Reverse micelles containing water

Micelles form when a suitable amphiphile [e.g., sodium bis(2-ethyl-hexyl)sulfosuccinate (AOT)], is introduced into a hydrocarbon solvent (e.g isooctane). Reverse micelles containing water form when water is taken up by an isooctane—AOT solution. At water contents exceeding what is needed to saturate the polar head groups forming the micelle wall, the system can properly be termed a water-in-oil microemulsion, in which water droplets stabilized by a monolayer of surfactant are dispersed in an organic solvent. For convenience, the terms reverse micelle and microemulsion are sometimes considered equivalent. There is a considerable literature on the properties of proteins, particularly enzyme activity, in reverse micelles (see Luisi and Steinmann-Hofmann, 1987, and references cited therein). 

The majority of practical micellar systems of Tionnal micelles use water as tire main solvent. Reverse micelles use water immiscible organic solvents, altlrough tire cores of reverse micelles are usually hydrated and may contain considerable quantities of water. Polar solvents such as glycerol, etlrylene glycol, fonnamide and hydrazine are now being used instead of water to support regular micelles [10]. Critical fluids such as critical carbon dioxide are... 

It is generally accepted that the soft-core RMs contain amounts of water equal to or less than hydration of water of the polar part of the surfactant molecules, whereas in microemulsions the water properties are close to those of the bulk water (Fendler, 1984). At relatively small water to surfactant ratios (Wo < 5), all water molecules are tightly bound to the surfactant headgroups at the soft-core reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons, and altered pHs. The solubilization properties of these two systems should clearly be different (El Seoud, 1984). The advantage of the RMs is their thermodynamic stability and the very small scale of the microstructure 1 to 20 nm. The radii of the emulsion droplets are typically 100 nm (Fendler, 1984 El Seoud, 1984). 

In reverse micelles, the water molecules in the internal pool can be divided into two subensembles. One of the ensemble consists of those water molecules which are near the charged head groups and involved in stronger polar interaction with head groups. This subensemble is termed the shell as these water molecules form the outer shell of the nano-pool water. The other suhensemble contains water molecules in the core of the reverse micelle and has dynamical character similar to those found in bulk water. This subensemble is termed the core (see Figure 17.6). 

Direct micelles contain lyophilic component of surface-active substance, whereas the reverse micelles contain lyophobic one. The miceUes can be formed in the presence and absence of water. In the case of reverse miceUes, for instance, in the hydrocarbon medium, water is easily solubilized, forming a water pool . Its size is characterized by the ratio of the water and surfactant volumes. Thus, a limited amount of water inside the micelle determines the kinetics and thermodynamics of the nanoparticles formation in a small micro/nanoreactor volume. 

Babu et al. (2003) studied encapsulating proteins in reverse micelles and dissolving it in a low-viscosity solvent that can lower the rotational correlation time of the protein. They examined the applicability of several strategies for the preparation and characterization of encapsulated proteins dissolved in low-viscosity fluids that were suitable for high-performance NMR spectroscopy. Ubiquitin was used as a model system to explore various issues such as the homogeneity of the encapsulation, characterization of the hydrodynamic performance of reverse micelles containing protein molecules, and the effective pH of the water environment of the reverse micelle. 

The formation of the metal oxide/hydroxide nanoparticles commenced upon the addition of the precipitating agent, the NaOH solution, to reverse micelles containing the dissolved salt. The addition of the aqueous solution disrupts the equilibrium momentarily as the reverse micelles re-group to accommodate the added solution [27,28]. This dismption is manifested by the appearance of immediate cloudiness. Reaction R3.1 proceeds in the bulk water pools, upon re-grouping of the reverse micelles, and metal hydroxide nanoparticles form by means of intramicellar [27,28,56,57] and intermicellar [57,58] nucleation and growth. 

