Second microemulsion

The double microemulsion-mediated process also provides a convenient method for preparing a metal-containing sihcate coating. The two microemulsion systems contained two common components anionic sirrfactant AOT and cyclohexane [134]. The difference was that the first microemulsion consisted of an aqueous solution of sodiirm metasihcate (0.2 M) and 10 wt% SDS as the co-surfactant, while the second microemulsion consisted of an aqueous solution of copper nitrate (0.1 M) and 10 wt% SDS. The copper-ion microemulsion was added to the silicate-ion microemulsion with constant stirring. After 8 h of gel-lation, and ageing for an additional 24 h, copper nitrate crystals were identified within the sihcate network. SUica-copper composite powders with various copper contents (4-20 wt%) and surface areas of 200-400 m /g were synthesized. 

Cerium (IV) oxide nanoparticles were synthesized by Masui et al [236] by use of a two-microemulsion technique. One of the microemulsions contained polyoxyethylene(lO) octylphenyl ether (OP-10) as the surfactant, n-hexyl alcohol as the co-surfactant, cyclohexane as the continuous phase, and an aqueous solution of cerium nitrate as the droplet phase. The second microemulsion was the same except that the droplet phase was an aqueous ammonia solution. The two were mixed to cause precipitation the particles thus obtained were gathered by centrifugation and washing under sonication with methanol, deionized water and acetone. The final treatment involved freeze-drying and vacuum drying. The mean particle size varied with experimental conditions in the range 2.5-4.0 nm. 

The same method as described above was used by the authors to produce Ce02"Coated Y-AI2O3 particles. Here, the second microemulsion (see above) contained not only aqueous ammonia solution but also Y-AI2O3 particles as substrates for deposition of Ce02. 

The lamellar area is followed by a second microemulsion are at point (c). This microemulsion is an oil-rich gel with a viscosity of 20,000 mPa-s at 1 s" (12% surfactants, 8% coemulsifier, 20% oils) and is suitable as a refatting foam bath. The Cg j -APG/SLES mixture contributes toward (cleansing performance and foam while the oil mixture acts as a refatting skin care component. 

FIG. 7 Order parameter for disperse pseudophase water derived from self-diffusion data for water, decane, and AOT reverse microemulsion illustrated in Fig. 6. The Op and arrow denote the approximate onset of percolation in low-frequency conductivity and a breakpoint in water self-diffusion increase. The arrow labeled AOT shows a second continuous transition corresponding to the onset of AOT self-diffusion increase. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

Fatty alcohol- (or alkyl-)ethoxylates, CoE, are considered to be better candidates for LLE based on their ability to induce rapid phase separation for Winsor II and III systems. (Winsor III systems consist of excess aqueous and organic phases, and a middle phase containing bicontinuous microemulsions.) However, C,E,-type surfactants alone cannot extract biomolecules, presumably because they have no net negative charge, in contrast to sorbitan esters [24,26,30,31]. But, when combined with an additional anionic surfactant such as AOT or sodium benzene dodecyl sulfonate (SDBS), or affinity surfactant, extraction readily occurs [30,31]. The second surfactant must be present beyond a minimum threshold value so that its interfacial concentration is sufficiently large to be seen by... 

In a biodesulfurization process, there are actually three phases. For a liquid mixture containing the three phases - liquid fossil fuel, water, and the biocatalyst, more than one filter would be required. One filter will preferentially collect either the liquid fossil fuel or aqueous phase as the filtrate. The retentate will then flow to the second filter, which will collect the component not removed before. The remaining retentate, containing the biocatalyst, can then, preferably, be recycled. The process can be used to resolve an emulsion or microemulsion of the liquid fossil fuel and aqueous phase resulting from a... 

BDS process. The pore size of the filter (0.2-1.0 xm) is selected such that the liquid phase, which is miscible with the liquid that is used to wet the filter, passes through the filter, while the second liquid phase remains. Thus, an aqueous filter is wet with a liquid, which is miscible with water, but immiscible with oil. The flow rate is chosen so as to prevent solid deposition through the filter. Although, such a separation process can be applied to any oil/water emulsion, it was particularly envisioned as part of a BDS process. One may ask, whether it would be more efficient to break a macroemulsion by filtering than it is by any other means Second, in the case of microemulsions, how efficient would such a filtration process be ... 

