Aqueous microemulsions system

Figure C2.3.8. Self-diffusion coefficients at 45°C for AOT ( ), water ( ) and decane ( ) in ternary AOT, brine (0.6% aqueous NaCl) and decane microemulsion system as a function of composition, a. This compositional parameter, a, is tire weight fraction of decane relative to decane and brine. Reproduced by pennission from figure 3 of [46].

As outlined in Chapter 5, Section 5.2.3.2 various approaches to overcoming the low rates of the hydroformylation of long chain alkenes in aqueous biphasic systems have been proposed. Some of these, such as the use of microemulsions [24-26] or pH dependent solubility [27], have provided improvements often at the expense of complicating the separation process. Perhaps the most promising new approaches involve the introduction of new reactor designs where improved mixing allows for... 

A paper contributed by J. E. Desnoyers, R. Beaudoin, C. Roux, and C. Perron described the use of microemulsions as a possible tool for the extraction of oil from tar sands. Using a technique called flow microcalorimetry recently developed at the University of Sherbrooke, these researchers studied the structure and stability of organic microphases in aqueous media. These microphases can be stabilized by surfactants and can dissolve large quantities of oil. In a similar vein, D. F. Gerson, J. E. Zajic, and M. D. Ouchi (University of Western Ontario) described the extraction of bitumen from Athabasca tar sands by a combined solvent-aqueous-surfactant system. 

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. 

Different shapes of CdS nanomaterials such as quasi-nanospheres, nanoshuttles and nanotubes have been prepared in the w/o microemulsion system of nonionic surfactant (Tween-80, polyetheneoxy(20)octadecyl ether, polyoxyethylele(9)dodecyl ether (C12E9) or Tx-100)/ -pentanol/aqueous so-lution/cyclohexane [191]. Another kind of QDs, PbS, have also been prepared in nonionic CnEg/water/cyclohexane microemuisions [192]. 

Khomane et al. prepared dodecanethiol-capped CdS QDs of 4 nm size by using a Winsor II microemulsion system [242], which are soluble in solvents such as n-heptane, toluene, n-hexane, thus demonstrating the dual role of the anionic surfactant, viz., forming the microemulsion and facilitating the extraction of oppositely charged ions from the aqueous to the organic phase. 

Silica Gels. The acid-catalyzed alkoxide sol-gel process produces gels (17). Frib-erg and coworkers (40-50) pioneered the extension of this process to silica synthesis in microemulsions both aqueous and nonaqueous microemulsions were used. For aqueous microemulsions, experiments were conducted mostly with the SDS/ pentanol/water/acid system. A representative flow diagram is shown in Figure 2.2.9. The nonaqueous microemulsion systems utilized included CTAB/decanol/ decane/formamide and AOT/decane/glycerol (44-46,49,50). The experimental approach followed the sequence nonaqueous microemulsion preparation, water addition, and then TEOS addition. 

If the objective is to keep the enzyme active and stable in an aqueous phase but otherwise to use as much organic phase as possible, microemulsions are an option as a reaction medium. In contrast to ordinary emulsions they are thermodynamically stable and, at a particle diameter of 1-20 nm, accommodate most often only one enzyme molecule (Figure 12.5). The microemulsion droplets communicate rapidly and exchange their contents through elastic collisions. The boundary between microemulsions and reversed micelles is not clearly delineated, and the two notions are often used interchangeably. Enzyme of almost all classes and structures have been solubilized in microemulsion systems and used for reactions (Shield, 1986). 

In another microemulsion system, ° the colorant is incorporated into the ink as an aqueous pigment dispersion-based inkjet ink composition by formulating the ink to comprise at least one aqueous pigment dispersion and a microemulsion with at least one water-insoluble organic compound, one hydrotropic amphiphile, and water. This ink system was reported to improve waterfastness and bleed control, "" providing a fast drying ink. 

Semiconductor nanoparticles have been extensively studied in recent years owing to their strongly size-dependent optical properties. Among these nanomaterials, CdS and PbS are particularly attractive due to their nonlinear optical behavior and unusual fluorescence or photoluminescence properties [ 136,137]. A number of studies have been published recently regarding the preparation of CdS, PbS and ZnS nanoparticles in inverse microemulsion systems [138-143]. In these works, NP-5/NP-9 was the most commonly used surfactant and petroleum ether the most commonly used oil. The aqueous phase for each inverse microemulsion consisted of cadmium nitrate (0.1 M) and ammonia sulfide (0.1 M) respectively. CdS was recovered from the mixture of double microemulsions [141]. Electron microscopy revealed that the spherical particles were aroimd 10-20 nm in diameter, as seen in Fig. 14. 

