Micelles nonpolar solvents

The traditional association colloid is of the M R" type where R" is the surfactant ion, studied in aqueous solution. Such salts also form micelles in nonaqueous and nonpolar solvents. These structures, termed inverse micelles, have the polar groups inward if some water is present [198] however, the presence of water may prevent the observation of a well-deflned CMC [198,199]. Very complex structures may be formed in nearly anhydrous media (see Ref. 200). 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Eicke, F. H. Surfactants in Nonpolar Solvents. Aggregation and Micellization, in Topics in Current Chemistry (ed. Boschke, F. L.), p. 85, Berlin—Heidelberg—New York, Springer 1980... 

Use of a nonpolar solvent such as hexane or benzene in place of water should give rise to the formation of inverse micelles. 

Dendrimers can also be prepared with an inverse relationship between their hydrophobic and hydrophilic constituents, i.e. with a hydrophobic periphery and a hydrophilic interior. They can then behave as reverse micelles and are able to concentrate polar molecules from solutions of nonpolar solvents. The shape of these molecules, when dissolved in a solvent that matches the hydrophobic nature of the periphery, is spherical with chain-ends extended towards the solvent. The interior may then collapse to a minimum volume, so that unfavourable interactions that might result from penetration by solvent molecules are minimized. 

In the past few years, a range of solvation dynamics experiments have been demonstrated for reverse micellar systems. Reverse micelles form when a polar solvent is sequestered by surfactant molecules in a continuous nonpolar solvent. The interaction of the surfactant polar headgroups with the polar solvent can result in the formation of a well-defined solvent pool. Many different kinds of surfactants have been used to form reverse micelles. However, the structure and dynamics of reverse micelles created with Aerosol-OT (AOT) have been most frequently studied. AOT reverse micelles are monodisperse, spherical water droplets [32]. The micellar size is directly related to the water volume-to-surfactant surface area ratio defined as the molar ratio of water to AOT,... 

In mixtures of nonpolar solvents with little water, surfactants form spherical reverse micelles. They have a reversed orientation of the molecules with the hydrophilic groups in the interior and a drop of enclosed water in the middle. Starting from a precursor material, metal oxides in the form of uniform nanosized spheres can be obtained by hydrolysis under controlled conditions (pH, concentration, temperature). For example, titanium oxide spheres are obtained from a titanium alkoxide, Ti(OR)4 + 2 H20 —t Ti02 + 4 ROH. 

Thus far reference has only been made to the so-called normal micelles that are formed in polar solvents such as water. However, when the surfactants are dissolved in organic nonpolar solvents, the hydrophilic groups are found in the interior of the aggregate, while the hydrophobic chains extend toward the... 

It appears from a survey of the literature that the essential properties of micelles in nonpolar solvents are understood, namely their stability and variations of size, the dissociation behavior, and their solubilizing capacities. Reverse micelles can dissolve relatively large amounts of water (1-10% w/v depending on emulsion formula) as well as polar solutes and, of course, water-soluble compounds. Consequently, they can be used as media for a number of reactions, including enzyme-catalyzed reactions. Very few attempts to investigate such reverse micelles at subzero temperatures are known, in spite of the fact that hydrocarbon solutions present very low freezing points. 

Critical Micelle Concentration (cmc) is the surfactant concentration below which the formation of reverse micelles does not occur, while the number of surfactant molecules per micelle is referred to as the aggregation number, n. The cmc is obtained through physical measurements, and varies from 0.1-1.0 mmol dm in water or the nonpolar solvents. 

It can therefore be concluded that the aggregation of the monofunctional molecules in dilute solution of a nonpolar solvent leads to the formation of star-type inverse micelles of narrow molecular weight distribution. In addition, the extremely small CMC shows that the relative amount of material dissolved as unimers can be neglected at concentrations above 1%. 

Eicke, H.-p. Surfactants in Nonpolar Solvents. Aggregation and Micellization. 87, 85—145... 

Surfactants can aggregate in nonpolar solvents in the presence of small amounts of water with the tails oriented towards the bulk nonpolar solution and head groups interacting with water in the center (Fig. 2). The water pool formed in reverse micelles has been used as a medium to study chemical and biological reactions [22]. 

Reversed micelles 3D 20-40 A diameter Dissolution of surfactant in a nonpolar solvent in the presence of a small amount of water Concentration-, surfactant-, solubili-zate-, and solvent-dependent dynamic equilibria Particles could be generated and derivatized in aqueous pools and, subsequently, separated from surfactants 55, 101... 

