Microemulsion, defined

The surfactant mass fraction in a microemulsion defines the size of the interfacial area between the water and oil. The reaction rate of organic reactions in microemulsions can be dramatically enhanced by increasing the specific interfacial area [95]. Enzyme catalysis in microemulsions is usually not influenced by the size of the interfacial area because only a small fraction of the reverse micelles are hosting a bio-molecule. Most investigations published so far were made with low enzyme concentrations resulting in a low population of enzymes per reverse micelle. 

Many investigations have been undertaken regarding the effect of the water concentration in the microemulsion on the catalytic behaviour of enzymes. The surfactant concentration of the microemulsion defines the size of the internal interface but it often has no measurable influence on the enzyme kinetics. On the other hand, the physical properties of the water located inside the reverse micelles differ from those of bulk water, and the difference becomes progressively smaller as the water concentration, expressed in the w -value, increases. 

Microemulsions, defined as clear solutions obtained by tritrating normal coarse oil-in-water emulsions with a medium chain alcohol and composed of the... 

However, the formal differences between microemulsions and macroemulsions are well defined. A microemulsion is a single, thermodynamically stable, equihbrium phase a macroemulsion is a dispersion of droplets or particles that contains two or more phases, which are Hquids or Hquid crystals (48). 

For diffuse and delocahzed interfaces one can still define a mathematical surface which in some way describes the film, for example by 0(r) = 0. A problem arises if one wants to compare the structure of microemulsion and of ordered phases within one formalism. The problem is caused by the topological fluctuations. As was shown, the Euler characteristic averaged over the surfaces, (x(0(r) = 0)), is different from the Euler characteristics of the average surface, x((0(r)) = 0), in the ordered phases. This difference is large in the lamellar phase, especially close to the transition to the microemulsion. x((0(r)) =0) is a natural quantity for the description of the structure of the ordered phases. For microemulsion, however, (0(r)) = 0 everywhere, and the only meaningful quantity is (x(0(r) = 0))-... 

Recently an alternative approach for the description of the structure in systems with self-assembling molecules has been proposed in Ref. 68. In this approach no particular assumption about the nature of the internal interfaces or their bicontinuity is necessary. Therefore, within the same formahsm, localized, well-defined thin films and diffuse interfaces can be described both in the ordered phases and in the microemulsion. This method is based on the vector field describing the orientational ordering of surfactant, u, or rather on its curlless part s defined in Eq. (55). 

This equation defines the internal surfaces in the system. The model has been studied in the mean held approximation (minimization of the functional) [21-23,117] and in the computer simulations [77,117,118], The stable phases in the model are oil-rich phase, water-rich phase, microemulsion, and ordered lamellar phase. However, as was shown in Refs. 21-23 there is an infinite number of metastable solutions of the minimizahon procedure ... 

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. 

Figure 12.15. Fluorescence lifetime of IR-140 in Aerosol OT (AOT)/iso-octane microemulsions as a function of water pool size to, defined as the molar fraction of water to AOT. (From Ref. 58.)...

On the other hand, with microemulsions based on an anionic surfactant and a long chain alcohol, was fairly low for certain concentrations, indicating that distinct water droplets in a hydrophobic medium may form. The system investigated by Lindman et al (29-34) was based on octanoic acid - decanol -octane-water. This means that the anionic "surfactant" used contains only seven carbon atoms in the alkyl chain which is fairly short. With longer chain surfactants, one would expect well defined "water cores" provided the alcohol is also long-chain. Such well defined "water cores" have also been confirmed by Lindman et a (34) for the Aerosol OT - hydrocarbon system. 

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 microemulsion is defined as a thermodynamically stable and clear isotropic mixture of water-oil-surfactant-cosurfactant (in most systems, it is a mixture of short-chain alcohols). The cosurfactant is the fourth component, which effects the formation of very small aggregates or drops that make the microemulsion almost clear. 

As pointed out in Chapter 1, supramolecular chemistry comprises two broad, partially overlapping areas covering on the one hand the oligomolecular supermolecules and, on the other, the supramolecular assemblies, extended polymolecular arrays presenting a more or less well-defined microscopic organisation and macroscopic features depending on their nature (layers, films, membranes, vesicles, micelles, microemulsions, gels, mesomorphic phases, solid state species, etc.). 

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. 

Co is called the spontaneous curvature. The spontaneous curvature is a more general parameter than the surfactant parameter Ns, defined by Eq. (12.4). It makes it easier to discuss the phase behavior of microemulsions because we get away from the simple geometric picture. 

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