Micellar orientation

Lyotropic nematic phases were first reported by Lawson and Flautt (62) for mixtures of Cg and Cio alkyl sulfates, together with their corresponding alcohols in water. They are somewhat less common than the mesophases discussed so far. When they do form, they occur at the boundary between an isotropic micellar phase (LQ and the hexagonal phase (Li/HQ, or between Li and the lamellar phase (Li/L ). As their name implies, they have a similar micellar order to that of the molecules in a thermotropic nematic phase. This long-range micellar orientational and translational order is lower than in the other lyotropic phases described above. Like the thermotropic phases, they are of low viscosity and can be aligned in a magnetic field. It is possible to identify nematic phases optically because of their characteristic schlieren optical texture. 

Figure 28a displays a typical three-dimensional plot of the neutron intensity scattered by a nematic lyotropic solution in the (qv,qvv)-plane. The data were obtained on the SDS/Dec calamitic phase at 50 s (concentration c = 29.5 wt. % and R = [Dec]/[SDS] = 0.33). As shown in the iso-intensity contour plot (Fig. 28b), the patterns are characterized by two crescent-like peaks aside from the velocity axis. The maximum scattering corresponds to the first order of the structure factor, from which the distance between the center-of-mass of the micelles can be estimated (here 6 nm for a radius of nm). The modulation of the azimuthal intensity is also of interest since it reflects the distribution of micellar orientations. The spectra were analyzed in terms of angular distribution of the scattered intensity. The scattering was integrated over an elementary surface dgvdgvv = where corresponds typically to the half width at half... 

Caputo FE, Ugaz VM, Buighardt WR, Berret JF (2(X)2) Transient 1-2 plane smtdl-angje X-ray scattering measurements of micellar orientation in tdigning tmd tumbling nematic surfactant solutions. J Rheol 46(4) 927-946... 

Figure 11.26 Schematic representation of micellar oriented vinylpyridinium salts in water.

Berrett J F, Molino F, Porte G, Diat O and Lindner P 1996 The shear-induced transition between oriented textures and layer-sliding-mediated flows in a micellar cubic crystal J. Phys. Condens Matters 9513-17... 

Likewise, Grieco, while working with amphiphile-like reactants, observed an enhanced preference for endo-adduct in aqueous solutions, which he attributed to orientational effects within the micelles that were presumed to be present in the reaction mixture ". Although under the conditions used by Grieco, the presence of aggregates cannot be excluded, other studies have clearly demonstrated that micelle formation is not the reason for the improved selectivities . Micellar a peg tes even tend to diminish the preference for endo adduct. ... 

Jaeger and co-workers studied the regioselectivity of the reaction of a surfactant diene with a surfactant dienophile in micellar media" ". The orientational effects in the aggregates could result in an increase in the regjoselectivity in aqueous solutions of these compounds as compared to the reaction in organic media. 

The interfacial activity is determined by the sterical properties of the molecule. At the interface the spatial demand A0 of the hydrophobic part of the molecule is higher because of the second chain of the internal sulfonate compared with the terminal sulfonate. Thus, the surface concentration of the surfactant molecules is lower. That means that the hydrocarbon chains are laterally oriented and therefore cover the interface between the solution surface and air more completely. Because the ratio of the spatial demand of the head group to the volume of the alkyl chain governs the radius of the micellar surface, it... 

Micellar medium has received great attention because it solubilizes, concentrates and orientates the reactants within the micelle core and in this way accelerates the reaction and favors the regio- and stereoselectivity of the process [68], In addition the micellar medium is cheap, can be reused, is more versatile than cyclodextrins and more robust than enzymes. With regard to Diels Alder reactions, we may distinguish between (i) those in which one or both reagents are surfactants which make up the micellar medium, and (ii) those that are carried out in a micellar medium prepared by a suitable surfactant. 

The main peculiarity of solutions of reversed micelles is their ability to solubilize a wide class of ionic, polar, apolar, and amphiphilic substances. This is because in these systems a multiplicity of domains coexist apolar bulk solvent, the oriented alkyl chains of the surfactant, and the hydrophilic head group region of the reversed micelles. Ionic and polar substances are hosted in the micellar core, apolar substances are solubilized in the bulk apolar solvent, whereas amphiphilic substances are partitioned between the bulk apolar solvent and the domain comprising the alkyl chains and the surfactant polar heads, i.e., the so-called palisade layer [24],... 

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. 

The similarity for many reactions of second-order rate constants in aqueous and micellar pseudophases, and the observation that substrate hydrophobicity usually affects binding and not inherent reactivity in the micelle, suggests that substrate location or orientation is relatively unimportant. This conclusion is strongly supported by a quantitative analysis of the effects of CTABr micelles on the reaction of OH- and arylsulfonylalkyl arenesulfonates (16) (van der Langkruis and Engberts, 1984). 

X and Y. This behavior was not observed, suggesting that the substrate had considerable freedom of motion at the micellar surface. However, product selectivity is observed in some reactions in aqueous surfactants, and may be related to orientation of reactants in micelles (Sections 9 and 10). 

The changes of stereochemistry in these systems are due to changes in the preferred direction of nucleophilic attack upon a carbocationic intermediate or a related ion pair. These results therefore indicate that the substrates or reaction intermediates have preferred orientations at the micellar surface. 

There is no reason to believe that these surfactant effects upon regioselecti-vity are related to differences in molecularity. In this respect they differ from micellar control of elimination to substitution ratios. The controlling factors may be the orientation or conformation of reactants or intermediates in micelles or similar aggregates and changes in the lifetimes of intermediates (cf. van der Langkruis and Engberts, 1984). 

Elod and Schmid-Bielenberg86 observed that the speed of acetylation of dry native fibers increases with decreasing degree of micellar (crystallite) orientation. Arranged in order of increasing reactivity the dry native fibers were flax, hemp, ramie and cotton. On being pretreated with water or acetic acid, however, the fibers were alike in rates of reac-... 

Liquid crystals are a fascinating topic of study in their own right, but we limit our discussion to a brief description of the ordering in some of the possible structures. In all cases the amphipathic molecules are oriented in such a way as to minimize the contact between water and the alkyl chains. Whether the polar head points outward or not depends on which component dominates the continuous phase the minor component is solubilized inside the micellar structures. 

These results can be rationalized by picturing the crystal violet carbonium ion as solubilized and oriented in the micelle, followed by attack by the aqueous hydroxide ion. The catalytic effect of the micellar solution has a twofold origin a concentration effect and an... 

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