.02 im. The submicrometer systems (nanoparticles, nanocapsules, and emulsion) increased by more than three-fold the indomethacin concentration in the cornea, aqueous humor, and iris-ciliary body at 0.S and 1.0 h post instillation. Furthermore, an increased indomethacin bioavailability of 300% was observed in comparison with the value obtained for an aqueous commercial solution. Confocal laser scanning microscopy studies indicated (hat the submtcion particles penetrated into the comeal epithelium cells by endocyiosis (he audiors further suggested that the vehicle components (lecithin in the case of emulsions) may act as penetration enhancers or as endocytotic stimulators. 

Contrary to instillation of standard eyedrops, unilateral application of emulsified pilocarpine induced a biphasic effect in (he contralateral eye, presumably due to systemic absorption. The authors concluded tha(, in spite of (his drawback, (he pilocarpine submicrometer emulsion might serve as a long acting antiglaucoma preparation, requiring a single daily instillation. In another... 

Different architectures, such as block copolymers, crosslinked microparticles, hyperbranched polymers and dendrimers, have emerged (Fig. 7.11). Crosslinked microparticles ( microgels ) can be described as polymer particles with sizes in the submicrometer range and with particular characteristics, such as permanent shape, surface area, and solubility. The use of dispersion/emulsion aqueous or nonaqueous copolymerizations of formulations containing adequate concentrations of multifunctional monomers is the most practical and controllable way of manufacturing micro-gel-based systems (Funke et al., 1998). The sizes of CMP prepared in this way vary between 50 and 300 nm. Functional groups are either distributed in the whole CMP or are grafted onto the surface (core-shell, CS particles). 

The term microemulsion to describe such systems is not well chosen it conveys the idea of an actual emulsion characterized by submicrometer (below 0.1 tun) droplets. As is well known, an emulsion is not thermodynamically stable and cannot be represented by a single-phase domain in a thermodynamic phase diagram. The so-called microemulsions must be considered as real micellar solutions containing oil in addition to water and surfactants. These solutions, although very far from ideal in the thermodynamic sense, are nevertheless always real in the thermodynamic sense. Another important difference between microemulsions and emulsions is that, in general, a microemulsion requires significantly more surfactant than an emulsion. 

Fine solids are frequently mentioned as being responsible for W/0 emulsions, especially in systems involving various crude oils. The solids fraction of bitumen consists of fine submicrometer clay particles that have been rendered asphaltene-like due to the adsorption of highly aromatic, polar material on the particle siufaces (45). To determine if this solid fraction played a role in the film stability, a solids-free bitumen was prepared where all solid material larger than lOOnm was removed from the sample. Films of toluene- and heptane-diluted solids-free bitumen showed little or no change in both the drainage patterns and the film thickness, indicating that the fine solids had little or no effect on the behavioiu or stability of water/diluted bitu-men/water films. This is consistent with the observation described in Sec. II, where removal of fine solids from diluted bitumen had no effect on subsequently formed water in diluted bitumen emulsion. 

The diversified porous patterns of diatomaceous silicas are on the nano- to submicrometer scale (< 10-300 nm) and these meso- and macropores cannot be mediated by single macromolecules, not even proteins. To mimic these meso- and macroporous structures, a different approach can be applied based on a phase separation process as in the vesicle-mediated macromorphogenesis processes extensivily reviewed in Pickett-Heaps et al.I 1 In this case oil-in-water (0/W) emulsions are applied as a model system. 0/W emulsions are isotropic and thermodynamically stable liquid media with a continuous water domain and an oil domain, which are thermodynamically stabilized by a surfactant as micrometer-sized liquid entities. 

Emulsion Polymerization. The generation of radicals by ultrasound can also be applied in emulsion polymerization, which comprises a free-radical polymerization in a heterogeneous reaction system, yielding submicrometer polymer particles in a continuous aqueous phase. In literature a number of publications have appeared over the years on the application of ultrasound in emulsion polymerization (36-46). Ultrasoimd can be applied both for emulsification purposes as well as at higher conversions in the emulsion polymerization process (see Heterophase Polymerization). 

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