Hydrophilic domain microemulsions

Thus, in summary, self diffusion measurements by Lindman et a (29-34) have clearly indicated that the structure of microemulsions depends to a large extent on the chain length of the oosurfactant (alcohol), the surfactant and the type of system. With short chain alcohols (hydrophilic domains and the structure is best described by a bicontinuous solution with easily deformable and flexible interfaces. This picture is consistent with the percolative behaviour observed when the conductivity is measured as a function of water volume fraction (see above). With long chain alcohols (> Cg) on the other hand, well defined "cores" may be distinguished with a more pronounced separation into hydrophobic and hydrophilic regions. 

A broad variety of enzymes have been used to catalyze organic reactions in microemulsions. In the majority of cases the enzyme retains both activity and stability in a satisfactory way. Special attention has been given to the use of lipases in W/O microemulsions where the enzyme is located in water droplets of a size not much larger than the hydrodynamic diameter of the protein. Such systems are biomimetic in the sense that lipases in biological systems operate at the interface between hydrophobic and hydrophilic domains, with these interfaces being stabilized by polar lipids and other natural amphiphiles. 

Since some structural and dynamic features of w/o microemulsions are similar to those of cellular membranes, such as dominance of interfacial effects and coexistence of spatially separated hydrophilic and hydrophobic nanoscopic domains, the formation of nanoparticles of some inorganic salts in microemulsions could be a very simple and realistic way to model or to mimic some aspects of biomineralization processes [216,217]. 

The solubility characteristics of the substrates are important. The hydrophilic reactant must have negligible solubility in the non-polar domain and vice versa. If the lipophilic substrate is soluble in water to an appreciable extent, a bulk reaction in the water domain will accompany the reaction at the interface. This aspect has been investigated in some detail for another substitution reaction, reaction between potassium iodide and four different benzyl bromides using an oil-in-water microemulsion based on D2O, decane and Ci2E5 as reaction medium [ 16]. The lipophilic components were unsubstituted benzyl bromide, 4-methylbenzyl bromide, 4-isopropylbenzyl bromide and 4-fert-butylbenzyl bromide. As... 

The fluorescence emission spectra of hydrophilic probes solubilized in the polar domains of reverse micelles provide a convenient means of ascertaining the nature of the solubilized water (25) and thus the expected reactivity of TEOS with these water molecules. For example, the emission spectra of Ru(Bpy)32+ is blue-shifted with respect to water in media of reduced polarity (26). Figure 5 shows the spectra of Ru(Bpy)32+ obtained at different R values. At low R values (below about 0.9) the emission spectra have a maximum intensity at about 575 nm with a red shoulder at 615 nm. As the water content increases, the intensity at 615 nm increases and the emission at 575 nm decreases. For R values above 1, the spectra resemble that of the probe in water (26). These results agree very well with previous results (14) for the fluorescence of Ru(Bpy)32+ in NP-5-cyclohexane microemulsion samples of constant surfactant concentration. In addition,... 

TEOS-Microemulsion System, In these experiments the overall concentrations of water, ammonia, and TEOS were kept constant. The water-to-surfactant molar ratio R was varied by changing the total surfactant concentration. Thus, the observed effect of R on particle size and size distribution is due to the presence of surfactant aggregates, which result in the localization of reagents in well-defined polar (hydrophilic) and nonpolar (hydrophobic) domains. The NP-5-cyclohexane-NH40H system... 

To overcome most of solubilization problems, colloidal surfactant systems (e.g. micelles, liquid crystals, microemulsions, vesicles, emulsions, etc.) are attracting a great deal of attention as alternative reaction media (Walde 1996 Holmberg 1997 Antonietti 2001). Their advantages are they possess micro- and nanostmctures consisting of well-defined hydrophilic and lipophilic domains separated by surfactant films with very large interfacial area, the exchange between chemical species... 

