Micelles nonaqueous

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). 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes include compHcated phase changes, viscosity effects, gel formation, and Hquefication of Hquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

In spite of the potentialities of reversed micelles entrapping nonaqueous highly polar solvents [34], very few investigations on the solubilization in such systems are reported in the literature. An example is the study of the solubilization of zinc-tetraphenylporphyrin (ZnTPP) in ethylene glycol/AOT/hydrocarbon systems by steady-state and transient... 

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

Liu T, Liu LZ, Chu B (2000) Formation of amphiphilic block copolymer micelles in nonaqueous solution. In Alexandridis P, Lindman B (eds) Amphiphilic block copolymers self-assembly and applications. Elsevier, Amsterdam... 

The tendency of apolar side chains of amino acids (or lipids) to reside in the interior nonaqueous environment of a protein (or membrane/micelle/vesicle). This process is accompanied by the release of water molecules from these apolar side-chain moieties. The effect is thermodynamically driven by the increased disorder (ie., AS > 0) of the system, thereby overcoming the unfavorable enthalpy change (ie., AH < 0) for water release from the apolar groups. 

An overview of other forms of micellar systems follows in the next three sections. Formation of reverse micelles, in nonaqueous media, is discussed briefly in Section 8.8. Sections 8.9 and 8.10 present an introduction to microemulsions (oil, or water, droplets stabilized in water or oil, respectively) and their applications. 

We see in Section 8.8 that surfactants undergo aggregation in nonaqueous solvents also, but the degree of aggregation is very much less (n < 10), and the threshold for aggregation is far less sharp than in water. The mass action model for micellization seems preferable for nonaqueous systems. 

In considering the structure of micelles, we continue to base our discussion on aqueous, anionic surfactant solutions as prototypes of amphipathic systems. Cationic micelles are structured no differently from anionics, and nonionics are described parenthetically at appropriate places in the discussion. We summarize present thinking about the structure of micelles at surfactant concentrations equal to or only slightly above the CMC. We see that in nonaqueous systems (Section 8.8) and in concentrated aqueous systems (Section 8.6), the surfactant molecules are organized quite differently from the structure we describe here. 

Another striking difference between aqueous and anhydrous, nonaqueous systems is the size of the aggregates that are first formed. As we have seen, n is about 50 or larger for aqueous micelles, while for many reverse micelles n is about 10 or smaller. A corollary of the small size of nonaqueous micelles and closely related to the matter of size is the blurring of the CMC and the breakdown of the phase model for micellization. Instead, the stepwise buildup of small clusters as suggested by Reaction (D) is probably a better way of describing micellization in anhydrous systems. When the clusters are extremely small, the whole picture of a polar core shielded from a nonaqueous medium by a mantle of tail groups breaks down. 

Figure 330 The amphiphilic nature of phospholipids in solution drives the formation of complex structures. Spherical micelles may form in aqueous solution, wherein the hydrophilic head groups all point out toward the surrounding water environment and the hydrophobic tails point inward to the exclusion of water. Larger lipid bilayers may form by similar forces, creating sheets, spheres, and other highly complex morphologies. In nonaqueous solution, inverted micelles may form, wherein the tails all point toward the outer hydrophobic region and the heads point inward forming hexagonal shapes.

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). 

The same class of surfactants was used by Kitahara12s to investigate solubility, critical aggregation, micelle formation and its temperature dependence in nonaqueous... 

Becher, P. Micelle formation in aqueous and nonaqueous solutions. In Nonionic surfactants. Schick, M. J. (ed.). New York Dekker 1967... 

Kitahara, A., Kon-no, K. Micelle formation in nonaqueous media. In Colloidal dispersions and micellar behavior. ACS symposium series No. 9. 225, 1975... 

Setua, R, Chakraborty, A., Seth, D., Bhatta, M.U., Satyam, P.V., and Sarkar, N. 2007. Synthesis, optical properties, and surface enhanced Raman scattering of silver nanoparticles in nonaqueous methanol reverse micelles. Journal of Physical Chemistry C, 111 3901. 

Similarly In the emulsion system the potentials are grouped around the oxidation potential of Hyamlne indicating a chemical oxidation of the compounds by the electrolytlcally oxidized %amlne. However, In the micelle system the oxidations are spread over a wide range of potentials Indicating direct electrochemical oxidation of the compounds. This Is very similar to the results obtained In nonaqueous solutions, once more showing the hydrophobic nature of the electrode interface. 

The most important property of micelles in aqueous or nonaqueous solvents is their ability to dissolve substances that are insoluble in the pure solvent. In aqueous systems, nonpolar substances are solubilized in the interior of the micelles, whereas polar substances are solubilized in the micellar core in nonaqueous systems. This process is called solubilization. It can be defined as the formation of a thermodynamically stable isotropic solution with reduced activity of the solubilized material (8). It is useful to further differentiate between primary and secondary solubilization. The solubilization of water in tetrachloroethylene containing a surfactant is an example of primary solubilization. Secondary solubilization can be considered as an extension of primary solubilization because it refers to the solution of a substance in the primary solubilizate. 

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