Anomalous micellization

Anomalous micellization (large particles observed at onset of micellization) SLS, sed. vel. Tiizar el al. (1977)... 

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

Zhou, Z. and B. Chu. 1988. Anomalous micellization behavior and composition heterogeneity of a triblock ABA copolymer of (a) ethylene oxide and (b) propylene oxide in aqueous solUM laaromolecules 21 2548-2554. 

J. P. Bouchaud, A. Ott, D. Langevin, W. Urbach. Anomalous diffusion in elongated micelles and its Levy flight interpretation. J Phys II (Erance) 2 1465-1482, 1991. 

The molecular collective behavior of surfactant molecules has been analyzed using the time courses of capillary wave frequency after injection of surfactant aqueous solution onto the liquid-liquid interface [5,8]. Typical power spectra for capillary waves excited at the water-nitrobenzene interface are shown in Fig. 3 (a) without CTAB (cetyltrimethy-lammonium bromide) molecules, and (b) 10 s after the injection of CTAB solution to the water phase [5]. The peak appearing around 10-13 kHz represents the beat frequency, i.e., the capillary wave frequency. The peak of the capillary wave frequency shifts from 12.5 to 10.0kHz on the injection of CTAB solution. This is due to the decrease in interfacial tension caused by the increased number density of surfactant molecules at the interface. Time courses of capillary wave frequency after the injection of different CTAB concentrations into the aqueous phase are reproduced in Fig. 4. An anomalous temporary decrease in capillary wave frequency is observed when the CTAB solution beyond the CMC (critical micelle concentration) was injected. The capillary wave frequency decreases rapidly on injection, and after attaining its minimum value, it increases... 

In order to illustrate the potential applications of rheo-NMR five examples have been chosen. The first example deals with wormlike micelles [22] in which NMR velocim-etry is used to profile anomalous deformational flow and deuterium NMR spectroscopy is used to determine micellar ordering in the flow. The second example concerns flow in a soft glassy material comprising a solution of intermittently jammed star polymers [23], a system in which flow fluctuations are apparent. The third... 

It is more difficult to interpret micellar effects upon reactions of azide ion. The behavior is normal , in the sense that k /kw 1, for deacylation, an Sn2 reaction, and addition to a carbocation (Table 4) (Cuenca, 1985). But the micellar reaction is much faster for nucleophilic aromatic substitution. Values of k /kw depend upon the substrate and are slightly larger when both N 3 and an inert counterion are present, but the trends are the same. We have no explanation for these results, although there seems to be a relation between the anomalous behavior of the azide ion in micellar reactions of aromatic substrates and its nucleophilicity in water and similar polar, hydroxylic solvents. Azide is a very powerful nucleophile towards carboca-tions, based on Ritchie s N+ scale, but in water it is much less reactive towards 2,4-dinitrohalobenzenes than predicted, whereas the reactivity of other nucleophiles fits the N+ scale (Ritchie and Sawada, 1977). Therefore the large values of k /kw may reflect the fact that azide ion is unusually unreactive in aromatic nucleophilic substitution in water, rather than that it is abnormally reactive in micelles. 

McBain pointed out that this seemingly anomalous behaviour could be explained in terms of organised aggregates, or micelles, of the surfactant ions in which the lipophilic hydrocarbon chains are orientated towards the interior of the micelle, leaving the hydrophilic groups in contact with the aqueous medium. The concentration above which micelle formation becomes appreciable is termed the critical micelle concentration (c.m.c.). 

Pal, S., Balasubramanian, S., and Bagchi, B. Anomalous dielectric relaxation of water molecules at the surface of an aqueous micelle, /. Chem. Phys., 120, 1912, 2004. 

Some clear solutions of cellulose, however, are in fact micellar. Clear, apparently purely physical solutions of cellulose are formed in 7V-methyl-morpholine A -oxide (NMNO) and anhydrous dimethylacetamide-LiCl, but they can give rise to anomalously high DP values, because of association of the glucan chains in so-called fringe micelles , in which a group of chains associate chain-to-chain but the ends are largely random chain (Figure 4.34b and c). 

Amphiphilic molecules, when dissolved in organic solvents, are capable of self-assembly to form reversed micelles. The reversed micelles are structurally the reverse of normal micelles in that they have an external shell made up of the hydrocarbon chains of the amphiphilic molecules and the hydrophilic head-groups localized in the interior of the aggregate. Water molecules are readily solubilized in this polar core, forming a so-called water pool. This means that reversed micelles form microcompartments on a nanometer scale. The reversed micelles can host all kinds of substrate molecules whether hydrophilic, hydrophobic, or amphiphilic due to the dynamic structure of the water pool and the interface formed by the surfactant layer, in contrast with a liposome system. The properties of water molecules localized in the interior of reversed micelles are physicochemically different from those of bulk water, the difference becoming progressively smaller as the water content in the micellar system increases [1,2]. The anomalous water at low JVo =[water]/[surfactant] obviously influences the chemical behavior of host molecules in the water pools. 

The anomalous activity characteristics have been attributed to conformational changes of the solubilized enzyme [49], but more recent spectroscopic studies seem to indicate that this is not the main cause. Solubilization of an enzyme into microemulsion droplets does not normally lead to major conformational alterations, as indicated, e.g., by fluorescence and phosphorescence spectral investigations [28,50]. The situation is complex, however, and it has been shown by circular dichroism (CD) measurements that the influence of the oil/water interface on enzyme conformation may vary even between enzymes belonging to the same class [51]. In the case of human pancreatic lipase, the conformation of the polypeptide chain is hardly altered after the enzyme is transferred from a bulk aqueous solution to the microenvironment of reverse micelles. Conversely, the CD spectra of the lipases from... 

The second reason for the anomalous change of surface tension of a solid polymer containing surfactant lies in the structural and conformational conversions of the polymer itself under the influence of the surfactant. Such factors as the increase of the polymer surface tension when surfactant is added cannot be explained by the surfactant adsorption on the polymer surface only (see Fig. 2.13). Later we will consider this in detail. As was noted above, if the rate of aggregation of the surfactant molecules is higher than or equal to the rate of polymerization, the system surface tension alters during polymerization in the same way as in the coiu-se of the equilibrium process. At a high rate of polymerization, the formation of micelles of the maximum possible size can be hindered by the rapid increase of the system viscosity. In this case, when an IS substance is applied the split into two phases is not observed and the system appears to be more oversaturated by surfactant than in the first case. 

For AB-type polymers, the A-B junction point is taken at the micelle interface (Inoue et al., 910a,b), as before. The ends of the molecules are, admittedly, unrealistically placed for example, for spheres, the free end of the A block is at the center of the micelle and the bound end of the B block is constrained to a position on a sharply defined micelle spherical surface. Corresponding restrictions are placed on cylinder and lamellae formation. These assumptions, of course, lead to anomalous values for calculated densities, being high in the middle and low near the surface of the micelle. 

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