Structure micellar

The shape of the micelle formed by a particular surfactant is influenced to a large extent by the geometry of the surfactant molecule, as can be seen if we consider the packing of space-filling models of the surfactants. The dimensionless parameter of use in these considerations is called the critical packing parameter (CPP) and is defined as 

Equation (6.21) and (6.22) are important in that they can be used to predict the variation of both monomers and micelles with total solution concentration. 

In nonaqueous media, reverse (or inverted) micelles may form, in which the hydrophilic charge groups form the micellar core shielded from the nonaqueous environment hy the hydrophobic chains such stmctures are generally formed when CPP 1 (see Fig. 6.25). 

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

The outer surface of the Stern layer is the shear surface of the micelle. The core and the Stern layer together constitute what is termed the kinetic micelle. Surrounding the Stern layer is a diffuse layer called the Gouy-Chapman electrical double layer, which contains the aN counterions required to neutralise the charge on the kinetic micelle. The thickness of the double layer is dependent on the ionic strength of the solution and is greatly compressed in the presence of electrolyte. 

Ctouy Chapman layer Shear surface Stern layer Core 

In polyoxyethylated non-ionic micelles the core is surrounded by a layer composed of the polyoxyethylene chains to which solvent molecules may be hydrogen bonded. This region of the micelle is often termed the palisade layer. 

Recent years have seen a renewed interest in the investigation of the nature of the micellar regions as more sophisticated experimental techniques have become available. An understanding of the detailed nature of the micellar micro- 

Early deductions from observations on the macroscopic properties of micelles suggested an essentially liquid-like hydrocarbon core. Thus, for example, there was observed to be a similarity between the heat capacities [4] and compressibilities [5] of the micelles and those of the bulk hydrocarbon of which the core was composed. Mukerjee [6], however, drew attention to the irregular variation of the CMC with chain length in a homologous series of sodium alkyl sulphates which suggested a partial structuring of the core. 

Two main fluorescent techniques have been employed fluorescent depolarization and excimer fluorescence. The molecular motion of a fluorescent probe within the lifetime of its excited state results in a diminution of the extent of polarization of the fluorescent radiation. The degree of polarization, P, of the fluorescence emitted from a probe molecule is given by 

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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. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

Mizell, n., Mizelle,/. micell(e), micella. Mizellenaufbau, m. micellar structure, mk., abbrev. (mikroskopisch) microscopic(al). Mk., abbrev. (Mark, Marken) mark, marks. M.K., abbrev. (Meterkerze) meter candle, mkr, abbrev. (mikroskopisch) microscopic, Mdbel, 71, piece of furniture, -lack, m. furniture varnish or lacquer, cabinet varnish, -leder, 71. upholstery leather, furniture leather, -po-litur,/. furniture polish, -stoff, m. upholstery fabric,... 

The Smith-Ewart theory has been modified by several researchers [13,20-24]. These researchers argued against the Smith-Ewart theory that (1) the particle formation also occurs in the absence of micellar structure, (2) the predictions on particle number with the Smith-Ewart theory are higher relative to actual case. 

The transformation of the hydrophobic periphery composed of bromo substituents into a hydrophilic wrapping of carboxylic acid functions was achieved by reacting 31 with (i) n-butyllithium and (ii) carbon dioxide. The polymer-analogous transformation provides water soluble, amphiphilic derivatives of 31 which constitute useful covalently bonded unimolecular models for micellar structures. 

As indicated earlier, clear cellulose solutions are not necessarily molec-ularly dispersed they may contain aggregates of still ordered cellulose molecules [107]. The structure of these aggregates has been described in terms of a fringed micellar structure. Figure 2a shows a schematic possi-... 

Fig. 2 Schematic representation of cellulose structures in solution Part A shows the fringed micellar structure. Parts B and C show possible chain conformations of celluloses of different DP. For high molecular weight cellulose, C, intra-molecular hydrogen bonding is possible...

On a microscopic scale, a microemulsion is a heterogeneous system and, depending on the relative amounts of the constituents, three main types of structures can be distinguished [69] oil in water (OAV, direct micellar structure), water in oil (W/O, reverse micellar structure) and a bicontinuous structure (B) (Figure 6.1). By adding oil in water, OAV dispersion evolves smoothly to a W/O dispersion via bicontinuous phases. 

Apart from liposomes, other vehicles for delivery of antigens to the immune system, such as iscoms, emulsions, and micellar structures, are presently under investigation (Jiskoot et al., 1986b Allison and Byars, 1986 Kersten et al., 1988a,b). 