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. 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelleforming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/water systems is already well documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system. A The spherical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now well-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

Leak-off or loss of acid through the walls of worm holes often results in worm holes being too short to provide significant productivity increase. Therefore, effective stimulation often requires retardation of the mineral dissolution rate. The use of microemulsions is one method to accomplish this retardation. The hydrochloric acid is injected as an water-in-oil microemulsion. The diffusion rate of the dispersed aqueous acid to the rock surface is slower than molecular diffusion of acid from a totally aqueous system. Thus the rate of limestone dissolution is retarded with the microemulsion system. 

The goal of the present work is to obtain a consistent structural model for a microemulsion system. In particular, we are interested in carrying this model down to the molecular level so that the intermolecular effects which are responsible for the stability of these systems can be elucidated. We have studied the system consisting of water, SLS and MMA with and without n-hexanol or n-pentanol. We have determined the phase boundaries of the isotropic microemulsion and Lj phases and determined how these are affected by surfactant concentration and alcohol chain length. Measurements were also made of the vapor pressure of MMA over these systems to determine the concentration of MMA in the water surrounding the microemulsion droplets. From these data, the energetics of transfer of the MMA from aqueous to micellar solution were determined. Finally, a 1,C NMR chemical shielding study was performed to find how the MMA and the alcohol were distributed within the microemulsion. 

Table I displays the mole fraction of the MMA which is in the aqueous phase for the Lj phase and microemulsion systems studied. These fractions were found to be reproducible to within 0.04. As can be seen, MMA favors the micellar phase by at least a four-to-one ratio. The free energy of transfer, calculated for systems less than 0.53 M in MMA was found to be -14.0 kJ/mole with an uncertainty of 10%.

Figure 6 shows the chemical shift of the a-carbon of hexanol as a function of composition in three different kinds of solutions. The first represents hexanol in a 14.7 wt% SLS aqueous solution as a function of hexanol concentration. The concentration range covered runs from very dilute in hexanol up to systems in which a fairly viscous phase results. Next are the microemulsions containing a 25, 50, and 67% excess of MMA over that which saturates the corresponding L1 phase system. Lastly are water-saturated MMA solutions of hexanol in which the hexanol/MMA volume ratios bracket those found in the microemulsion. The a-carbon shifts of hexanol measured in the micellar solutions are about 0.6 ppm downfield from those measured in MMA solution. There is only a slight variation in shift for either the micellar or the MMA solutions with various hexanol concentrations. The hexanol in the microemulsion systems appears approximately halfway between that in the other two solutions. 

The shift of the carbonyl carbon of MMA was determined in aqueous solution at a concentration of 0.15 M, as well as in Lj phase systems where the wt% of MMA was varied up to 12.7%, which lies at the phase boundary as shown in Figure 7. In addition, the same microemulsions and MMA solutions of hexanol were examined as were used above for the hexanol shift studies. The signal from MMA dissolved in water is downfield by 3.8 ppm from those of MMA/hexanol mixtures. The latter systems show a small downfield shift with increasing hexanol concentration. Once again, the Lj phase and microemulsion systems show an intermediate behavior but now there is a continuous upfield shift with increasing MMA content in the Lj phase which is continued in the microemulsions. 

The results of the head space analysis are evidence for the applicability of the swollen micelle model to these microemulsion systems. Despite the solubility of MMA in water, vapor pressure measurements of L phase systems find that the concentration in the aqueous phase is small. Thus the majority of the MMA must be bound in micelles which behave as a distinct phase. Near the saturation limit in the Lj phase and in the microemulsions, the measured vapor pressure is about what one would expect from an aqueous solution saturated with MMA. This also is consistent with our picture of distinct aqueous and micellar phases. In fact, the composition of the continuous phase in water-in-oil microemulsions has been determined in just this way (20). 

Microemulsions are thermodynamically stable isotropically clear dispersions composed of a polar solvent, oil, and a surfactant(s). Labrafil and Gelucire 44/14 are all-in-one self-emulsifying surfactants which are in many oral products throughout the world. Microemulsions have much potential for drug-delivery since very hydrophobic molecules can be solubilized and formulated for oral administration (Tenjarla, 1999). All of the commercial products are actually nonaqueous microemulsions, also known as microemulsion preconcentrates or self-emulsifying drug delivery systems (SEDDS), since the polar solvent is not water. Upon contact with aqueous media, such as gastrointestinal fluids, a SEDDS formulation spontaneously forms a fine dispersion or aqueous microemulsion. 

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