Aqueous normal micelles

Figure 3. Simplified cross section of an aqueous normal micelle showing possible solubilization sites. A charged solute (A) would be electrostatically repelled from the micelle surface if it were of the same charge-type as the ionic micelle while an oppositely charged solute (B) would be electrostatically attracted to the micellar surface. Nonpolar solutes (C) would partition to the outer part of the more hydrophobic core region. Amphiphilic solutes (D) would attempt to align themselves so as to maximize the electrostatic and hydrophobic interactions possible between itself and the surfactant molecules. "Reproduced with permission from Ref. 49. Copyright 1984, Elsevier. "...

Aqueous Normal Micelles and Vesicles Formed by Surfactants.463... 

However, often the identities (aqueous, oleic, or microemulsion) of the layers can be deduced rehably by systematic changes of composition or temperature. Thus, without knowing the actual compositions for some amphiphile and oil of poiats T, Af, and B ia Figure 1, an experimentaUst might prepare a series of samples of constant amphiphile concentration and different oil—water ratios, then find that these samples formed the series (a) 1 phase, (b) 2 phases, (c) 3 phases, (d) 2 phases, (e) 1 phase as the oil—water ratio iacreased. As illustrated by Figure 1, it is likely that this sequence of samples constituted (a) a "water-continuous" microemulsion (of normal micelles with solubilized oil), (b) an upper-phase microemulsion ia equiUbrium with an excess aqueous phase, ( ) a middle-phase microemulsion with conjugate top and bottom phases, (d) a lower-phase microemulsion ia equiUbrium with excess oleic phase, and (e) an oA-continuous microemulsion (perhaps containing iaverted micelles with water cores). 

A microemulsion droplet is a multicomponent system containing oil, surfactant, cosurfactant, and probably water therefore there may be considerable variation in size and shape depending upon the overall composition. The packing constraints which dictate size and shape of normal micelles (Section 1) should be relaxed in microemulsions because of the presence of cosurfactant and oil. However, it is possible to draw analogies between the behavior of micelles and microemulsion droplets, at least in the more aqueous media. 

Two main microemulsion microstructures have been identified droplet and biconti-nuous microemulsions (54-58). In the droplet type, the microemulsion phase consists of solubilized micelles reverse micelles for w/o systems and normal micelles for the o/w counterparts. In w/o microemulsions, spherical water drops are coated by a monomolecular film of surfactant, while in w/o microemulsions, the dispersed phase is oil. In contrast, bicontinuous microemulsions occur as a continuous network of aqueous domains enmeshed in a continuous network of oil, with the surfactant molecules occupying the oil/water boundaries. Microemulsion-based materials synthesis relies on the availability of surfactant/oil/aqueous phase formulations that give stable microemulsions (54-58). As can be seen from Table 2.2.1, a variety of surfactants have been used, as further detailed in Table 2.2.2 (16). Also, various oils have been utilized, including straight-chain alkanes (e.g., n-decane, /(-hexane),... 

MECC separations are generally limited to compounds which are reasonably soluble in the mobile phase. In the case of normal micelles the sample components to be separated must have some solubility in the aqueous phase. It is interesting to note that good sample solubility in the aqueous-micelle mixture does not assure an effective separation. The addition of micelles to an aqueous solution can greatly increase the solubility of hydrophobic compounds. For example, the solubility of pyrene in water is enhanced by 10s when the water is made 0.07 M in SDS (2). However, all of the hydrophobic compounds in a mixture tend to be nearly completely solubilized by the micelles and elute from a MECC column poorly resolved, with retention times near tm. 

As surfactant is added to the second system of Figure 10, however, the tieline that terminates at the CMC for formation of inverted micelles in the oleic phase is reached before the normal CMC is encountered. Once the former tieline is reached, further additions of surfactant merely increase the concentration of inverted micelles. Only with much larger additions of surfactant will normal micelles begin to form. If only aqueous phases were studied, this could lead to the belief that normal micelles and the associated wettability would be encountered in a flow experiment, when, in fact, inverted micelles and different wettabilities would actually occur. 

Reverse Mieelles. Reverse Micelles in supercritical fluids are currently being studied for several distinct applications (15-18). Normal micelles and microemulsions in aqueous solutions are known to be capable of increasing solution viscosity in several applications including the surfactant flooding of petroleum reservoirs.(19) If reverse micelles or microemulsions can be formed in C02> an increase in solution viscosity could possibly occur. The surfactants chosen as candidates for CO2 flooding application should be characterized by low water solubility and a strong CO2 solubilityi minimal adsorption onto the porous media and stability at reservoir conditions. (20)... 