Not only is aggregation favored as R increases, but there is also a change in the nature of the solubilized water. Initially, water is tightly hydrogen-bonded to the oxyethylene groups of the nonionic surfactant Further water addition induces aggregation (dipole-dipole interactions), and a point is reached where unbound or free water molecules are present in the hydrophilic (polar) domain. The state of water in the polar domain is relevant to the formation of particles, because the initial hydrolysis of TEOS in the reverse micelle is expected to be favored as more free water molecules are available. Such an effect is expected because the hydrolysis of titanium alkoxides (naturally more reactive than TEOS toward water) is strongly inhibited in reverse microemulsions formulated with nonionic or anionic surfactants at low R values [10,16]. 

Surprisingly, esterification of fatty acids with simple sugars, such as glucose and mannitol, in AOT-based microemulsions did not take place at all [82]. No reaction was seen with either of two different lipases. This is probably due to poor phase contact between the very hydrophilic sugar molecule in the water pool and the fatty acid that resides in the hydrocarbon domain. Sugar monoesters can be produced in high yields by lipase-catalyzed esterification in a water-free medium [90]. 

The lowest minimum value of y is observed for the microemulsion containing the most hydrophilic cosurfactant and the sulfonate blend which has the largest fraction of TRS 18 allowed by the locus domain. 

Microemulsions are clear, thermodynamically stable liquid mixtures of two immiscible fluids, consisting of nanometer-sized aqueous or oil droplets dispersed in the continuous phase [11], These nanodroplets can be used as nanoreactors because of their characteristic interfacial properties allowing an intimate contact, at nanoscale level, of hydrophilic and hydrophobic domains. The relatively well understood self-assembly dynamics of these materials, and the ease of formation of enclosed structures with specified size (and, in some cases, shape) are very attractive for a comprehensive understanding of chemical reactions carried out in these media [12],... 

Since the appearance of micellar enzymology as a new field of research interest, proteases, being hydrophilic and relatively small, have been considered as good model systems for the effectuation of both hydrolytic and synthetic reactions in w/o microemulsions [44,55] (Table 13.2). These enzymes can be easily dissolved in the aqueous domains of w/o microemulsions and maintain their catalytically active conformation since they remain protected from the denaturating effect of both organic solvent and synthetic emulsifiers. In addition, the coexistence of aqueous, organic, and amphiphilic domains, which characterize w/o microemulsions, enables the contact of the enzymes with substrates of different polarities. 

In microemulsions, oil and water mix over small length scales, and thus an extraordinarily large interfacial area spans the oil and water domains. In order to thermodynamically stabilize such fine structures, the surfactant must generate an ultra-low free energy per unit of interfacial area between oil and water microdomains within the microemulsion phase. Such low free energies result from a precise balancing of the hydrophilic-lipophilic nature of the surfactant. As a consequence of this precise balancing, the macroscopic interfacial tensions between microemulsion phases and excess oil phases are also ultra-low (of the order of 10-3 mN/m) (15, 16). 

With this type of nonionic surfactant, the presence of a cosurfactant is not needed to obtain a large microemulsion single-phase domain at a fixed temperature. Moreover, by varying the relative volumes of the polar and apolar parts of the surfactant, it is easy to modify the hydrophilic/hydrophobic balance (expressed as the spontaneous curvature //q, surfactant packing parameter Pq, or HLB balance R). These quantities can be modified by temperature variations only because the hydration of the polar head is temperature-dependant. Therefore, the spontaneous curvature turns from oil to water as the temperature is increased. Shinoda, Kunieda and co-workers, as well as Kahlweit s group, have given a general description of the phase behaviour obtained with C/Ey species (49). 

Amphiphiles, the representatives of which are soap, surfactant and lipid, have a hydrophilic polar head and lipophilic nonpolar tails. They always remain on the interface between water and oil and form monolayers of surfactants in a water/oil/amphiphile ternary system. This monolayers or interfacial film reduce the surface tension between water and oil domains. In a three-component system the surfactant film exists in various topologically different structures such as micelles, vesicles, bicontinuous microemulsions, hexagonal arrays of cylinders or lamellar structures depending upon the pressure, temperature and the concentration of the components [1,2]. Microemulsions are thermodynamically stable, isotropic and transparent mixtures of ternary amphiphilic systems. When almost equal volume fractions of water and oil are mixed with a dilute concentration of surfactants, they take... 

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