Swollen reversed micelle

The issue of water in reverse micellar cores is important because water swollen reverse micelles (reverse microemulsions) provide means for carrying almost any water-soluble component into a predominantly oil-continuous solution (see discussions of microemulsions and micellar catalysis below). In tire absence of water it appears tliat premicellar aggregates (pairs, trimers etc.) are commonly found in surfactant-in-oil solutions [47]. Critical micelle concentrations do exist (witli some exceptions). 

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]. 

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. 

There have been many studies devoted to characterization of these practically important systems. Reverse emulsion droplets have been used as chemical micro-reactors to produce nanosize inorganic and polymer particles with special properties that are not found in the bulk form (38-42). These microemulsion systems have also been a topic of research for biological systems and the AOT head groups have been found to influence the conformation of proteins and increase enzyme activity (43-6). The unique environment created in the small water pools of swollen reverse micelles allows for increased chemical reactivity. The increase in surface area with decreased in size of the droplets also can significantly increase reactivity by allowing greater contact of immiscible reactants. 

At R = 10 the acoustic method gave a slightly larger diameter than expected. This could be as a result of the constrained state of the bound water in the swollen reverse micelles. The water under these conditions may exhibit different thermal properties from those of the bulk water used in ttie particle size calculations. Also, at the low R values R< 10 or < 2.4% water), the attenuation spectrum is not very large as compared to the background heptane signal. 

The distinction between reversed micelles (RMs) and reversed miCToemul-sions is often ill-defined. Water molecules added to an RM are not distributed evenly throughout the hydrocarbon continuum but are found associated with the surfactant head groups. This is also described as swollen reversed micelle. The volume of water that can be taken up and be stabilized in the swollen reversed micelle is limited. The swollen reversed micelles are also often called reversed microemulsion. In RMs, the amount of solubilized water is less than or equal to the amount necessary to hydrate the surfactant head groups. Solubilization of water over and above this threshold results in the formation of an isotropic and thermodynanucally stable water-in-oil microemulsion. ... 

These types of surfactants become lipophilic with increasing temperature because of dehydration of the polyoxyethylene chains. At low temperature, the surfactant monolayer has a large positive spontaneous curvature forming oil-swollen micellar solution phases (or OAV microemulsions), which may coexist with an excess oil phase. At high temperatures, the spontaneous curvature becomes negative and water-swollen reverse micelles (or W/O microemulsions) coexist with excess water phase. At intermediate temperatures, the hydrophile-lipophile balance (HLB) temperature, the spontaneous curvature becomes close to zero and a bicontinuous, D phase, micro-... 

With increasing water content the reversed micelles change via swollen micelles 62) into a lamellar crystalline phase, because only a limited number of water molecules may be entrapped in a reversed micelle at a distinct surfactant concentration. Tama-mushi and Watanabe 62) have studied the formation of reversed micelles and the transition into liquid crystalline structures under thermodynamic and kinetic aspects for AOT/isooctane/water at 25 °C. According to the phase-diagram, liquid crystalline phases occur above 50—60% H20. The temperature dependence of these phase transitions have been studied by Kunieda and Shinoda 63). 

It is convenient to differentiate between oil-in-water (o/w) microemulsions and water-in-oil (w/o) microemulsions in which water and oil are the respective major components. It is reasonable to regard (o/w) microemulsions as akin to swollen normal micelles and w/o microemulsions as reverse micelles (Section 1). 

These fluorescent probes have been successful in reporting the structural parameters of surfactant assemblies such as micelles [103], reverse micelles [104], ternary systems [105], swollen micelles [106], microemulsion [107], vesicles [108], liposomes [109], hemimicelles [110], monolayers [111] and bilayers [111]. 

Note RMs, reversed micelles SMs, swollen micelles MEs, W/O microemulsions. 

In contact with water, HN03 and metal salts such as uranyl nitrate, thorium nitrate, or zirconium nitrate, extractants dissolved in the diluent form small reverse micelles. Upon extraction of metal salts, the swollen micelles interact through attractive forces... 

Table I shows the effect of various systems such as micelles, swollen micelles (achieved by adding hexanol to CTAB), microemulsion systems, vesicles formed from a double-chain CTAB surfactant, and reversed micelles with water cores formed with benzyl dimethylcetylammonium bromide in benzene. Hie active chromophore exists either as pyrene, pyrene sulfonic acid or pyrene tetrasulfonlc acid. Essentially the concept here is that the polar derivatives of pyrene will always locate pyrene at the surface of the micelle as these anionic species of pyrene complex with the positively charged surface. Dimethylaniline is used as an electron donor in each case, it can be seen that for pyrene, a continual decrease in the yield of the pyrene anion (ion yield of unity in the micelle) is observed on going from micelle to swollen micelle, to microemulsion, and no yield of ions is observed in a reversed micelle system. With pyrene tetrasulfonic acid the yield of ions over the different systems is fairly constant, even across to the reverse micellar system. However, the lifetime of the ions is extremely short in the reversed micellar system. An explanation for such behavior can be given as follows as we transverse across the...

The structure of the AOT micellar system, as well as the state of water entrapped inside swollen micelles, have been characterized using different techniques, such as photon correlation spectroscopy (25), positron annihilation (26), NMR (27, 28), fluorescence (29-32) and more recently small angle neutron scattering (33). The existence of reversed micelles has been demonstrated in the domain of concentrations explored by protein extraction experiments. Their size (proportional to the molar ratio of water to surfactant known as wo), shape and aggregation number have been determined. Furthermore, the micelle size distribution is believed to be relatively monodisperse. 

These are transparent or translucent systems covering the size range from 5 to 50nm. Unlike emulsions and nanoemulsions (which are only kinetically stable), microemulsions are thermodynamically stable as the free energy of their formation is either zero or negative. Microemulsions are better considered as swollen micelles normal micelles can be swollen by some oil in the core of the micelle to form O/W microemulsions. Reverse micelles can be swollen by water in the core to form W/O microemulsions. 

Figure 5.13 Section of the phase prism at constant surfactant concentration. Different structures within the one-phase region are indicated by hatching. In the water-rich region, swollen micelles solubilise oil. In the oil-rich region, reverse micelles of nanometre size exist. Bicontinuous structures are found in the intermediate range. (From Ref. [45], reprinted with permission of Elsevier.)...

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