Spherical ionic micelle

Numerous books and reviews have been published on this subject (e.g. Fendler and Fendler, 1975 Mittal, 1977). Therefore, the structural characteristics of micelles will be presented only to the extent that is necessary for the subsequent discussions. These surfactants form micelles at concentrations above the cmc (critical micelle concentration). Such micelles have average radii of 12-30 A and contain 20-100 surfactant molecules. The hydrophobic part of the aggregate forms the core of the micelle while the polar head groups are located at the micellar surface. Micelles at concentrations close to their cmc are assumed to possess spherical and ellipsoidal structures (Tanford, 1973, 1978). A schematic representation of a spherical ionic micelle is shown in Fig. 1. 

Eig. 2. A two-dimensional schematic representation of the regions of a spherical ionic micelle. The counterions (x), the head groups ( ), and the hydrocarbon chains (naxs/v) are schematically indicated to denote their relative locations but not their number, distribution, or configuration. 

Fig. 3. Schematic representation of the solubilization of nonane (upper left), n-pentanol (lower left) and a small ionic species (right) by a spherical ionic micelle (Kavanau, 1965).

Fig. 2-13. Schematic two-dimensional representation of the solubilization of (b) n-nonane as a nonpolar substrate, and (c) 1-pentanol as another amphiphile, by a spherical ionic micelle (a) of an -decanoic acid salt in water.

Rguie 6.26 (a) Diagrammatic representation of a spherical ionic micelle and (b) partial cross-section of an anionic micelle showing charged layers. 

FIGURE 1 Highly simplified pictorial model of a spherical ionic micelle. Symbols indicate counterions (x), headgroups (O), and hydrocarbon chains (wv). [From Fendler, J. H., and Fendler, E. J. (1975). Catalysis in Micellar and Macromolecular Systems, Academic Press, New York.]... 

FIGURE 7.1 A model representing the cross section of a spherical ionic micelle. 

The micelles are spherical, but when the concentration of surfactant increases, the shape of the ionic micelles changes following the spherical sequence cylindrical-hexagonal-laminar [22], In the case of nonionic micelles the shape... 

It is also known that additives may change the size and shape of micelles.At a certain point, as the surfactant or additive concentrations change, ionic micelles may change shape from spherical or nearly spherical to rodlike or other elongated forms. This may also affect the solubilization of the additive. It appears that alkane solubilization increases as the micelles become large, rodlike aggregates, whereas for polar additives like alcohols the solubilization decreases. ... 

In order to rationalize the effect whereby the activity in the Rh/2-catalyzed hydroformylation of 1-tetradecene goes through a maximum as a function of the tail length of the surfactant 2, the model of simplified spherical (Hartley) ionic micelle [9a-c] (Figure 1) was proposed [14, 15], The core of the micelle is probably composed of the hydrophobic tail of the tenside phosphine 2 where 1-tetradecene is solubilized (Figure 1, stippled part). 

Fig. 1 Representation of a simplified model of a spherical (Hartley) ionic micelle containing the Rh/2 catalyst. The solubilized 1-tetradecene in the core (stippled area), the tail of the tenside (CH3(CH2) CHCHy), the head (SOf), the counter ions (Na+, OH, depicted as X) schematically indicate their relative locations and not the relationship to their molecular size, distribution, number, or configuration.

Although at low concentrations the surfactant molecules behave independently, at higher concentrations they aggregate to form micelles. The micelles are roughly spherical and typically contain about 50 to 100 molecules. An ionic micelle is shown schematically in Fig. 10.1.3. The polar heads are in contact with the water, and surrounded by a double layer shell, while the central core of the micelle is essentially water free, being made up of the hydrocarbon tails. The concentration at which micelle formation begins is called the critical micelle concentration. Above this critical value the concentration of free surfactant molecules is essentially unchanged and, therefore, so is the surface tension. Any further addition of surfactant molecules would only go into micelle formation (Hiemenz 1986). 

Scaling Theory of Non-ionic Block Copolymer Micelles 3.1 Spherical Non-ionic Micelles... 

Figure 1 Cartoon of the surfactant unimer distribution in aqueous solution and the spontaneous self-assembly of surfactant unimers into spherical micelles just above the cmc. The two sets of arrows represent the concept of dynamic equilibrium in which the exchange rates of unimer between the air/water interface and micelles are equal. The cross section of the spherical micelle shows the core region containing the tails, the interfacial region containing hydrated headgroups (and a fraction of the counterions for ionic micelles, not shown), and the surrounding aqueous region. Such iconic images of micelles are unrealistic because experiments show that micelles are fluids and the tails are almost randomly distributed, and headgroups move at near diffusion-controlled rates that do not define a smooth surface.

---