Effectiveness of selective adsorption of phenanthrene in Triton X-100 solution depends on surface area, pore size distribution, and surface chemical properties of adsorbents. Since the micellar structure is not rigid, the monomer enters the pores and is adsorbed on the internal surfaces. The size of a monomer of Triton X-100 (27 A) is larger than phenanthrene (11.8 A) [4]. Therefore, only phenanthrene enters micropores with width between 11.8 A and 27 A. Table 1 shows that the area only for phenanthrene adsorption is the highest for 20 40 mesh. From XPS results, the carbon content on the surfaces was increased with decreasing particle size. Thus, 20 40 mesh activated carbon is more beneficial for selective adsorption of phenanthrene compared to Triton X-100. 

AtCCD7 (Schwartz et al. 2004). Organic solvent addition (dioxane, DMSO, methanol or acetone) improved activity under low concentrations (Mathieu et al. 2007). Short chain aliphatic alcohols activated the enzymes although the reason for this activation is unclear (probably due to influences on substrate accessibility or micellar structure). An increase in activity was observed for all aliphatic alcohols tested, although the optimal concentration lessened with increasing log P values (Schilling etal. 2007). 

Chemistry of micelles is an important area of dendrimer research. A micellar structure depends on numerous factors, such as temperature, concentration, and mainly the molecular framework of the given amphiphiles. Revolutionary research in micellar chemistry is exhibited in the work of Menger et al. [57] and by Shinkai et al. [58]. 

A number of diblock copolymers of NIPAM and hydrophobic comonomers have been prepared by various groups and assessed in terms of micellar structure, thermosensitivity, and applications. For example, PS-fo-PNIPAM was shown to form either micelles consisting of a PS core and a PNIPAM corona, or vesicles. The assemblies were colloidally stable at elevated temperature [262-266]. 

Molecules that possess both hydrophilic and hydrophobic structures may associate in aqueous media to form dynamic aggregates, commonly known as micelles. The properties of micellar structures have been discussed in great detail [66-69], but thejr main pharmaceutical application lies in their ability to provide enhanced solubility to compounds lacking sufficient aqueous solubility [70], The ability of a micelle to solubilize compounds of limited aqueous solubility can be understood from consideration of the schematic drawing of Fig. 10a. Above the critical micelle concentration, these molecules orient themselves with the polar ends in interfacing with the aqueous solution and the nonpolar ends at the interior. A hydrophobic core is formed at the interior of the micelle, and hydrophobic solute molecules enter and occupy this region. 

Figure 1. Interaction of polymer and foam an alcohol group in the foam reacts with an anhydride function on the polymer chain and is thus linked to the polymer chain. The hydro-phobic R group is subsequently incorporated into the micellar structure.

The potential influence of the dendrylation on the functional core unit includes sometimes a drastically increased molecule size as well as a steric shielding (encapsulation) and a micro-environment different and isolated from its external surroundings, eg., unimolecular micellar structures, electron-rich shells, solubilization. It is even possible to activate the core unit by both energy and electron transfer processes. In the following subsections, these design possibilities will be dealt with in more detail. 

Upon reaction, the heterogenized catalyst can be easily separated from the reaction mixture by filtration and then recycled. The hydro-phobic substrate is microemulsified in water and subjected to an orga-nometallic catalyst, which is entrapped within a partially hydrophobized sol-gel matrix. The surfactant molecules, which carry the hydrophobic substrate, adsorb/desorb reversibly on the surface of the sol-gel matrix breaking the micellar structure, spilling their substrate load into the porous medium that contains the catalyst. A catalytic reaction then takes place within the ceramic material to form the desired products that are extracted by the desorbing surfactant, carrying the emulsified product back into the solution. 

Rationalization of Micellar Structure From Theory to Experiments. 98... 

From a morphological point of view, block copolymer micelles consist of a more or less swollen core resulting from the aggregation of the insoluble blocks surrounded by a corona formed by the soluble blocks, as decribed in Sect. 2.3. Experimental techniques that allow the visualization of the different compartments of block copolymer micelles will be presented in Sect. 2.4. Other techniques allowing micellar MW determination will also be briefly discussed. Micellar dynamics and locking of micellar structures by cross-linking will be commented on in Sects. 2.5 and 2.6, respectively. 

Recent progress in novel micellar structures, including micelles containing exotic blocks such as natural or synthetic polypeptides and metal-containing segments, micelles from ABC triblock copolymers, Janus micelles and other noncentrosymmetric micelles, micelles based on interpolyelectrolyte or other noncovalent complexes, and metallosupramolecular micelles, will be discussed in Sect. 7. 

It should, however, be mentioned that the transfer of a bulk-organized system into solution can lead to very interesting structures, as will be demonstrated in Sect. 7.3 in the case of Janus micelles [33]. In this case, a micellar structure is preformed in the bulk, its core is stabilized by cross-linking, and... 

The micellar structure depicted in Fig. 2 is of course only valid for simple AB diblock copolymers. The situation can be much more complex for micelles prepared from block copolymers with complex architectures, as will be discussed later. 

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