There is also a problem in defining the volume element of reaction in a microemulsion droplet, but despite these uncertainties second-order rate constants in the droplet are similar to those in cationic micelles for reactions of anionic nucleophiles in alcohol-swollen droplets (Bunton et al., 1983b). Thus, the rate enhancements seem to be due to concentration of reactants in the droplet. 

CTABr, n-C6H13NH2, octane. Second-order rate constants in the microemulsion droplets calculated... 

Before describing how microemulsion nature and structure are determined by the structure and chain length of surfactant and cosurfactant, it is necessary first to briefly review the theories of microemulsion formation and stability. These theories will highlight the important factors required for microemulsion formation. This constitutes the first part of this review. The second part describes the factors that determine whether a w/o or o/w microemulsion is formed. This is then... 

These systems were referred to by Clausse t a (21) as Type U systems. On the other hand, with cofurfactants with chain length Cg to Cy (Figur 3 e-g), the Winsor IV domain is split into two disjointed areas that are separated by a composition zone over which viscous turbid and birifringent media are encountered. This second class of systems was referred as Type S systems (24). It can also be seen that the Winsor IV domain reaches its maximum extension at reducing in size below and above C. Moreover, at C, one observes a small monophasic region near the W apex (probably o/w microemulsion of the Schulman s type) which vanishes as the alcohol chain length is increased to Cg. 

Table I. Phase volume corrected second order rate constants (k .) as a function of phase volume (( )) for the reaction of cyanide with N-dodecyl-3-carbamoyl-pyridinium ion in CTAB and BriJ 96 microemulsions...

Three different interacting phases can be distinguished in ACE the stationary, pseudostationary, and mobile phases. First, the interaction can take place at the surface of a coated capillary wall or at a stationary phase present in the capillary. This approach is analogous to CEC, as discussed previously. Second, the interaction can take place in pseudostationary phases, such as micelles, microemulsions, and liposomes. Third, the interaction can take place when both the solute and the affinity molecule are in free solution. For studying these interactions, two analysis methods have been developed. 

With micelles, microemulsions, or liposomes, a second phase is introduced into the separating system. As in chromatography, exchange of the analyte between the mobile and the stationary phases controls the separation process. Contrary to classical chromatography, both phases are mobile, moving with different velocities. As in all electrophoresis techniques, the net mobility of an analyte is the mean mobility of its fraction in the aqueous and the micellar phases ... 

This method involves formation of reverse micelles in the presence of surfactants at a water-oil interface. A clear homogeneous solution obtained by the addition of another amine or alcohol-based cosurfactant is termed a Microemulsion. To a reverse micelle solution containing a dissolved metal salt, a second reverse micelle solution containing a suitable reducing agent is added reducing the metal cations to metals. The synthesis of oxides from reverse micelles depends on the coprecipitation of one or more metal ions from... 

At low water contents (-10-20%) the mixtures will generally be milky and at some composition will become clear - at this composition a microemulsion is produced and the boundary point has been ascertained and can be plotted. On increasing the water content a second transition is reached (at typically about 60% water), which is more difficult to observe. This is the formation of a gel of high viscosity and marks the other boundary of the microemulsion region. 

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. 

Unlike micelles, an emulsion is a liquid system in which one liquid is dispersed in a second, immiscible liquid, usually in droplets, with emulsiLers added to stabilize the dispersed system. Conventional emulsions possess droplet diameters of more than 200 nm, and are therefore optically opaque or milky. Conventional emulsions are thermodynamically unstable, tending to reduce their total free energy by reducing the total area of the two-phase interface. In contrast, microemulsions with droplet diameters less than 100 nm are optically clear and thermodynamically stable. Unlike conventional emulsions that require the input of a substantial amount of energy, microemulsions are easy to prepare and form spontaneously on mixing, with little or no mechanical energy applied (Lawrence and Rees, 2000). 

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