Reverse micellar transport

Additional instability mechanisms and release pafliways have been demonslrated and discussed in detail by various authors. These mechanisms include transport through thinned lamella (Fig. 7b), transport of adducts or complexes that are formed in die oil phase, and other variations of these mechanisms. It seems, however, that the main instability and release mechanisms are the parallel or simultaneously occurring phenomena of reverse micellar transport and coalescence. 

Multiple emulsions are usually not empty. Soluble active materials are entrapped during the emulsification in the inner oily phase. Because of the osmotic pressure gradient, the active matter tends to diffuse and migrate from the internal phase to the external interface mostly through a controlled reverse micellar transport mechanism (Figure 7.10a) (Garti and Bisperink, 1998 Garti and Benichou, 2001). The dilemma that researchers were faced with was how to control the diffusion of oil molecules, as well as the emulsifier molecules... 

Figure 7.10 Schematic of three possible transport mechanisms in double emulsions, (a) Reverse micellar transport, (b) lamellar thinning transport of marker from the inner aqueous phase to the continuous aqueous phase.

Most of release studies are done in W/OAV multiple-emulsion systems where an active water soluble molecule is present in the inner aqueous phase. Several attempts have been made to explain the transport phenomena of entrapped addenda from the inner to the outer phase of multiple-emulsion droplets. It has been demonstrated that for lipid soluble material dissolved in the oil phase, the release obeys first-order kinetics and is diffusion controlled with excellent accordance to Pick s law. Two mechanisms for the permeation through the oil intermediate phase are well accepted, the first being via the reverse micellar transport (Figure 7.10 ) and the second via diffusion across a very thin lamellae of surfactant phase formed in areas where the oil layer is very thin (Figure 10b). 

At the inner phase, BSA provides a mechanical barrier to the release of small molecules from the internal interface. The release proceeds mainly via reverse micellar transport. The presence of BSA reduces the chance of reverse micelle formation and thus decreases the release rate of entrapped addenda within the emulsion droplets. 

Dungan et al. [186] have measured the interfacial mass transfer coefficients for the transfer of proteins (a-chymotrypsin and cytochrome C) between a bulk aqueous phase and a reverse micellar phase using a stirred diffusion cell and showed that charge interactions play a dominant role in the interfacial forward transport kinetics. The flux of protein across the bulk interface separating an aqueous buffered solution and a reverse micellar phase was measured for the purpose. Kinetic parameters for the transfer of proteins to or from a reverse micellar solution were determined at a given salt concentration, pH, and stirring... 

Kinugasa T, Tanahashi S, Takahashi S, and Takeuchi H. Transport of proteins through reversed micellar solution layer as a liquid membrane. In Proceedings of ISEC 90, 16-21 July 1990 Kyoto, Japan, Vol. 13 pp. 1839-1846. 

A further technique to overcome the mass transport limitations in biphasic catalysis is the method to work in micellar [187] or reverse micellar [188] systems, that means to enhance the surface area decisively via addition of surfactants. Ren-ken found higher reaction rates and selectivities than in non-micellar systems and could hydroformylate also olefins with a long hydrocarbon chain up to C16 (see also Section 4.5). 

Two system-dependent interpretative pictures have been proposed to rationalize this percolative behavior. One attributes percolation to the formation of a bicontinuous structure [270,271], and the other it to the formation of very large, transient aggregates of reversed micelles [249,263,272], In both cases, percolation leads to the formation of a network (static or dynamic) extending over all the system and able to enhance mass, momentum, and charge transport through the system. This network could arise from an increase in the intermicellar interactions or for topological reasons. Then all the variations of external parameters, such as temperature and micellar concentration leading to an extensive intermicellar connectivity, are expected to induce percolation [273]. 

This chapter describes the electrochemistry of small reactants dissolved in micellar solutions and microemulsions. A major influence of these microheteroge-neous fluids on reversible reactants is slowing down mass transport. These phenomena enable electrochemical probes to be used to characterize aggregate mass transport and size in the fluids. Tuning the compositions of micelles and microemulsions can control pathways and kinetics of direct organic reactions, polymerizations, and mediated electrochemical reactions. 

Abstract Faradaic electron transfer in reverse microemulsions of water, AOT, and toluene is strongly influenced by cosurfactants such as primary amides. Cosurfactant concentration, as a field variable, drives redox electron transfer processes from a low-flux to a high-flux state. Thresholds in this electron-transport phenomenon correlate with percolation thresholds in electrical conductivity in the same microemulsions and are inversely proportional to the interfacial activity of the cosurfactants. The critical exponents derived from the scaling analyses of low-frequency conductivity and dielectric spectra suggest that this percolation is close to static percolation limits, implying that percolative transport is along the extended fractal clusters of swollen micellar droplets. and NMR spectra show that surfactant packing... 

NBD probes are often used to assay flip-flop. Flip-flop refers to the reversible transversal diffusion of lipids from one leaflet to the other leaflet of a lipid bilayer membrane. In intact membranes, this transversal diffusion is very slow (fi/2 on the order of hours to days). However, it can be accelerated by biological or synthetic flippases, which are a special class of membrane transporters related to ion carriers. Alternatively, micellar pores are synthetic ion channels and pores with flippase activity and can thus be identified with flip-flop assay (Figure 2 interfacial location of the transporter, as second distinctive characteristic of micellar pores, can be identified by fluorescence depth quenching experiments with DOXYL probes). 

Two types of micellar chromatography are possible. For neutral solutes, a normal micelle with nonpolar interior and ionic surface, in an aqueous mobile phase against a reversed-phase solid support is used (10, 130, 402). The solute components, which may not be water soluble and would be adsorbed irreversibly on the reversed-phase support, are transported by partitioning into the nonpolar interior of the micelle. This type of micellar chromatography has been successful in separating pesticides, polynuclear aromatic compounds, and other model solutes (10, 130, 402). 

Whenever block copolymers are dissolved in a selective solvent that is a thermodynamical good solvent for one block and a precipitant for the other, the copolymer chains may associate reversibly to form micellar aggregates in deep analogy with the situation observed for classical low-MW surfactants, hi this respect, a CMC can be defined and experimentally measured for block copolymer micelles, as discussed in Sect. 2.1. Compared to low-MW surfactants, the values of the CMC are much lower in the case of block copolymer macrosurfactants. This motivates, e.g., the use of block copolymer micelles as nanocontainers for drug dehvery. In contrast to low-MW surfactants, these block copolymer nano containers do not dissociate into unimers whenever they are diluted in the blood stream and can therefore transport the drugs to a specifically targeted area provided that they are functionalized by suitable mo cities for site-recognition [3]. Nevertheless, macromolecular chains can encounter some dissolution problems whenever they are placed in a se-... 

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