Disk-like micelle

In the lumen of the small intestine, dietary fat does not only meet bile salt but the much more complex bile in which bile salts are about half saturated with lecithin in a mixed micellar system of bile salt-lecithin-cholesterol. On dilution in the intestinal content, the micelles grow in size as the phase limit is approached and large disk-like micelles form which fold into vesicles [49]. These changes are due to the phase transition that occurs when the bile salt concentration is decreased and the solubility limit for lecithin in the mixed micelles is exceeded. The information is mostly derived from in vitro studies with model systems but most probably is applicable to the in vivo situation. What in fact takes place when the bile-derived lamellar bile salt-lecithin-cholesterol system meets the partly digested dietary fat can only be pictured. Most probably it involves an exchange of surface components, a continuous lipolysis at the interphase by pancreatic enzymes and the formation of amphiphilic products which go into different lamellar systems for further uptake by the enterocyte. Due to the relatively low bile salt concentration and the potentially high concentration of product phases in intestinal content early in fat digestion, the micellar and monomeric concentration of bile salt can be expected to be low but to increase towards the end of absorption. 

Consider an ensemble of disk-like micelles composed of a single surfactant species. By analogy with the calculation of cylindrical micelles, find the probability distribution for finding a disk of a given size as a function of the surfactant volume fraction. Contrast the probability distribution for disks with that of cylindrical micelles (where a broad distribution of sizes exists) and comment on the reason for the difference. 

Various studies suggested the existence of stmctures intermediate between micelles and vesicles. These structures—perforated vesicles, bilayer fragments, giant worm-like micelles, ring-like micelles, and disk-like micelles, depending on the investigated system—were... 

More and more disk-like micelles-that could be the promoters of the lamellar liquid-crystal phase-are formed with further addition of the cosurfactant, up to phase separation. In this region, then, the medium mode is the predominant one. 

For the macroscopic orientation of lyotropic elastomers in mechanical fields the same principles as for thermotropic elastomers can be applied. In some ways the situation is even simpler as the rod-like micelles of the hexagonal phase are compatible with an overall prolate network chain conformation. In addition, the lamellar phase built up from disk-like micelles requires an overall oblate chain conformation to form a liquid single-crystal hydrogel (LSCH) [98]. 

Li), the lamellar Ld), and the nematic one (No), which in this case consists of disk-like micelles. One outstanding feature of the system is the extremely wide range of stability of the lamellar as well as of the nematic phase. Moreover, the capability to perform phase transitions lamellar/nematic/ isotropic by temperature variation makes the handling of the samples easy. The short, perfluorinated chains of the surfactant are responsible for the unusual properties of this lyotropic liquid crystal which in some aspects behaves very similar to a thermotropic phase. Both of the briefly introduced achiral lyotropics have been used extensively as host phases for the induction of phase chirality by means of chiral dopants. 

Figure 14.7. Helical arrangement of rod-like and disk-like micelles in the and Np chiral nematic phases.

The stripes arise by the influence of the boundary layer for the Nl type of chiral nematics, due to the fact that disk-like micelles prefer a flat orientation on many, if not all, solid surfaces adsorption layers of the surfactant form on the solid. Initially hydrophobic surfaces will be covered by a monolayer, the hydrophilic ones by a bilayer. Therefore no spontaneous formation of fingerprints occurs with N -type chiral nematics instead, Grandjean textures are observed. The periodicity of the fingerprints equals half of the pitch, if the sample is prepared correctly. Especially the sample thickness must be large compared to the pitch otherwise there will be helix unwinding by the action of the solid boundary. 

Cellulose derivatives have been mixed with achiral nematic host phases made up by anionic or cationic surfactants. Only for the noncharged cellulose and only for disk-like micelles a strong HTP could be observed. The HTP of the polymer is much higher than that of low molecular weight dopants, presumably a kind of necklace of individual micelles forms, see Figure 14.17 [42]. 

Both models are sketched in Figure 14.18. A rod-like micelle can be imagined to have a shape like a corkskrew, for a disk-like micelle it is not as obvious how it can be twisted without disruption. 

Several experimentally observed similarities between thermotropic and lyotropic induced phase chirality suggested a structural relationship. This was the origin of the hypothesis of an intramicellar chirality in disk-like micelles. At first, a disk-like micelle is simplified to a piece of a double-layer of finite... 

Key words Micelle - small-angle scattering - fluorinated surfactant -disk-like micelle... 

This function is plotted for a selected value of (p and a in Fig. 6.20. This type of analysis can also be applied to interpret association into spherical or flat aggregates. The case of spherical micelles is considered (in a simplified form) in Section 4.6.5. The key difference compared with the case of cylindrical aggregates (Fig. 6.20) for spherical and disk-like micelles is that the distribution of micelle sizes does not extend to small aggregates in fact it can be described by a Gaussian distribution centred around a mean aggregation number. This is a consequence of the fact that monomers added above the cmc go to form new micelles. 

Besides the critical aggregation concentration, temperature or pH and specific morphology, the polymeric self-assembled stmctures are characterized by particle molar mass and aggregation number, dimensional parameters such as hydrodynamic radius, radius of the core or membrane thickness for the polymersomes and disk-like micelles, radius of the cylindrical micelles and thickness of the corona, radius of gyration. A distinctive feature of the polymeric self-assembled particles is the core-corona stmcture. Both, the core and corona, can be considered as separate entities which are able to accommodate active substances of appropriate nature and to serve as... 

An extreme case of multicompartment nanostmcture has been recently described by Shi et al. [21]. They have studied the case of a linear triblock copolymer where the two end blocks were poly(N-(2-methacryloyloxyethyl)pyrroIidone) (PNMEP), while the middle macromolecular chain was high density grafted poly(t-butyl acrylate)-b-polystyrene (PBA-b-PS). The aforementioned copolymer was found to form soft disk-like micelles by hexagonaUy packing of the middle block (Fig. 2.6). It has to be noted that the hexagonal pattern of the molecular brushes aligned perpendicularly to the disk plane, while this pattern demonstrates a periodic... 

Fig. 2.6 Multicompartment nanostmcture of soft disk-like micelles with perpendicular areas of hexagonaUy packing blocks. Reproduced from [21] with permission from American Chemical Society...

The amphiphilic copolymers in selective solvents self-assemble into nano-sized structures due to differences in the physicochemical characteristics of the constituent moieties. Spherical and cylindrical core-corona micelles as well as vesicles or polymersomes are the most commonly observed morphologies. Other morphologies such as toroids, bicontinuous, and multicompartment micelles, tubules, disk-like micelles, etc., are considered somewhat exotic. These stmctures are less frequently observed, however, they are not worth less than the commonly observed ones. The constituent moieties can be made sensitive to variations of parameters of the surrounding media such as temperature, pH, ionic strength, presence of specific additives/substances. 

Shi Y, Zhu W, Yao D, Long M, Peng B, Zhang K, Chen Y (2014) Disk-like micelles with a highly ordered pattern from molecular bottlebrushes. ACS Macro Lett 3 70-73... 

Figure 5. Aggregation stnictures of amphiphilic molecules. A spherical micelle, B rod-like micelle, C disk-like micelle, D inverse micelle, E hexagonal I phase of infinite rod-like micelles, F lamellar phase of infinitely stacked bilayer sheets, G hexagonal 11 phase of infinite inverse rod-like micelles (adapted from reference [7]).

---