Micellar aggregate

At low concentrations the micelles arrange in a liquid structure (no long-range translational order). It is called Lj phase. 

In an intermediate concentration range, where the materials cannot decide if the water or the amphiphile aggregate structure should be the continuous matrix, often so-called bicontinuous structures form. In such bicontinuous cubic phases the interfaces have saddle-splay fype sfructures characterized by nonzero negative mean curvature and negative Gaussian curvature. The most common bicontinuous cubic phase is called gyroid 

Normal structures and the lamellar phase, (a) Normal micellar cubic structure (Ij) (b) normal hexagonal structure (Hj). 

If the amphiphile concentration is larger than about 50 wt%, the amphiphile aggregates may revert to inverse structures (inverse hexagonal or inverse cubic structures). In these cases, the solvent becomes the minority phase. The sequence in increasing amphiphile concentrations is typically the following  

Although lyotropic liquid crystals are characterized by the fact that concentration is the determining factor in their phase transitions, temperature also plays an important role. This can be seen on the phase diagram of a soap-water system, where the vertical axis is the concentration of amphiphilic molecules and the vertical axis is the temperature. The concentration at which micelles form in solution, called the critical micelle concentration, is shown as a dotted line. The line at low temperatures is called Krafft temperature T. It separates the crystal in water part from the liquid crystalline structures. Above the solutions have milky appearances, since the micelles scatter light (the larger the micelle, the milkier the solution). Below the solution becomes clear, as only crystals are suspended in the solvent. 

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Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

Mortensen K 1996 Structural studies of PEO-PPO-PEO triblock copolymers, their micellar aggregates and mesophases a small-angle neutron scattering study J. Phys. Condens Matters A103-A104... 

Further evidence for an increased efficiency of complexation in the presence of micellar aggregates with bivalent metal counterions is presented in Table 5.4. The apparent rate constants of the reaction of 5.1c with 5.2 in the presence of micelles of Co(DS)2, Ni(DS)2, Cu(DS)2 and Zn(DS)2 are compared to the rate constants for the corresponding bivalent metal ion - dienophile complexes in the absence of micelles. The latter data are not dependent on the efficiency of the formation of the catalyst - dienophile complex whereas possible incomplete binding will certainly be reflected in the former. The good correlations between 1 and and the absence of a correlation between and... 

Another consequence of the above analysis is, that the surprising inefficiency of micellar aggregates to catalyse Diels-Alder reactions can now be rationalised. Obviously, micelles are able to bind diene and dienophile efficiently but in different parts of the micelle. The reactions seems to take place at the surface of the micelle in a rather aqueous environment, where the concentration of diene is low. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Elnulsifler Magg Balance. The overall emulsifier concentration in the system, Cgt. is constant however, it is distributed among the aqueous liiase (Cg ), the polymer particles and the monomer droplets intertaces (Cga) and the micellar aggregates (Cgm), according to the sinple balance ... 

Cgm can be easily evaluated The value of the concentration emulsifier in the micellar aggregates is required so as to properly evaluate the total surface of the micelles, In... 

The conformational dynamics of chain segments near the head groups is more restricted than that of those far from the micellar core [8]. Moreover, to avoid the presence of energetically unfavorable void space in the micellar aggregate and as a consequence of the intermolecular interactions, surfactant molecules tend to assume some preferential conformations and a staggered position with respect to the micellar core [9]. A schematic representation of a reversed micelle is shown in Figure 1. 

Solutions of surfactant-stabilized nanogels share both the advantage of gels (drastic reduction of molecular diffusion and of internal dynamics of solubilizates entrapped in the micellar aggregates) and of nonviscous liquids (nanogel-containing reversed micelles diffuse and are dispersed in a macroscopicaUy nonviscous medium). Effects on the lifetime of excited species and on the catalytic activity and stability of immobilized enzymes can be expected. 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

Fig. 6 Reaction of p-nitrophenyl diphenyl phosphate in non-micellar aggregates of tri-n-octyl ethylammonium mesylate (TEAMs) at pH 10.7 , 10 4M naphth-2,3-imidazole and O, 10-4 and 2 x KT4 M benzimidazole, respectively. (Reprinted with permission of the American Chemical Society)...

An interesting application of static quenching is the determination of micellar aggregation numbers (see Box 4.2). 

Box 4.2 Determination of micellar aggregation numbers by means of fluorescence quenching3 ... 

Quenching of pyrene by excimer formation (Py + Py —> (PyPy) —> 2Py) (see Section 4.4.1) is widely used for the determination of micellar aggregation numbers for new surfactant systems. An example is given in Figure B4.2.1. 

AP has been used to probe micellar media (Saroja et al., 1998). The probe is located at the micellar interface and is well suited to monitoring micellar aggregation. In fact, the sharp change in the fluorescence intensity versus surfactant concentration allows the critical micellar concentration (CMC) to be determined. Excellent agreement with the literature values was found for anionic, cationic and nonionic surfactants. The electroneutrality of 4-AP and its small size are distinct advantages over ionic probes like ANS or TNS. 

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