Microemulsions aqueous phase

The phase behavior studies reported in Figure 4 were performed in a brine of similar salinity as in Figure 3, but containing additives to ensure long-term chemical stability of the polymer among these additives was 0.4% isopropanol. These additives have a marginal effect on phase behavior and the same increase in microemulsion/ aqueous phase interfacial tension has been observed. However, we did not find the drastic increases in viscosity reported in Figure 3. 

In Figure 1, the pairs (or triad) of phases that form ia the various multiphase regions of the diagram are illustrated by the corresponding test-tube samples. Except ia rare cases, the densities of oleic phases are less than the densities of conjugate microemulsions and the densities of microemulsions are less than the densities of conjugate aqueous phases. Thus, for samples whose compositions He within the oleic phase-microemulsion biaodal, the upper phase (ie, layer) is an oleic phase and the lower layer is a microemulsion. For compositions within the aqueous phase-microemulsion biaodal, the upper layer is a microemulsion and the lower layer is an aqueous phase. When a sample forms two layers, but the amphiphile concentration is too low for formation of a middle phase, neither layer is a microemulsion. Instead the upper layer is an oleic phase ("oil") and the lower layer is an aqueous phase ("water"). 

In three-phase systems the top phase, T, is an oleic phase, the middle phase, Af, is a microemulsion, and the bottom phase, B, is an aqueous phase. Microemulsions that occur ia equiUbrium with oae or two other phases are sometimes called "limiting microemulsions," because they occur at the limits of the siagle-phase regioa. 

The locations of the tietriangle and biaodal curves ia the phase diagram depead oa the molecular stmctures of the amphiphile and oil, on the concentration of cosurfactant and/or electrolyte if either of these components is added, and on the temperature (and, especially for compressible oils such as propane or carbon dioxide, on the pressure (29,30)). Unfortunately for the laboratory worker, only by measuriag (or correcdy estimatiag) the compositions of T, Af, and B can one be certain whether a certain pair of Hquid layers are a microemulsion and conjugate aqueous phase, a microemulsion and oleic phase, or simply a pair of aqueous and oleic phases. 

However, often the identities (aqueous, oleic, or microemulsion) of the layers can be deduced rehably by systematic changes of composition or temperature. Thus, without knowing the actual compositions for some amphiphile and oil of poiats T, Af, and B ia Figure 1, an experimentaUst might prepare a series of samples of constant amphiphile concentration and different oil—water ratios, then find that these samples formed the series (a) 1 phase, (b) 2 phases, (c) 3 phases, (d) 2 phases, (e) 1 phase as the oil—water ratio iacreased. As illustrated by Figure 1, it is likely that this sequence of samples constituted (a) a "water-continuous" microemulsion (of normal micelles with solubilized oil), (b) an upper-phase microemulsion ia equiUbrium with an excess aqueous phase, ( ) a middle-phase microemulsion with conjugate top and bottom phases, (d) a lower-phase microemulsion ia equiUbrium with excess oleic phase, and (e) an oA-continuous microemulsion (perhaps containing iaverted micelles with water cores). 

Tjandra et al. (1998) have proposed an interfacial reaction model for the kinetics of the reaction between 1-bromo octane and sodium phenoxide to give 1-phenoxyoctane in a nonionic microemulsion. In this model the microemulsion is assumed to consist of the aqueous phase and the interface is covered by a monolayer of surfactant molecules. It is thus possible to assess the interfacial area from the concentration of the surfactant in the microemulsion medium. 

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. 

Depicted in Fig. 2, microemulsion-based liquid liquid extraction (LLE) of biomolecules consists of the contacting of a biomolecule-containing aqueous solution with a surfactant-containing lipophilic phase. Upon contact, some of the water and biomolecules will transfer to the organic phase, depending on the phase equilibrium position, resulting in a biphasic Winsor II system (w/o-ME phase in equilibrium with an excess aqueous phase). Besides serving as a means to solubilize biomolecules in w/o-MEs, LLE has been frequently used to isolate and separate amino acids, peptides and proteins [4, and references therein]. In addition, LLE has recently been employed to isolate vitamins, antibiotics, and nucleotides [6,19,40,77-79]. Industrially relevant applications of LLE are listed in Table 2 [14,15,20,80-90]. 

The high sulfur-containing feedstock and the biocatalyst, usually suspended in the aqueous phase have to be contacted with each other in a bioreactor. A homogeneous, continuous phase would be preferred, which would imply formation of an emulsion, preferably a microemulsion. Several bioreactor designs have been suggested for biodesulfurization of petroleum feedstocks including impeller-mixed systems [65,202], electro-spray bioreactor [220,261,262], and draft tube air-lift bioreactor [263],... 

The draft-tube airlift bioreactor was studied using water-in-kerosene microemulsions [263], The effect of draft tube area vs. the top-section area on various parameters was studied. The effect of gas flow rates on recirculation and gas carry over due to incomplete gas disengagement were studied [264], Additionally, the effect of riser to downcomer volume was also studied. The effect of W/O ratio and viscosity was tested on gas hold-up and mass transfer coefficient [265], One limitation of these studies was the use of plain water as the aqueous phase in the cold model. The absence of biocatalyst or any fermentation broth from the experiments makes these results of little value. The effect of the parameters studied will greatly depend on the change in viscosity, hold-up, phase distribution caused due to the presence of biocatalyst, such as IGTS8, due to production of biosurfactants, etc., by the biocatalyst. Thus, further work including biocatalyst is necessary to truly assess the utility of the draft-tube airlift bioreactor for biodesulfurization. 

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... 

