Oil microdroplets

Microemnlsions with high surfactant concentrations provide a wide-range variation in the component ratio, which can result in essential changes in their structural behaviour and properties, in rearrangements of the interface, and in phase inversions. As a result, transitions become possible between three different phases (i) reverse microemnlsions (water/ oil w/o) composed of water microdroplets dispersed in a continuous oil phase and stabilized by the surfactant monolayer including a co-surfactant if necessary (ii) bicontinu-ous systans with both water and oil as continuous phases and surfactant molecules intertwined in a three-dimensional network (iii) direct microemnlsions (oil/water o/w) composed of the oil microdroplets dispersed in a continuous aqueous phase. ... 

M. Mazur, Polypyrrole containers grown on oil microdroplets Encapsulation of fluorescent dyes, Langmuir, 24(18), 10414-10420 (2008). 

FIGURE 13.1 Surfactant and cosurfactant molecules oriented at the surface of the oil microdroplet. 

In most reactions it is reasonable to analyze rate data in terms of reactions in water or in the interfacial region of the association colloids. This simplification fails for reactions of very hydrophobic substrates in O/W microemulsions. These substrates may be solubilized in a region of the oil microdroplets that is inaccessible to ionic or polar reactants, and it is then necessary to consider the partitioning of substrates between aqueous, interfacial, and oil regions [110,111]. 

S. Higashi and T. Setoguchi, Hepatic arterial injection chemotherapy for hepatocellular carcinoma with epira-bicin aqueous solution as numerous vesicles in iodinated poppy-seed oil microdroplets Clinical application of water-in-oil-in-water emulsion prepared nsing a membrane emulsification system, Adv. Drug Deliv. Rev. 45 (2000) 57-64. 

S. Higashi, K. Iwata, S. Tamura, Arterial-injection chemotherapy for hepatocellular carcinoma using monodispersed poppyseed oil microdroplets containing fine aqueous vesicles of epirubidn. Cancer 1995, 75,1245. 

S. Higashi, Y. Maeda, M. Kai.T.Kitamura, H. Tsubouchi, S. Tamura, T. Setogudii, A case of hepatocellular carcinoma effectively treated with epirubidn aqueous vesides in monodispersed iodized poppy-seed oil microdroplets, Hepato-Gastwenlerology 1996, 43, 1427. 

Although supercritical CO2 is an effective solvent for oils, fats, and similar substances, it is a poor one for nonvolatile hydrophilic (water-loving) substances such as proteins or metallic salts. Adding water as such to the supercritical CO2 is of little help, as the solubility of water in it is limited. Johnson and co-workers216 overcame the latter limitation by forming water-in-C02 emulsions with the aid of an added nontoxic perfluoropolyether surfactant that forms reverse micelles around the water microdroplets, in effect combining the special properties of supercritical CO2 with the solvent power of water. These emulsions can dissolve a variety of biomolecules at near-ambient temperatures, without loss of their biological activity. 

In the previous section, we demonstrated the micrometer droplet size dependence of the ET rate across a microdroplet/water interface. Beside ET reactions, interfacial mass transfer (MT) processes are also expected to depend on the droplet size. MT of ions across a polarized liquid/liquid interface have been studied by various electrochemical techniques [9-15,87], However, the techniques are disadvantageous to obtain an inside look at MT across a microspherical liquid/liquid interface, since the shape of the spherical interface varies by the change in an interfacial tension during electrochemical measurements. Direct measurements of single droplets possessing a nonpolarized liquid/liquid interface are necessary to elucidate the interfacial MT processes. On the basis of the laser trapping-electrochemistry technique, we discuss MT processes of ferrocene derivatives (FeCp-X) across a micro-oil-droplet/water interface in detail and demonstrate a droplet size dependence of the MT rate. 

It was observed that the titration of a coarse emulsion by a coemulsifier (a macromonomer) leads in some cases to the formation of a transparent microemulsion. Transition from opaque emulsion to transparent solution is spontaneous and well defined. Zero or very low interfacial tension obtained during the redistribution of coemeulsifier plays a major role in the spontaneous formation of microemulsions. Microemulsion formation involves first a large increase in the interface (e.g., a droplet of radius 120 nm will disperse ca. 1800 microdroplets of radius 10 nm - a 12-fold increase in the interfacial area), and second the formation of a mixed emulsifier /coemulsifier film at the oil/water interface, which is responsible for a very low interfacial tension. 

