Vesicles dynamic processes

Models with increasing sophistication for the analysis of dynamic processes in supramolecular systems, notably micelles, as well as for the determination of other parameters have been developed over the past two decades. The basic conceptual framework has been described early on [59,60,95,96] and has been classifred into different cases which take into account the extent of quencher mobility and the mechanism of quenching [95]. Two of those cases lead to information about mobility and will be discussed. It is important to emphasize that this analysis is only applicable to self-assembled system such as micelles and vesicles it cannot be applied to host-guest complexes. This model assumes that the probe is exclusively bound to the supramolecular system and that no probe migration occurs during its excited state lifetime. The distribution of probe and quencher has been modeled by different statistical distributions, but in most cases, data are consistent with a Poisson distribution. The Poisson distribution implies that the quencher association/dissociation rate constants to/from the supramolecular system does not depend on how many... 

Figure 1 Top Small unilamellar vesicles (SUVs), large unilamellar vesicles (LUVs), giant unilamellar vesicles (GUVs), and multilamellar vesicles (MLVs). Bottom Dynamic processes in bilayer membranes involve exchange (fcex), lateral diffusion (jfcdiff), and flip-flop (%).

Harvested after controlled swelling, vesicles have a broad spectrum of different sizes and shapes. Obviously, they originate in local microconditions (crystal defects) during swelling. Spherical shapes, however, are most frequently encountered because these shapes have minimal bending energy [25]. The richness of all the shapes clearly indicates that lipid swelling and liposome detachment is a dynamic process. 

The temperatiu-e at which the gel-to-hquid crystalline phase transition of lipid and surfactant bilayers occrus is an important characteristic of these systems. Maybrey and Sturtevant observed an increase in the phase transition temperature when palmitic acid was added to DMPC, but both increases and decreases can be observed with different fatty acids. It has been shown in Section II.A that the lateral and transverse motions of lipids in vesicle bilayers become more rapid as the temperature is increased from below to above Tq. This section reviews dynamic processes that occur in bilayers at around Tq. 

The study of vesicle dynamics is now a mature area of research. The main processes involved in bilayer mobility and solute transport are quite well vmderstood, but this is not so with the spontaneous formation and breakdown of vesicles. There is a significant difference between the d5mamics of micelle forma-tion/breakdown and vesicle formation/breakdown. Micelle phenomena occur on a very short time scale, with processes for most micellar systems taking place in time scales less than 1 sec. For vesicles of synthetic surfactants like the alkylbenzene-sulfonates, the relevant processes are in the second to minute time range, although surfactant monomer exchange between vesicles and aqueous solution may well take place in the mil-... 

The absolute values of the interfacial tensions varied between different amphi-philes and solvents (Table 1). AOT, which is well known in the literature for the formation of microemulsions, showed the lowest surface tension at the interface of both solvents. The other nonionic snrfactants mentioned here. Span 80 and Brij 72 showed shghtly higher valnes. This was also observed for Lecithine, but this lipid precipitated partly during the spinning-drop measurements. Due to this phenomenon, it was not possible to measure accurate data for this emulsifying compound. The interfacial tension had also some influence on the mean size of the emulsion droplets and on the stability of the vesicles (Table 3). In addition to the stationary values of the surface tension, dynamic processes as the surfactant diffusion represented another important factor for the process of stimulated vesicle formation. If an aqueous droplet passed across the fluid interface it carried-over a thin layer of emulsifiers and thereby lowered the local surfactant concentration in the vicinity of the oil-water interface. In the short time span, before the next water droplet approached the interface, the surfactant films should entirely reform and this only occurred, if the surfactant diffusion was fast enough. 

This chapter limits itself to P-NMR studies on phospholipids and lysophospholipids in monomeric and micellar states and focuses on the identification of species and aggregation states, as well as on the dynamic processes of migration and reaction kinetics. For this purpose, micelles and mixed micelles are defined as dilute isotropic solutions of phospholipids, either with or without detergents that form spontaneously and are at thermodynamic equilibrium diis definition specifically excludes sonicated or small unilamellar vesicles as well as membranes. These are covered by Smith and Ekiel (Chapter 15). This limitation in scope necessarily requires heavy reliance on work from the laboratory of the authors of this chapter. 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

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