Hydrophobic domain microemulsions

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... 

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],... 

The aforesaid limitations of dissolving apolar solutes in ILs can be overcome by incorporating hydrophobic domains in the IL-containing solution by the formation of microemulsions. A microemulsion is a thermodynamically stable dispersion of two immiscible liquids, a polar and an apolar phase, stabilized by an adsorbed surfactant film at the liquid-liquid interface [32,33]. Water has been conventionally used as the polar phase in microemulsions, since amphiphilic surfactants spontaneously self-assemble in water [34-A6]. In 1982, Evans and coworkers showed, for the first time, the formation of self-assembled micellar structure in the RTTL ethylammo-nium nitrate (EAN) [47]. Since then, a large number of protic and aprotic ILs have been shown to support the self-assembly of surfactants in IL-surfactant binary... 

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]. 

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. 

In 1968, Stober et al. (18) reported that, under basic conditions, the hydrolytic reaction of tetraethoxysilane (TEOS) in alcoholic solutions can be controlled to produce monodisperse spherical particles of amorphous silica. Details of this silicon alkoxide sol-gel process, based on homogeneous alcoholic solutions, are presented in Chapter 2.1. The first attempt to extend the alkoxide sol-gel process to microemul-sion systems was reported by Yanagi et al. in 1986 (19). Since then, additional contributions have appeared (20-53), as summarized in Table 2.2.1. In the microe-mulsion-mediated sol-gel process, the microheterogeneous nature (i.e., the polar-nonpolar character) of the microemulsion fluid phase permits the simultaneous solubilization of the relatively hydrophobic alkoxide precursor and the reactant water molecules. The alkoxide molecules encounter water molecules in the polar domains of the microemulsions, and, as illustrated schematically in Figure 2.2.1, the resulting hydrolysis and condensation reactions can lead to the formation of nanosize silica particles. 

Another important aspect of the formulation is illustrated in the studies aimed at preparing porous materials. In this case, one has to formulate systems containing large amounts of hydrophobic monomers (up to 70 wt%) either in the continuous phase of globular microemulsions [32-42] or in the oil domains of bicontinuous microemulsions [43-55]. A typical example of a phase diagram is given in Fig. 4. It shows four detectable... 

There have not been such detailed formulation studies for O/W microemulsions based on hydrophobic monomers. The main reason is that the monomer most investigated so far is styrene trapped in droplets stabilized by aliphatic surfactants. According to the criterion defined above, there is a chemical mismatch between styrene (aromatic) and the hydrophobic tail of the surfactant. In addition, styrene has no amphiphilic character and cannot act as a cosurfactant. As a result, the domain of existence of microemulsions is very limited. 

Certain large, hydrophobic proteins, such as bacteriorhodopsin, seem to induce a more complicated microstructure inW/O microemulsions. It has been claimed that the molecule extends over several water droplets, adapting a conformation such that polar parts of the molecule are incorporated into the water droplets and nonpolar parts are exposed to the continuous hydrocarbon domain [16]. 

Corresponding treatment of the kinetics for hydrophobic compounds residing in the continuous hydrocarbon domain predicts that both /Teat and are independent of the volume fraction of water in the microemulsion [55]. The K t value in microemulsion is predicted to be larger than the value in aqueous solution by a factor approximately equal to the oil-water partition coefficient of the substrate. The predicted kinetics seems to be supported by previously published experimental data. 

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. 

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). 

Let us turn now to the consideration of surfactants in mixtures of oil and water. With proper choice of the amphiphiles,the surfactants may solubilize the oil and water into single thermodynamic transparent, low viscosity phases called microemulsions. Microemulsions and their phase behavior may be understood in terms of surfactant decorated interfaces separating microphase separated oil and water domains. Locally, these interfaces may resemble the monolayers sketched in Fig. 4. The localization of the surfactants at the interfaces is driven by the amphiphilic character of the molecules, i.e., hydrophilic (often polar) head groups and hydrophobic (alkyl) tails. Indeed, Schulman has suggested that the total interfacial area is related to the number of surfactant molecules by the following argument. 

Potential determining salts, also referred to as phase transfer agents for a future objective of electrochemistry at the oil-water interface in microemulsions are considered. Reasearchers have studied these salts, composed of a hydrophilic and a hydrophobic ion, in microemulsion stabilized by nonionic surfactants with an oligo ethylene oxide head-group. NMR measurements show that the salts preferentially dissoc. across the surfactant interface between the oil and water domains, and hence create a potential drop across the surfactant film, and back to back diffuse double layers in the oil and water phases. These observations are also supported by Poisson-Boltzmann calcns. ... 

Figure 10 Local dynamic domain structure model for the bicontinuous microemulsion of NaDEHP-n-heptane-water. Hatched and blank regions represent water and oil, and the small black and large open circles represent the hydrophilic and hydrophobic portions, respectively, of the surfactant molecules.

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