Amphiphiles molecular assembly

In this paper, UV-visible absorption spectra and X-ray diffraction experiments of single crystals and solvent cast films of the azobenzene amphiphiles, CnAzoCmN+Br, were systematically investigated. Structural characterization of the cast bilayer films are discussed in comparison with aqueous solutions and single crystals. Some novel functional properties of the cast films are described, too. We also emphasize that the two-dimensional molecular assemblies, cast films and crystals of bilayer-forming amphiphiles, are suitable candidates for "crystal engineering" because of their simple structures compared with usual three-dimensional molecular crystals. 

The size of molecular assembly of six synthetic dialkyl amphiphiles as determined by a quasi-elastic light scattering is varied in the presence of nonionic MEGA-n surfactants (N-D-gluco-N-methylalkanamide C = 7-9). 

Dialysis causes a deficiency in MEGA surfactant which has patched hydrophobic portion of dialkyl amphiphilic molecular assembly. 

It has been shown,therefore, that the behavior of molecular assemblies can be divided into three regions depending upon the amount of MEGA-n surfactant (1) at higher MEGA-n concentration where dlalkyl amphiphile is... 

Most molecular assemblies are usually formed by amphiphilic molecules (also called surfactant or detergent molecules) consisting of a polar head and of one or more nonpolar tails . Head groups can be cationic, anionic, non-ionic... 

Aqueous molecular assemblies such as micelles and bilayer membranes are formed by the self-assembly of amphiphihc compounds (Figure 11.la, b) [10]. Aqueous micelles have been utihzed for a variety of apphcations in surfactant industry, including emulsification, washing, and extraction processes [11]. BUayer membranes are basic structural components of biomembranes, and their structures are maintained even in dilute aqueous media. This is in contrast to micelles that show dynamic equihbrium between aggregates and monomeric species. Thus bilayers are more stable and sophisticated self-assemblies, and they require suitable molecular design of the constituent amphiphiles. BUayer membranes and vesicles have wide-ranging applications, as exemphfied by drug dehvery [12], sensors [13], and bilayer-templated material synthesis [14]. 

Although the formation of amphiphilic molecular assemblies in water is facilitated by hydrophobic interactions, micelles and bilayers are available even in... 

It should be now be clear that the molecular assembly in ionic liquids is governed by (1) the balance of ionophilicity and ionophobicity of the constituent molecules and (2) the chemical structure of the ionic liquids. It may well be that an increase in intermolecular interactions in the bilayer-forming amphiphiles makes their... 

Micelles - Dynamic Supramolecular Assemblies The micelle is the simplest molecular assembly. It is composed of amphiphilic molecules and has a dynamic nature. 

An electron donor or acceptor site is usually needed in organic synthons for covalent synthesis. The covalent connection of both leads to a new molecule in an essentially irreversible synthetic reaction. Organic synkinons for non-covalent synkinesis usually contain a hydrophilic and a hydrophobic part and/or proton donor or acceptor sites. Non-covalent connection of such amphiphiles leads to a molecular assembly in a reversible synkinetic reaction. Amphiphiles are not only surface active molecules ( surfactant, detergent ), but much more important, they create surfaces. This becomes particularly evident in microemulsions and in suspensions of vesicles and micellar fibres, but is also true in nanoholes and pores, on monolayer surfaces and for many other supramolecular structures. 

Later on we shall portray several examples where very similar amphiphiles, even diastereomers and enantiomers, produce a variety of molecular assemblies under identical conditions. It is not the shape of a molecule that determines the shape of its molecular assemblies, but the degree of binding and repulsion between them. The more binding interactions occurring between molecules, the larger will be the assemblies formed, because monomers are less likely to... 

As a general result, we conclude that micelle and vesicle formation cannot be explained by cone or cylinder shapes of the monomeric amphiphiles. The key criterion for the curvature of molecular assemblies lies in the saturation solubility or cmc of the amphiphile. A cmc above 10 M usually means appreciable dissociation leading to small aggregation numbers of micelles. A cmc below 10 M means large planar bilayers or, upon their disruption, vesicles. 

The above methodology using crystal structures as a basis and CPMAS- C-NMR and infrared spectroscopies as major tools is generally useful for the determination of molecular conformations in molecular assemblies. It is not necessary to use a crystal structure of an amphiphile, which is often difficult to obtain. It is sufficient to start with the crystal structure of the head group component of interest, e.g. of ethylgluconamide or gluconic acid itself. Such simple structures can usually be taken from the literature and the CPMAS- C-NMR spectrum of the same crystals can then be measured and taken as a reference. 

So far, we have described synkinesis only for molecular mono- and bilayer systems. 3D crystals are, in general, considered to constitute stable, chemically dead systems with no potential for the construction of reactive, supramolecular systems. There are, however, exceptions. First of all, the surface of crystals is, of course, as reactive as any other surface. Crystal engineering is considered here as the solid-state branch of synkinesis. Furthermore crystals with large cavities have recently been prepared. They contain inner surfaces which may have interesting receptor properties in co-crystallization and photochemical fixation processes, which constitute another type of planned synkinesis. Furthermore, spontaneous 3D crystallization may compete with the synkinesis of membranes and the molecular conformations and interactions in crystals are important standards for the study of membranes. The study of 3D crystal structures of amphiphiles is therefore mandatory as a basis for all structural work on molecular assemblies. 

The other chapters then lead from the simple to the more complex molecular assemblies. Syntheses of simple synkinons are described at first. Micelles made of 10-100 molecules follow in chapter three. It is attempted to show how structurally ill-defined assemblies can be most useful to isolate single and pairs of molecules and that micelles may produce very dynamic reaction systems. A short introduction to covalent micelles, which actually are out of the scope of this book, as well as the discussion of rigid amphiphiles indicate where molecular assembly chemistry should aim at, namely the synkinesis of solid spherical assemblies. Chapter four dealing with vesicles concentrates on asymmetric monolayer membranes and the perforation of membranes with pores and transport systems. The regioselective dissolution of porphyrins and steroids, and some polymerization and photo reactions within vesicle membranes are also described in order to characterize dynamic assemblies. 

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