Micelles in aqueous solutions

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Critical micelle concentration (Section 19.5) Concentration above which substances such as salts of fatty acids aggregate to form micelles in aqueous solution. 

In the homologous series of alkane 1-sulfonates, micellization in aqueous solutions begins with the pentanesulfonate at cM = 1 mol/L. The critical micelle concentrations of the technical alkanesulfonates are cM = 0.002 mol/L (sulfo-chlorination route) and cM = 0.44 g/L (sulfoxidation route). 

Hotta, J., Sasaki, K, Masuhara, H. and Morishima, Y. (1998) Laser-controlled assembling of repulsive unimolecular micelles in aqueous solution. J. Phys. Chem. B, 102, 7687-7690. 

Disperse systems can also be classified on the basis of their aggregation behavior as molecular or micellar (association) systems. Molecular dispersions are composed of single macromolecules distributed uniformly within the medium, e.g., protein and polymer solutions. In micellar systems, the units of the dispersed phase consist of several molecules, which arrange themselves to form aggregates, such as surfactant micelles in aqueous solutions. 

The cyclodextrins are stable bodies in aqueous solution, unlike the micelles, which are transitory and are in a state of dynamic equilibrium with the monomer surfactants. However, in many aspects the inclusion of analytes in the cyclodextrin cavity is reminiscent of the solubilization of hydrophobic molecules in micelles in aqueous solution. 

For some applications, it is desirable to lock the micellar structure by cross-Hnking one of the micellar compartments, as discussed previously in Sect. 2.6. Cross-Hnked core-shell-corona micelles have been prepared and investigated by several groups as illustrated by the work of Wooley and Ma [278], who reported the cross-linking of PS-PMA-PAA micelles in aqueous solution by amidation of the PAA shell. Very recently, Wooley et al. prepared toroidal block copolymer micelles from similar PS-PMA-PAA copolymers dissolved in a mixture of water, THF, and 2,2-(ethylenedioxy)diethylamine [279]. Under optimized conditions, the toroidal phase was the predominant structure of the amphiphilic triblock copolymer (Fig. 19). The collapse of the negatively charged cylindrical micelles into toroids was found to be driven by the divalent 2,2-(ethylenedioxy)diethylamine cation. 

Torchilin et al. synthesized an iodine-containing amphiphilic block-copolymer consisting of iodine-substituted poly-L-lysine which is able to form micelles in aqueous solution [37]. The two components of the block-copolymer were methoxy-poly(ethylene glycol) propionic acid (MPEG-PA) with a molecular weight of 12 kDa and poly[ ,M-(2,3,5-triiodobenzoyl)]-L-lysine. The particle size of the micelles was approx. 80 nm, and the iodine concentration was 20 mg mL . Biodistribution studies in rats showed significant and prolonged enhancement of the aorta, the liver and spleen. 

The ethylene oxide block is hydrophilic, whereas the propylene oxide block is (relatively) hydrophobic. The copolymer forms micelles in aqueous solutions with the hydrophilic portions pointing outward, interacting with the water, while the hydrophobic portions form the inner core, shielded from the water by the ethylene oxide-derived block. A micelle is also formed in organic liquids, but here the hydrophobic propylene oxide block faces outward, whereas the ethylene oxide bloek acts as the inner eore. 

MacKerell, Jr., A. D. (1995) Molecular dynamics simulation analysis of a sodium dodecyl sulfate micelle in aqueous solution decreased fluidity of the micelle hydrocarbon interior. J. Phys. Chem. 99, 1846-1855. 

Exploiting ATRP as an enabling technology, we have recently synthesised a wide range of new, controlled-structure copolymers. These include (1) branched analogues of Pluronic non-ionic surfactants (2) schizophrenic polymeric surfactants which can form two types of micelles in aqueous solution (3) novel sulfate-based copolymers for use as crystal habit modifiers (4) zwitterionic diblock copolymers, which may prove to be interesting pigment dispersants. Each of these systems is discussed in turn below. 

Sha et al. applied the commercially available dual initiator ATRP-4 for the chemoenzymatic synthesis of block copolymers. In a first series of publications, the group reported the successful synthesis of a block copolymer comprising PCL and polystyrene (PS) blocks [31, 32]. This concept was then further applied for the chemoenzymatic synthesis of amphiphilic block copolymers by macroinitiation of glycidyl methacrylate (GMA) from the ATRP functional PCL [33]. This procedure yielded well-defined block copolymers, which formed micelles in aqueous solution. Sha et al. were also the first to apply the dual enzyme/ATRP initiator concept to an enzymatic polycondensation of 10-hydroxydecanoic acid [34]. This concept was then extended to the ATRP of GMA and the formation of vesicles from the corresponding block copolymer [35]. 

Consider the formation of a mixed micelle in aqueous solution from a binary surfactant solution consisting of a nonionic and an anionic surfactant. The process is depicted as the aggregation of ng molecules of nonionic surfactant B, of n molecules of anionic surfactant A", and in addition there will be counterions, C" ", of the anionic surfactant in the amount of an where a is the fraction of the counterions associated or bound with the surfactant anions in the micelle. The process as depicted is... 

Thermodynamic Properties of Surfactant Micellization in Aqueous Solutions at 300°K... 

Our data, to date, show that molecular interaction between two surfactants, both in mixed monolayers at the aqueous solution/air interface and in mixed micelles in aqueous solution, increases in the order POE nonionic-POE-nonionic < POE nonionic-betaine < betaine-cationic < POE nonionic-ionic (cationic, anionic) betaine-anionic cationic-anionic. The greatest probability of synergism exists, therefore, in cationic-anionic mixtures, followed by betaine-anionic mixtures. Synergism can exist in POE nonionic-ionic mixtures only if the surfactants involved have the proper structures. 

Much recent work on micellization in block copolymers has been focussed on this industrially important type of polymer. We therefore describe experiments on micellization in aqueous solutions of poly(oxyalkylene) diblocks and triblocks in some detail. This serves to illustrate many of the important features of micellization of block copolymers, also observed in other systems such as the styrenic block copolymers covered in the following section. 

The PEO-rich PSt-h-PEO block copolymers form spherical micelles in aqueous solutions [63]. The DLS measurements indicate the presence of a bimodal size distribution - two very narrowly distributed species. The smaller more mobile species had Rh corresponding to the star model of block copolymer micelles. However, 99% or more of the block copolymer is present as simple micelles. 

The other surfactants include sodium dodecyl (lauryl) sulfate, the polysorbates, the laureths, Brijs and benzalkonium chloride (Figure 10.5). These are predominantly water soluble and can form associations (micelles) in aqueous solution. 

Figure 3.20 Illustration ofthe structure of a micelle in aqueous solution, showing three arrangements tails overlapping at the centre (a), water penetrating the core (b), and chain protrusion and bendingto correct the deficiencies ofthe first two arrangements (c). From Hiemenz and Rajagopalan [13]. Copyright 1997, Dekker.

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