A convenient way to describe microemulsions is to compare them with micelles. The latter, which are thermodynamically stable, may consist of spherical units with a radius that is usually less than 5 nm. Two types of micelles may be considered (i) normal micelles in which the hydrocarbon tails form the core and the polar head groups are in contact with the aqueous medium and (ii) reverse micelles (formed in nonpolar media) in which the water core contains the polar head groups and the hydrocarbon tails are now in contact with the oil. Normal micelles can solubiHse oil in the hydrocarbon core to form O/W microemulsions, whereas reverse micelles can solubilise water to form a W/O microemulsion. A schematic representation of these systems is shown in Figure 15.1. 

The reactions of organic compounds can be catalyzed markedly in micellar solution. Catalysis by both normal micelles in aqueous medium and by reversed micelles in nonpolar solvents is possible (Fendler and Fendler, 1975 Kitahara, 1980). In normal micelles in aqueous medium, enhanced reaction of the solubilized substrate generally, but not always, occurs at the micelle-aqueous solution interface in reversed micelles in nonaqueous medium, this reaction occurs deep in the inner core of the micelle. 

The Winsor R parameter and the Mitchell-Ninham VH /lca0 parameter are related to each other in that both specify that when the value of the parameter exceeds 1, normal micelles in aqueous media in the presence of excess nonpolar solvent will be converted into reverse micelles in nonpolar solvent in the presence of excess aqueous phase. The former concept bases this on molecular interactions, the latter on molecular geometry. 

Water is the bulk solvent in all the experiments described here, although normal micelles form in a variety of three-dimensional associated solvents including 1,2-diols, formamide, and 100% sulfuric acid (17-19), and some kinetic work has been done on micelles in aqueous 1,2-diols (20). 

Ionic surfactants are strong electrolytes in dilute aqueous solution, and non-ionic surfactants are monomers, but above the so-called critical micelle concentration (cmc) they spontaneously self-associate to form micelles [15]. Micellization in water is an example of the hydrophobic effect at work [18]. The phenomenon is more properly called the solvophobic effect, because it is important in associated solvents which have three-dimensional structure, and normal micelles form in 1,2-diols, or formamide [19] and micelles with a carbocationic head group form in 100% sulfuric acid [20], for example. However, we live in an aqueous world, and most normal micellar systems are studied in water, so we can reasonably retain the term hydrophobic with the hydrophobic bond dictated by water association. 

Quantitative treatments of micellar rate effects in aqueous solution The development of quantitative models of micellar effects upon reaction rates and equilibria was based on the concept that normal micelles in aqueous, or similar associated, solvents behave as a separate medium from the body of the solvent. 

Most investigators of micellar and related phenomena have used water as a solvent. It is abundant, cheap, and easily purified, and because biological reactions occur in aqueous media we are naturally interested in reactions in water which model biological reactions. However, micelles, or micelle-like aggregates, can form in non-aqueous solvents, and it is useful to distinguish between the normal micelles which form in solvents which have three-dimensional structure [19,20], and the so-called reverse micelles which form in apolar solvents [1,126,127]. 

The formation of normal micelles in water, or in diols or triols or sulfuric acid is well established, as is the formation of aggregates in apolar solvents containing traces of water. But there are examples of aggregation in dipolar aprotic solvents or in aqueous-organic solvents of relatively low water content. In some cases reaction rates have been studied in these media, but generally only physical studies have been reported. 

In these systems, as in normal micelles, it is useful to distinguish between an aqueous and a non-aqueous pseudophase, except that the alcohol or amine in a microemulsion droplet may act as a nucleophile. Thus, as in micellar solutions, the aggregates can bring reactants together or keep them apart. For example, the complexing of metal ions to amino acid derivatives can be controlled by microemulsions [152], and the interior of a w/o microemulsion in these systems has been compared with the metal binding site of a protein. 

A major problem in interpreting rate effects in microemulsions is that a hydro-phobic reagent may bind in the non-polar part of the microemulsion droplet whereas a hydrophilic ion, e.g., OH or F , will be largely in the aqueous part of the medium. Therefore one can ask whether treatments similar to those applied to reactions in aqueous micelles are applicable to those in microemulsions. In addition it would be useful to know whether the interface between water and a microemulsion droplet has properties similar to those of a normal micelle in water. 

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