Figure 1 shows schematically the monomeric amphipathic particle, in this case an ionic one, with its polar head and its hydrophobic tail which is curled up in the aqueous medium. This is in equilibrium with a micelle formed by many monomers, all oriented with their heads outward toward the water and their tails randomly intertwined in the interior. A microdroplet of oil with an ionic hydrophilic surface is thus formed. The cooperative action of the many charged polar heads binds tightly a substantial fraction of the counterions thus effectively reducing the surface charge. 

A first order dependence of the rate on the quinoline concentration Is observed, as compared with a second order dependence In a similar benzene In water mlcroemulslon. The nature of the oil also has a significant effect on the electrochemical reduction of Cu(II), the half-wave potentlal(E2/2) being about 0.9 volts more negative In the mineral oil mlcroemulslon. The addition of quinoline causes a positive shift In Ej /2 which Is a-scrlbed to the formation of a four coordinate Cu(I) complex. Although aqueous Inorganic Ions are normally repelled by a microdroplet Interface of the same charge, It Is found that cadmlum(II) Ion Is bound to a droplet stabilized by the cationic surfactant cetyltrlmethyl ammonium bromide. This behavior Is Interpreted as arising from the formation of anionic species such as CdBr In the Stern layer. 

The solubility of TPP is much greater in benzene than in mineral oil, and It is therefore likely that its average location (10. 11)is nearer to the Interface and the copper does not have to be transported (e.g., as a complex) into the droplet Interior. Since the microdroplet has a net negative surface charge, it is expected that the local concentration of hydroxide Is lower, and that hydroxide cannot effectively penetrate very deeply into the surface region. This is consistent with the effect of hydroxide on an alkylation reaction, to be discussed below. This can account for its failure to Increase the rate of the base removal component, but Its role In promoting the dependence of k on copper ion remains unexplained. 

The microemulsion components, particularly the nature of the oil, has been shown to have a dramatic effect on the interaction of metal ions in the microdroplet Interfaclal region. In particular, the change from benzene to mineral oil causes a change in the quinoline dependence of the rate of metalloporphyrin formation and a 0.9 volt shift in copper(II) half- ave potential. The... 

For a simple extraction of a solute (X) from a water phase into a single spherical micro-oil-droplet, the time (t) dependence of the amount of the solute transferred across the microdroplet/water interface (m) is obtained by Equation (1) [41]. 

For chemical and physical processes across microdroplet/solution interfaces, obs having dimensions of s or dm mor s is often proportional to r " ( = 0, 1 or 2). A linear relationship between obs and has been reported for the extraction of a neutral compound such as ferrocene derivatives from water into a micro-oil-droplet without adsorption at the microdroplet/water interface [18,19] and for a photographic dye formation reaction in an oil-in-water emulsion [23]. The proportionality of a kobs versus r plot has been reported for a relatively slow process such as a photographic dye formation reaction [23,29,42], electron transfer [43-45] and adsorption at the micro-oil-droplet/water interface [19,20]. For the chemical reaction with the rate-determining step in a solution phase or a microdroplet in a microdroplet/solution system, fcobs is independent of r[23]. Based on the droplet size dependence of the reaction rate, the rate-determining step of the overall reaction processes across a microdroplet/solution interface is analysed and the reaction mechanism can be discussed in detail. 

For ion-pair extraction, a cation is extracted with an anion into oil. In this case, individual ions or the ion pair species transfer across a microdroplet/water interface and the extraction rate is expected to depend on the Galvani potential between the microdroplet and water, the ion transfer potentials across the liquid/liquid interface, the association constant of the ions in the solution and so forth [46-54]. Therefore, the mass transfer processes are complicated even in the absence of adsorption of an ion at the microdroplet/water interface. In this section, the kinetic analysis of a simple ion-pair extraction without adsorption is described and the extraction mechanism is discussed on the basis of the single microdroplet technique. 

---