Reverse micelles containing water solubilization

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). 

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. 

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... 

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. 

In recent decades, many investigations have been carried out on the solubilization and on the physicochemical characterization of a wide variety of substances confined in water-containing reversed micelles. Even if these studies have not produced a general theory to predict a priori all the effects accompanying the solubihzation process, some general aspects nonetheless have been underhned. In the following, the results of some of these investigations, selected to show the extent of some peculiar behaviors, will be reported. 

Electrolytes are obviously solubilized only in the aqueous micellar core. Adding electrolytes in water-containing AOT-reversed micelles has an effect that is opposite to that observed for direct micelles, i.e., a decrease in the micellar radius and in the intermicellar attractive interactions is observed. This has been attributed to the stabilization of AOT ions at the water/surfactant interface [128]. 

FIG. 6 Representation of spherical water-containing reversed micelles solubilizing a polar molecule (p) in the micellar core (A) or an amphiphilic molecule (a) in the palisade layer (B). 

In addition to the degree of hydrophilicity of the solubilizates, their size and structure, the size of the host microregions, or the occurrence of specific processes must be taken into account in order to rationalize the driving forces of the solubilization process and of the solubilization site within water-containing reversed micelles [25,138,139],... 

By IR spectroscopy it was emphasized that the solubilization of amino acids or ohgopeptides in water-containing lecithin-reversed micelles involves structural changes in the aqueous micellar core [159]. 

The solubilization of enzymes and proteins in water-containing reversed micelles has attracted a great deal of interest for their selective separation, purification, and efficient refolding and for bioreactions involving a wide class of polar, apolar, and amphiphilic reactants and products [13,44,162-164]. 

Reverse micelles form in aprotic organic solvents, such as hydrocarbons or CCI4, and can be seen as a core containing water (the water pool) solubilized in an oily environment (for example hydrocarbons) by the hydrophobic tails. Figure 9.9 also shows the structure of AOT (from aerosol octyl), which is the most popular surfactant for reverse micelles. A typical reverse micellar system appears as a clear... 

In the studies described here, we examine in more detail the properties of these surfactant aggregates solubilized in supercritical ethane and propane. We present the results of solubility measurements of AOT in pure ethane and propane and of conductance and density measurements of supercritical fluid reverse micelle solutions. The effect of temperature and pressure on phase behavior of ternary mixtures consisting of AOT/water/supercritical ethane or propane are also examined. We report that the phase behavior of these systems is dependent on fluid pressure in contrast to liquid systems where similar changes in pressure have little or no effect. We have focused our attention on the reverse micelle region where mixtures containing 80 to 100% by weight alkane were examined. The new evidence supports and extends our initial findings related to reverse micelle structures in supercritical fluids. We report properties of these systems which may be important in the field of enhanced oil recovery. 

As shown in Figure 3.9, the L2 phase is able to solubilize a very large amount of a hydrocarbon such as decane or hexadecane. In fact, a composition containing up to 75% decane and water/surfactant/cosurfactant proportions corresponding to the L2 phase is still clear, fluid and isotropic, forms spontaneously, and is thermodynamically stable. The structure of this microemulsion can be (to some extent) regarded as a dispersion of tiny water droplets (reverse micelles) in a continuous phase of the hydrocarbon. The surfactant and cosurfactant are mainly located at the water/oil interface. This type of system is often referred to as a w/o microemulsion. 

The anionic surfactant AOT was chosen for the initial experiments because its behavior in liquid solvents such as isooctane had been extensively studied [15]. AOT readily forms one-phase reverse micelle systems containing large amounts of solubilized water without the need for a cosurfactant. The added water gathers at the centers of the reverse micelles and forms water pools in which hydrophiles can be solubilized. When water and surfactant concentrations are high, AOT reverse micelle systems can cross a percolation boundary and behave as bicontinuous microemulsions [16]. 

Reverse micelles of AOT in liquid solvents such as isooctane have been the focus of applied studies designed to exploit their special interior features such as the water pool and the palisade region near the AOT headgroups. For example, reverse micelle extraction, especially of biomolecules, has been studied by several investigators [77,78]. The process consists of contacting an aqueous solution containing valuable biological products, such as a fermentation broth, with a nonpolar reverse micelle phase. The biomolecule of interest transfers to the interior of the reverse micelles, and the unwanted components remain in the aqueous phase. A second extraction is then carried out on the reverse micelle solution to recover the solubilized biomolecules. 

To begin with, we consider microemulsions that contain individual aggregates as they are typically observed in Winsor I and II systems. In principle, these systems can be considered as oil-swollen micelles (Winsor I) or water-swollen reverse micelles (Winsor II), where, of course, the distinction between a microemulsion droplet and a (reverse) micelle that contains solubilized material is somewhat arbitrary [46]. 

Only limited work has been reported on microemulsion-mediated synthesis of aluminum hydroxide [44,45]. In the two publications available [44,45], AOT served as the surfactant. It is possible to form reverse micelles in supercritical fluid media [130], and Matson et al. [44] used such a medium and the microemulsion-plus-reactant technique to synthesize A1(0H)3 particles at 110°C. With supercritical propane as the continuous phase, anhydrous ammonia was injected into the reversed micellar solution containing solubilized Al + [as an aqueous A1(N03)3 solution]. Referring to Fig. 1 and Table 2, the resulting precipitation process followed reaction path AP3 the added ammonia reacted with water molecules in the aqueous pseudophase of the microemulsion to generate hydroxide ions ... 

In the literature there is often no clear distinction between microemulsions and micellar systems. For instance, a system containing a small amount of water solubilized in hydrocarbon may be referred to as a W/O microemulsion (or an L2 microemulsion) or as a system of reverse micelles (or swollen reverse micelles). It has been suggested [2] that the borderline between reverse micelles and microemulsions droplets should be defined by the water-to-surfactant ratio above a molar ratio of 15, the system should be referred to as a microemulsion. In this chapter no such distinction is made. All systems containing oil and water together with surfactant are termed microemulsions, regardless of the relative component proportions. 

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