Use of reverse micelles in synthetic chemistry to improve the rate and the yield of reactions seems likely to be a fruitful area of research in the future. In addition to catalysis, several other applications of reverse micelles can be cited. Just as nonpolar dirt is solubilized in aqueous micelles, so, too, polar dirt that would be unaffected by nonpolar solvents may be solubilized into reverse micelles. This plays an important role in the dry cleaning of clothing. Motor oils are also formulated to contain reverse micelles to solubilize oxidation products in the oil that might be corrosive to engine parts. 

Sodium cholate is insoluble in chloroform and in nonpolar solvents in general, but it is very soluble in alcohol and in water. Lecithin, on the contrary, is soluble in chloroform and only swells in water without dissolving in it. These differences in solubility are evidently related to the molecular structure and to the position of the hydrophilic groups in each of these molecules. The lecithin molecule has two important paraffinic chains and a group of hydrophilic functions (choline phosphate) localized at one end. In the presence of water, the lecithin molecules are oriented with their hydrophilic groups toward the water, and they hide their paraffinic chains inside a structure formed of two superposed layers of molecules. Conversely, in a nonpolar solvent the paraffinic chains are turned toward the solvent, while the polar groups are hidden inside the micelle. 

A nonpolar solubilizate such as hexane penetrates deeply into such a micelle, and is held in the nonpolar interior hydrocarbon environment, while a solubilizate such as an alcohol, which has both polar and nonpolar ends, usually penetrates less, with its polar end at or near the polar surface of the micelle. The vapor pressure of hexane in aqueous solution is diminished by the presence of sodium oleate m a manner analogous to that cited above for systems in nonpolar solvents. A 5% aqueous solution of potassium oleate dissolves more than twice the volume of propylene at a given pressure than does pure water. Dnnethylaminoazobenzene, a water-insoluble dye, is solubilized to the extent of 125 mg per liter by a 0.05 M aqueous solution of potassium myristate. Bile salts solubilize fatty acids, and this fact is considered important physiologically. Cetyl pyridinium chloride, a cationic salt, is also a solubilizing agent, and 100 ml of its A/10 solution solubilizes about 1 g of methyl ethyl-butyl either m aqueous solution. 

When carboxylate salts are put into nonpolar solvents, reversed micelles often are formed, where the polar parts of the molecules are on the inside and the nonpolar parts are on the outside. 

Surfactants with very small head group areas such as cholesterol tend to form inverted micelles (Fig. 12.7). Their head groups point into the center of the micelle while the hydrophobic tails form the continuous, hydrophobic outer region. Inverted structures such as inverted liposomes, are also formed in nonpolar solvents such as toluene, benzene, cyclohexane instead of water [534]. 

Some surfactants undergo an aggregation process in hydrocarbon and other nonpolar solvents. Th forces involved in surfactant aggregation with nonaqueous solvents must differ considerably from those already discussed for water-based systems. The orientation of the surfactant relative to the bull solvent will be opposite to that in water therefore, these systems are referred to as reverse micelles, These micelles will not have any signiLcant electrical properties relative to the bulk solvent (Luisi etal., 1988). 

In nonaqueous solvents, the signiLcant energetic source of micelle formation is the reduction of unfavorable interactions between the ionic head group of the surfactant and the nonpolar solvent molecules. In these systems, small spherical micelles appearto be the most favored, especially wher the reduction of solvent/polar group interactions is important (Luisi et al., 1988 Huruguen et al., 1991). 

H. F. Eicke. Surfactants in nonpolar solvents. Aggregation and micellization. Topics Curr. Chem., 87 85-145, 1980. 

It may be expected that other, highly structured solvents with a tri-dimensional network of strong hydrogen bonds, would also permit micelle formation by surfactants, but little evidence of such occurrences has been reported. On the other hand, surfactants in non-polar solvents, aliphatic or aromatic hydrocarbons and halocarbons tend to form so-called inverted micelles, but these aggregate in a stepwise manner rather than all at once to a definite average size. In these inverted micelles, formed, e.g., by long-chain alkylammonium salts or dinonyl-naphthalene sulfonates, the hydrophilic heads are oriented towards the interior, the alkyl chains, tails, towards the exterior of the micelles (Shinoda 1978). Water and hydrophilic solutes may be solubilized in these inverted micelles in nonpolar solvents, such as hydrocarbons. 

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