Micelles vesicle comparison

The book describes the wealth of membrane structures realized by chemists mainly within the last two decades - micelles, vesicles, fibres, and surface monolayers - from the point of view of supramolecular chemistry. It puts new emphasis on "synkinetic" procedures for the preparation of non-covalent assemblies as well as their structural characterization by electron microscopy, solid state NMR spectroscopy, and comparison to X-ray crystal structures. 

FIGURE 7.31. (A) Schematic representation of the bilayer membrane formed by PEE37-b-PEO40 and for comparison a similar representation of a phopholipid bilayer. (B) Dispersion in water of PEE37-b-PEO40 showing next to vesicles, rod-like micelles (grey arrow) and spherical micelles (black arrow). Reproduced with permission of the American Association for the Advancement of Science. 

With the exception of a few examples, bimolecular reactions in micelles are largely controlled by the local concentration (and pH) realized at the micellar pseudophase. The data reported in Table 2 [28, 31] give a comparison of the second-order rate constants measured for a series of functional derivatives in aqueous and micellar pseudophases. The ratios of the two rate constants (taking into account concentration and deprotonation effects in micelles) are all close to unity, confirming the above assertion. Finally, although the quantification of rate accelerations has been done mainly with micellar aggregates, the reactivity in vesicles appears to follow basically the same rules with minor differences due to the different lipophilicity and/or order of the membrane [37]. 

Measurement of the influence of different micellar environments on proton transfer from excited states of 3-hydroxyflavone allows estimates to be made of micelle concentrations from measurement of the tautomer emission yield. Proton transfer reactions of benzimidazole excited singlet states have also been studied in ionic micelles. Magnetic fields are found to affect the behaviour of radicals generated by the photodissociation of benzil in micellar media. The starburst dendrites which are formed by anionic macromolecules in interaction with both anionic and cationic surfactants have been examined by pyrene fluorescence. Benzo[k]fluoranthrene fluorescence has served as a probe of the effects of metal salts on bile salt aggregation. The incorporation and distribution of benzoquinone into liposomes containing amphilic Zn(II) porphyrin has been followed by its effect on the quenching of the excited state °. A comparison of the photochromism of spirobenzpyran derivatives in unilamellar surfactant vesicles and solvent cast surfactant films has also been reported. ... 

From these phase diagrams, one important conclusion must be emphasized A comparison of the water-rich side and the oil-rich side of the phase diagram shows similar properties in their phase behavior and structures. In each side as the alcohol concentration is varied, the system exhibits the sequence micelle (or vesicle)-lamellar-sponge. This reveals that although the experimental situation seems opposite, the physics is the same and can be described with the flexible surface model [19]. Symmetry properties of phase behavior were found also with nonionic surfactants systems [104]. 

All these simulations refer to the Ising model on the simple cubic lattice. Of course, also more comphcated lattices have been studied, and Fig. 2 shows a comparison of a hep simulation with solid helium. Again model and reality agree nicely. The Ising model has also been used to study oil-water systems where amphiphilic molecules may form membranes, micelles, and vesicles [15]. 

This chapter is focused on spherical particles (micelles and vesicles) but should highlight that there has been increased recent interest in the preparation of cylindrical particles. This is due to recent realization of their advantages in comparison to spherical micellar morphologies with preferable properties being elicited in their application as templates for spherical micelles and nanowires as... 

In this chapter, we overviewed several fluorescence techniques suitable for studies of colloidal particles in aqueous solutions and discussed their application in the research of amphiphilic block copolymer micelles. Unlike surfactant micelles or phospholipid vesicles, amphiphilic block copolymer micelles have no sharp interface between the hydrophobic interior of the particles and the bulk solution, which results in greater heterogeneity of localization sites of fluorescent probes in the polymeric micelles and consequently in more difficult interpretation of data in comparison with surfactant micelles and phospholipid vesicles. 

In contrast with phospholipid vesicles, the oleic acid-oleate vesicle system is characterized by a relatively high monomer solubility. The critical sodium oleate concentration for micelle formation (cmc) at about pH 10.5 is in the range 0.7-1.4mM [10] it has been reported that the monomer solubility at pH 7.4 is around 10-20 pM [11]. (For comparison, the monomer concentration of 1,2-dipalmitoyl-5n-glycero-3-phosphocholine (DPPC) is known to be aroimd 10 M [12].) Due to the relatively high monomer concentration, several physicochemical properties of oleic acid-oleate vesicles are different from vesicles made from phospholipids, such as phosphatidylcholines. The kinetics of lipid exchange is expected to be faster in the... 

Jaeger et al. have studied the kinetics of hydrolysis of cationic ketal-based surfactants [41], A comparison was made between acid hydrolysis of surfactants in nonaggregated form and in the form of either micelles or vesicles. (Ketal surfactants with one hydrophobic tail formed micelles and those with two hydrophobic tails formed vesicles.) It was found that both types of aggregation caused about two orders of magnitude reduction of the hydrolysis rate. Aggregation is evidently a way to protect these acid-labile cationic species from acid hydrolysis just as aggregation is a way to speed up alkaline hydrolysis of cationic alkali-labile surfactants, such as esterquats. 

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