Vesicular aggregates

VESICULAR AGGREGATES AS NOVEL MEDIA FOR THE ISOLATION OF POLAR AND NON-POLAR ORGANIC COMPOUNDS PRIOR TO LIQUID CHROMATOGRAPHY... 

In many cases, under changing experimental conditions, water-containing reversed micelles evolve, exhibiting a wide range of shapes such as disks, rods, lamellas, and reverse-vesicular aggregates [15,107,108], Nickel and copper bis(2-ethylhexyl) sulfosucci-nate and sodium bis(2-ethylhexyl) phosphate, for example, form rod-shaped droplets at low water contents that convert to more spherical aggregates as the water content is increased [23,92,109,110],... 

The relation between head-group area and extended chain length and volume of the apolar groups favors formation of vesicles or bilayers from these twin-tailed surfactants. However, it seems that DDDAOH forms vesicles only in dilute solution, because the solution becomes viscous with increasing [surfactant] which is incompatible with the presence of approximately spherical vesicles. It appears that the solution then contains long, non-vesicular aggregates (Ninham et al., 1983). 

Hydrolysis of acetals yields aldehydes, which are intermediates in the biochemical /3-oxidation of hydrocarbon chains. Acid catalyzed hydrolysis of unsubstituted acetals is generally facile and occurs at a reasonable rate at pH 4-5 at room temperature. Electron-withdrawing substituents, such as hydroxyl, ether oxygen, and halogens, reduce the hydrolysis rate, however [50]. Anionic acetal surfactants are more labile than cationic [40], a fact that can be ascribed to the locally high oxonium ion activity around such micelles. The same effect can also be seen for surfactants forming vesicular aggregates. 

Tomotaka and coworkers [202] studied the photodimerization of cinnamic acids incorporated in vesicles (Fig. 39). They mixed equimolarly the cinnamic acid with alkyldimethylamine N-oxide (C DAO) to produce the ion pair 72 (Fig. 39). These ion pairs form stable vesicular aggregates in water. Whereas photoirradiation of the cinnamic acids in methanol resulted in only the cis-trans isomerization to form 71, in these vesicle medium, three dimers, 68-70, were obtained. 

Figure 7 Plot of the change in the product of the coupling and maximum saturation factors as a function of macromolecular structure. At lower pH values, the spin-labelled lipids are present as vesicles and vesicular aggregates, while at higher pH values, micelles are formed. The higher psmax values for the micelles imply greater water accessibility to the radical site. The solid circles represent 16-DS (16-doxyl stearic acid, spin-labelled at the end of the lipid tail) while the open circles represent 5-DS (5-doxyl stearic acid, spin-labelled near the polar head group). Reproduced with permission from Ref. [70].

Fig. 6.6.1. Model for the self-assembly of a poly(butadiene)4o-b-poly(L-glutamic acid)ioo diblock copolymer into vesicular aggregates. (Subscripts indicate the number-average degree of polymerization of the blocks. Adapted from Ref. [11 b]).

Fig. 15 A Chemical structures of Thy- and DAP-functionalized polymers, and schematic illustration of vesicle formation. B Differential interference contrast microscopy (DIG), C AFM, and D fractured TEM images of resultant vesicular aggregates. Reprinted with permission from [88]...

Similar to Zn-SPS, Cs-neutralized poly(styrene-ran-methacrylic acid) (Cs-SMAA) ionomers exhibit Cs-rich vesicular aggregates that are randomly distributed in a... 

Shapes of vesicular aggregates range from tubular to spherical, from more exotic large compound (LCV) and starfish vesicles to simpler extended lamellae. Both unilamellar [75] and multilamellar ( onions ) [47,76] vesicles have been observed. One of the possible morphologies formed in solution are tubular vesicles, also known as tubes (rods) [77,78], Soft, water-filled polymer tubes of nanometer-range diameters and several tens of millimeters in length have been prepared via self-assembly of amphiphilic ABA triblock copolymer in aqueous media (Fig. 4). The tubes were mechanically and chemically stable and could be loaded with water-soluble substances [23],... 

Diazocrown appended cholesterol gelators formed vesicular aggregates that have also been templated [59]. The spherical siHca structures obtained had two distinct diameters, 200 nm (interconnected spheres) and 2500 nm (isolated spheres) which closely resembled the initial gelator assembly. Poly(L-lysine), which also forms spherical aggregates (in the presence of a hydrophobic base additive), can be templated to give, as the end product, hollow siHca spheres of various diameters [60]. As yet no attempts have been made to control the aggregate size. 

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