Micellization in Non-Polar Solvents

Evidently, the situation that interests us most (as will be seen in Chapter 5) is where a micelle has been formed in a non-polar liquid, and water, otherwise insoluble in this liquid, has been incorporated in the center of the micelle. It is better to reiterate here that micelles in non-polar solvents (if they form at all [65]), should have a relatively small CMC. 

Other interesting examples of the organized molecular structures used to increase the quantum yield of charge photoseparation are micelles and vesicles. Micelles represent aggregates of surfactant molecules, one end of which is hydrophobic and the other hydrophilic. On reaching a certain critical concentration in a solution, these molecules group into spherical formations in which either the hydrophilic ends of the molecules are turned towards the micelle centre while their hydrophobic ends form its surface or vice versa. Micelles of the former type are usually formed in non-polar solvents and those of the latter type in polar solvents. The micelle is schematically represented in Fig. 1(d). 

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. 

In non-polar solvents, hydrophilic head groups interact due to dipole-dipole attractions and produce aggregates called reverse micelles. With this structure, head groups of surfactant molecules orientate towards the interior and the hydrophobic tails orientate towards the nonpolar solvents. In the absence of additives such as water, the aggregation numbers of reverse micelles are small (mostly less than 10). On the other hand, in polar solvents such as glycol,... 

In reverse micelles (formed in non-polar solvent systems containing surfactant), polar additives may be solubilized in the core where a polar interaction of head groups occurs. 

Micelles of amphiphatic surfactant molecules in non-polar solvents have the reverse structure when compared with micelles in aqueous solvents. The polar... 

This dimeric structure (12-17) comprising a polar central core with peripheral alkyl groups (or more poetically, highly polar stannoxane islands dispersed in a less polar ocean ),81 has the properties of a reversed micelle, and it is this structure that confers the high solubility in non-polar solvents. 

Gangliosides are colorless crystalLizabk substances which melt with decompn In sol in non-polar solvents. Soly in polar solvents increases with the size of the sugar residue and the sialic acid content. Forms micelles in aq soln, having mol wi of about 200,000-250,000, Forms molecular solns in DMF or tetrahydrofuran- having mol wt of 1000-3000. 

For simple amphiphilic compounds, the association is of low co-operativity in non-polar solvents and leads typically only to smaller and polydisperse aggregates. An illustration is given in Figure 19.16. However, introduction of even quite small amounts of water can induce a co-operative self-assembly leading to reverse micelles. 

Micelle formation and critical micelle concentration of different surfactants in aqueous solvents have been extensively studied from different angles and discussed at length [41,3]. Some discussion on the factors influencing the CMC in aqueous media has been made in Section 2.4. We therefore move now to the case of CMC in non-polar solvents. 

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... 

Physical properties Insoluble in non-polar solvents form micelles in aqueous solution... 

A micelle is an assembly of amphiphilic molecules dispersed in water. Such molecules are made of two parts, a polar head group and a non-polar tail . The polar head is for example a carboxylic acid which can dissociate into ions (—COO- and H+) the non-polar tail is a saturated hydrocarbon chain. Since the non-polar parts are insoluble in a polar solvent, these molecules aggregate in water to form micelles which are microscopic droplets with a non-polar interior and polar groups at the water interface. This picture of micelles is probably an oversimplification, because water penetrates to some extent between the molecules it is however sufficient for an understanding of the special properties of micellar suspensions in photochemistry. 

The micelles formed in the non-polar solvents would seem in general to be relatively small, the aggregation number being of the order of 4 to 30. When the micelles are nearly spherical, as they seem to be at low concentrations, the low aggregation numbers are due to steric factors. The space in a spherical soft-core micelle permits only the accommodation of a limited number of polar groups this also applies to the number of bulky hydrocarbon groups in the outer parts of the micelle (Eicke, 1980 Ekwall, 1972 Kertes and Gutman, 1976 Rounds, 1976). 

Aggregation number Three main factors play a critical role in aggregation of soft-core reverse micelles the interaction between polar groups, the interaction of the hydrophobic (non-polar) part, and environmental factors. Compare aggregation numbers of some surfactants in low polar solvents specified in Table 3.1. 

An inverse (or reverse) micelle, which forms in a non-polar solvent, will have the hydrophilic head groups oriented toward the inside of the sphere, where a water pool is formed and a hydrophilic probe can become associated [10]. Some surfactants commonly employed to stabilize reverse micelles include sodium diisooctylsulfosuccinate (AOT), benzylhexadecyldimethylammonium chloride (BHDC), and dodecylammonium propionate (DAP). Ionic surfactants induce formation of a larger water pool than non-ionic surfactants, but the size of the hydrophilic core also depends on temperature and on the ratio of water to surfactant. 

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