Typical approaches to this biphasic system have involved the immobilization of catalysts in the aqueous phase as colloids [53] or using water-soluble catalysts based on ligands such as the trisulfonated TPPTS [55, 56]. Particularly high reaction rates have been obtained with surfactant-stabilized microemulsions and emulsions that allow for intimate contact of all reagents with the catalyst during the reaction [57]. The emulsions separate readily into two phases by small pressure changes and the C02-phase is then vented to isolate the products. The catalyst RhCl(tppds)3 (tppds =... 

Based on the above results and discussion, the mechanism for the rhythmic oscillations at the oil/water interface with surfactant and alcohol molecules may be explained in the following way [3,47,48] with reference to Table 1. As the first step, surfactant and alcohol molecules diffuse from the bulk aqueous phase to the interface. The surfactant and alcohol molecules near the interface tend to form a monolayer. When the concentration of the surfactant together with the alcohol reaches an upper critical value, the surfactant molecules are abruptly transferred to the organic phase with the formation of inverted micelles or inverted microemulsions. This step should be associated with the transfer of alcohol from the interface to the organic phase. Thus, when the concentration of the surfactant at the interface decreases below the lower critical value, accumulation of the surfactant begins and the cycle is repeated. Rhythmic changes in the electrical potential and the interface tension are thus generated. 

DR. THOMAS For a solute the meaningful thing is the degree of contact with the aqueous phase. In a small micelle the solute is close to or on the surface, and it is in contact with the water. In a larger microemulsion the molecule is in the oil, the other volume. However, depending on the solute, it may penetrate further into the micelle and you can then talk of the water or lipid side of the surface. We can control the location by use of a co-surfactant [Thomas, J. K. Chem. Rev., op. cit.]. 

The rate of metal complex formation is often modified (usually enhanced) by the presence of a charged interface in the aqueous phase. This may be provided by ionic micelles, e. g., SDS, microemulsions or polyelectrolytes. jjjg reactions of Ni + and Co with hydrophobic ligands pan, pap and pad 14-16 are popular ones for examining effects, since they are well characterized in the bulk water. The simple model (4.126)... 

Winsor [15] classified the phase equilibria of microemulsions into four types, now called Winsor I-IV microemulsions, illustrated in Fig. 15.5. Types I and II are two-phase systems where a surfactant rich phase, the microemulsion, is in equilibrium with an excess organic or aqueous phase, respectively. Type III is a three-phase system in which a W/O or an O/W microemulsion is in equilibrium with an excess of both the aqueous and the organic phase. Finally, type IV is a single isotropic phase. In many cases, the properties of the system components require the presence of a surfactant and a cosurfactant in the organic phase in order to achieve the formation of reverse micelles one example is the mixture of sodium dodecylsulfate and pentanol. 

The Winsor II microemulsion is the configuration that has attracted most attention in solvent extraction from aqueous feeds, as it does not affect the structure of the aqueous phase the organic extracting phase, on the other hand, is now a W/0 microemulsion instead of a single phase. The main reason for the interest in W/0 microemulsions is that the presence of the aqueous microphase in the extracting phase may enhance the extraction of hydrophilic solutes by solubilizing them in the reverse micellar cores. However, this is not always the case and it seems to vary with the characteristics of the system and the type of solute. Furthermore, in many instances the mechanism of extraction enhancement is not simply solubilization into the reverse micellar cores. Four solubilization sites are possible in a reverse micelle, as illustrated in Fig. 15.6 [19]. An important point is that the term solubilization does not apply only to solute transfer into the reverse micelle cores, but also to insertion into the micellar boundary region called the palisade. The problem faced by researchers is that the exact location of the solute in the microemulsion phase is difficult to determine with most of the available analytical tools, and thus it has to be inferred. 

In the water-flooding process, mixed emulsifiers are used. Soluble oils are used in various oil-well-treating processes, such as the treatment of water injection wells to improve water injectivity and to remove water blockage in producing wells. The same method is useful in different cleaning processes with oil wells. This is known to be effective since water-in-oil microemulsions are found in these mixtures, and with high viscosity. The micellar solution is composed essentially of hydrocarbon, aqueous phase, and surfactant sufficient to impart micellar solution characteristics to the emulsion. The hydrocarbon is crude oil or gasoline. Surfactants are alkyl aryl... 

Consequently, the SDS microemulsion system is the best model for indirect measurement of log Pow. However, this is valid only for neutral solutes. We reported that the relationship between MI and log Pow for ionic solutes is different from that for neutral solutes (49). This would be caused by the ionic interaction between ionic solutes and the ionic microemulsion as well as ionic surfactant monomer in the aqueous phase. Kibbey et al. used pH 10 buffer for neutral and weak basic compounds and pH 3 buffer for weak acidic compounds (53). Although their purpose was to avoid measuring electrophoretic mobility in the aqueous phase, this approach is also helpful for measuring log Pow indirectly. 

To obtain a true k in MEEKC, it is important to trace the migration of the pseudostationary phase accurately. Sudan III, timepidium bromide, and quine, which have generally been used as tracers for micelles in MEKC, could not be employed as tracers for microemulsions consisting of sodium dodecylsulfate salt (SDS) or cetyltrimethylammonium bromide (CTAB), n-butanol and heptane (12). An iteration method based on a linear relationship between log k and the carbon number for alkylbenzenes (13) seems to provide a reasonable value of the migration time of the microemulsions. Dodecylbenzene shows a migration time larger than the value calculated by the iteration method and those of other hydrophobic compounds, such as phenanthrene, fluoranthrene, and Sudan III (Table 1). Methanol and ethanol were used as tracers for the aqueous phase. 

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