One of the structures proposed for an ionic reverse micelle containing solubilized water is shown in Fig. 5.28 (see Chapter 3 for alternative models). Since micelles in non-aqueous solvents have their polar groups directed inwards and their hydrophobic groups in contact with the solvent, water or small polar... 

Since CO2 is widely considered to be the desirable SCF because of its environmentally benign characteristics and ambient critical temperature, the use of supercritical CO2 for the preparation and processing of nanomaterials has naturally received considerable attention. However, as discussed in previous sections, the poor solubility of most solutes in supercritical CO2 represents a major limitation. Surfactants containing both C02-soluble and hydrophilic moieties are often added to CO2 to form reverse micelles. Such water-in-C02 microemulsions offer a convenient means of dissolving hydrophilic compounds, as demonstrated by the in situ methods for the preparation of nanoparticles (240,247-249). For the production of nanoscale metals and semiconductors via RESOLV, on the other... 

Some surfactants, as for instance the sodium diethylhex-ylsulfosuccinate (better known as AOT), are soluble in oil. These organic solutions may contain small surfactant aggregates. They are able to solubilize water, giving rise to reverse micelles that have an aqueous core. There is no clear or obvious difference between reverse micelles and water-in-oil microemulsions. As pointed out by Friberg there is continuity in phase diagrams between (direct or reverse) micellar solutions, solubilized systems, and microemulsions. 

Reverse micelles can be used as mini-reactors for enzyme-catalysed reactions. These reactions require an aqueous environment but often the reactants are only sparingly soluble in water. Consequently, the formation of reverse micelles in an organic solvent has been suggested. The reverse micelle contains the enzyme in a tiny pool of water. Reactants dissolved in the organic solvent diffuse into the reverse micelle and products diffuse out. For the design of... 

A. Structural and Dynamic Properties of Water-Containing Reversed Micelles... 

In the case of water-containing AOT-reversed micelles, less than 1 in 1000 intermi-cellar coUisions leads to micelle coalescence followed by separation and a material exchange process occurring in the microsecond to millisecond time scale [3,79]. 

Independent of the nature of the apolar solvent, nearly spherical and monodisperse water-containing reversed micelles are formed by AOT, whose size is quite independent of the surfactant concentration and regulated mainly by the molar ratio R(R = [water]/[sur-factant]) [5,84,85]. 

Experimentally, it has been found that for water-containing AOT-reversed micelles, Vs, Vw, and change with R, becoming nearly constant above / = 10 [86,87], It follows that only above / = 10 it can a linear relationship between r and R be expected [88],... 

Eqnation 4 shows that, at constant , a change of the external parameter/ affects not only the radins but also the concentration of water-containing reversed micelles. It is also of interest that, by increasing R, the fraction of bulklike water molecules located in the core (or the time fraction spent by each water molecule in the core) of spherical reversed micelles increases progressively, whereas the opposite occurs for perturbed water molecules located at the water-surfactant interface, as a consequence of the parallel decrease of the micellar surface-to-volume ratio. 

FIG. 4 Onion model of spherical water-containing reversed micelles. Solvent molecules are not represented. A, surfactant alkyl chain domain B, head group plus hydration water domain C, hulk water domain. (For water-containing AOT-reversed micelles, the approximate thickness of layer A is 1.5 nm, of layer B is 0.4 nm, whereas the radius of C is given hy the equation r = 0.17R nm.)... 

Incidentally, it is of interest to note that solutions of water-containing reversed micelles could be employed to study the physicochemical properties of nanosize solid water. 

FIG. 5 Representation of rodlike water-containing reversed micelles. 

In many cases, under changing experimental conditions, water-containing reversed micelles evolve, exhibiting a wide range of shapes such as disks, rods, lamellas, and reverse-vesicular aggregates [15,107,108], Nickel and copper bis(2-ethylhexyl) sulfosucci-nate and sodium bis(2-ethylhexyl) phosphate, for example, form rod-shaped droplets at low water contents that convert to more spherical aggregates as the water content is increased [23,92,109,110],... 

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