Micelle cylindrical structure

Schurtenberger, P, Magid, L. J., King, S. M. and Lindner, P. (1991). Cylindrical structure and flexibility of polymerlike lecithin reverse micelles. J. Phys. Chem., 95,... 

The insets in Figure 18.16 demonstrate that a break (that shows up better in the Cp graph) occurs at approximately 1/m = 5 kg-mol-1 or m = 0.2 mol-kg-1. This break is attributed to a change in the micelle known as the second transition. At this molality, the micelle apparently changes from a spherical to a cylindrical structure. ... 

Bilayers are preferentially formed for Ns = 0.5...1. Lipids that form bilayers cannot pack into micellar or cylindrical structures because of their small head group area and because their alkyl chains are too bulky to fit into a micelle. For bilayer-forming lipids this requires that for the same head group area a a, and chain length Lc, the alkyl chains must have twice the volume. For this reason lipids with two alkyl chains are likely to form bilayers. Examples are double-chained phospholipids such as phophatidyl choline or phophatidyl ethanolamine. Lipids with surfactant parameters slightly below 1 tend to form flexible bilayers or vesicles. Lipids with Ns = 1 form real planar bilayers. At high lipid concentration this leads to a so-called lamellar phase. A lamellar phase consist of stacks of roughly parallel planar bilayers. In some cases more complex, bicontinuous structures are also formed. As indicated by the name, bicontinuous structures consist of two continuous phases. 

The most reasonable explanation for the increase in apparent hydrodynamic diameter measured by DLS is the enhanced micelle-micelle interactions as the boundary of a two-phase system is approached (i.e., the pressure is lowered). Figure 4 illustrates this concept of micelle-micelle interactions, which is manifested as aggregation (or clustering) of the reverse micelle or microemulsion droplets. Since the solvent environment is essentially unchanged by this "macromolecular aggregation" (Ui) we exclude the possibility of (other than transitory) micelle-micelle coalescence to form stable, larger micelles. The micelles may coalesce briefly to form transitional species (which might be a "dumbbell" or more cylindrical structures), in which the water cores collide and intermix. 

Fig. 9. Self-organization structures of block copolymers and surfactants spherical micelles, cylindrical micelles, vesicles, fee- and bcc-packed spheres (FCC, BCC), hexagonaUy packed cylinders (HEX), various minimal surfaces (gyroid, F-surface, P-surface), simple lamellae (LAM), as well as modulated and perforated lamellae (MLAM, PLAM) (with permission from [5])...

P(S-b-BMA) and From dilute 2-propanol solutions, 42 wt% copolymer formed micelles with PS blends with PPE in the core. From THF solutions, ordered cylindrical structures were obtained. 

The lengths of the cylindrical structures can be increased to over 10 pm by controlling the preparation technique. The cylinders show considerable stability in solution and are unchanged in size after heating to 80 °C. However, ultrasonication of samples of the cylindrical micelles leads to a shortening of their length as indicated by lightscattering and TEM studies. 

In the case of ordered mesoporous oxides, the templating relies on supramolecular arrays micellar systems formed by surfactants or block copolymers. Surfactants consist of a hydrophihc part, for example, ionic, nonionic, zwitterionic or polymeric groups, often called the head, and a hydrophobic part, the tail, for example, alkyl or polymeric chains. This amphiphiUc character enables surfactant molecules to associate in supramolecular micellar arrays. Single amphiphile molecules tend to associate into aggregates in aqueous solution due to hydrophobic effects. Above a given critical concentration of amphiphiles, called the critical micelle concentration (CMC), formation of an assembly, such as a spherical micelle, is favored. These micellar nanometric aggregates may be structured with different shapes (spherical or cylindrical micelles, layered structures, etc. Fig. 9.8 Reference 70). The formation of micelles. 

Fig. C4.9 Four different microphase segregated structures which can exist in diblock-copolymer melt. Layers (lamellas), spherical micelles, cylindrical micelles, and bicontinuous phase are shown. The figure is courtesy of P.G. Khalatur.

The molecular arrangement of the bile acid-lipolytic product micelle is unknown but is probably similar to that of the bile acid-lecithin micelle, the structure of which, based on nuclear magnetic resonance studies, is a cylindrical bimolecular leaflet of lecithin molecules coated on the sides by bile acid molecules, their hydrophobic backs apposed to the paraffin chains of the phosphatide (65). All of the molecular species of the micelle are considered to be in rapid exchange with those of other micelles, as well as the concentration of molecularly dispersed lipolytic products and bile acids (at their CMC) in the bulk phase surrounding the micelles. Benzene molecules exchange rapidly between bile acid-monoglyceride micelles, a mean micellar... 

In addition to three canonical morphologies of aggregates (S, C, and L), more complex associations of block copolymer molecules could be found in certain regions of the diagram. In particular, a recent theoretical study [24] predicts the existence of branched cylinders in the vicinity of the S-C binodal line. The latter occupy a narrow corridor and coexist with cylindrical and spherical micelles. Branched structures and networks of aggregates formed by diblock copolymer with quenched PE block were also considered in [17]. 

Above the cmc, spherical or cylindrical structures are present, while a further increase in concentration can result in the self-organization of micelles into periodic hexagonal, cubic, or lamellar mesophases. Depending on their molecular structure, surfactants can also aggregate to develop and produce extended structures in water such as bilayers. 

A - A Truncated cone Cylindrical micelles, columnar structures... 

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