Micelles, spherical

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

Fig. XIII-11. Schematic diagram of a spherical micelle and a unilamellar vesicle. (From Ref. 118.)...

This expression can be generalized to two-dimensional aggregates (disclike micelles) and to spherical micelles, where... 

Figure 6.9 represents schematically the formation of a micelle by the association of n surfactant molecules. The cutaway view of the spherical micelle shows the hydrocarbon interior of these particles. Incidentally, it is this sort of reversible... 

Figure 6.9 Schematic illustration of the micellization process. Cutaway view of spherical micelle shows hydrocarbon interior with polar heads on surface.

Sta.g C I Pa.rtlcIeNucIea.tlon, At the start of a typical emulsion polymerization the reaction mass consists of an aqueous phase containing smaU amounts of soluble monomer, smaU spherical micelles, and much larger monomer droplets. The micelles are typicaUy 5—30-nm in diameter and are saturated with monomer emulsified by the surfactant. The monomer droplets are larger, 1,000—10,000-nm in diameter, and are also stabilized by the surfactant. 

Emulsification is the process by which a hydrophobic monomer, such as styrene, is dispersed into micelles and monomer droplets. A measure of a surfactant s abiUty to solubilize a monomer is its critical micelle concentration (CMC). Below the CMC the surfactant is dissolved ia the aqueous phase and does not serve to solubilize monomer. At and above the CMC the surfactant forms spherical micelles, usually 50 to 200 soap molecules per micelle. Many... 

Inserting this into Eq. (6), one obtains a size distribution for spherical micelles which is approximately Gaussian,... 

Hence the sizes of spherical micelles are distributed around a most probable aggregation number M, which depends only on molecular details of the surfactants in this simplest approximation. Indeed, micelle size distributions at concentrations beyond the CMC have shown a marked peak at a given aggregation number in many simulations [37,111,112,117,119,138,144,154,157]. 

At small N, correction terms come into play, which account for the ends of the cylinders. In particular, the aggregation number of cylindrical micelles in this simple picture must always be larger than M, the most probable aggregation number of a spherical micelle. Putting everything together, the expected size distribution has a peak at M which corresponds to spherical micelles, and an exponential tail at large N which is due to the contribution of cylindrical micelles. 

FIG. 10 Micelle size distribution for H2T2 surfactants within the Larson model. The dashed lines show fits to the expected form for spherical micelles (main peak) and cylindrical micelles (tail). Inset shows the tail of the distribution on a semi-logarithmic plot to demonstrate the exponential decay predicted for the cylindrical micelles. (From Nelson et al. [120].)... 

The model has been successfully used to describe wetting behavior of the microemulsion at the oil-water interface [12,18-20], to investigate a few ordered phases such as lamellar, double diamond, simple cubic, hexagonal, or crystals of spherical micelles [21,22], and to study the mixtures containing surfactant in confined geometry [23]. 

To simplify our discussion, we will consider two specific cases spherical micelles in a selective solvent and selective adsorption on to a solid surface from a selective solvent. 

Assuming spherical micelles, the volume Vm and the surface A of a micelle are given by... 

The different location of polar and amphiphilic molecules within water-containing reversed micelles is depicted in Figure 6. Polar solutes, by increasing the micellar core matter of spherical micelles, induce an increase in the micellar radius, while amphiphilic molecules, being preferentially solubihzed in the water/surfactant interface and consequently increasing the interfacial surface, lead to a decrease in the miceUar radius [49,136,137], These effects can easily be embodied in Eqs. (3) and (4), aUowing a quantitative evaluation of the mean micellar radius and number density of reversed miceUes in the presence of polar and amphiphilic solubilizates. Moreover it must be pointed out that, as a function of the specific distribution law of the solubihzate molecules and on a time scale shorter than that of the material exchange process, the system appears polydisperse and composed of empty and differently occupied reversed miceUes [136],... 

Figure 8.4 (a) Atypical molecule that behaves as lyotropic liquid crystal (b) schematic representation of a plate-shaped micelle (c) a spherical micelle (d) a cylindrical micelle. 

Further modification of the above nanostructures is useful for obtaining new functional materials. Thirdly, we apply the dopant-induced laser ablation technique to site-selectively doped thin diblock copolymer films with spheres (sea-island), cylinders (hole-network), and wormlike structures on the nanoscale [19, 20]. When the dye-doped component parts are ablated away by laser light, the films are modified selectively. Concerning the laser ablation of diblock copolymer films, Lengl et al. carried out the excimer laser ablation of diblock copolymer monolayer films, forming spherical micelles loaded with an Au salt to obtain metallic Au nanodots [21]. They used the laser ablation to remove the polymer matrix. In our experiment, however, the laser ablation is used to remove one component of block copolymers. Thereby, we can expect to obtain new functional materials with novel nanostmctures. 

The situation is similar to the exchange between the monomer and the micellar states. Usually, the exchange between the monomer and the micellar states is fast. The spectra at surfactant concentrations above CMC, therefore, consist of a single set of peaks whose chemical shifts are averaged between the monomer and micellar states. Such an example is shown by spherical micelles formed by lithium perfluoro-octylsulfonate (FOS )... 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

Prior to the addition of the silica precursor (TEOS), the acidic copolymer solution appears transparent and the SANS data shows that the copolymer forms spherical micelles of size 7.1 nm (figure 1-a). After the addition of TEOS, the solution becomes immediately turbid. Most probably, it is because TEOS is hydrophobic and forms an emulsion droplets under stirring when added to the solution [3], Then, the opacity increases with time (figure 1-b), until a thick white precipitate forms after about 23 minutes (figure 1-c). 

Figure 2 Evolution of the neutron scattering intensities with time. Only spherical micelles of P123 block copolymer are present in the synthesis mixture within the first few minutes of the reaction (300 s), during the hydrolysis of the silica precursor. Then, hybrid organic-inorganic cylindrical micelles are detected (300-1400 s). The SBA-15 hexagonal phase is formed when the precipitation occurs, after 1400 s.

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