Amphiphilic molecular assemblies

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

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

KEISHIRO SHIRAHAMA is Professor of physical chemistry of St a University, Saga, Japan. He received a doctor of science from Kyushu University. His academic interest is directed to amphiphilic molecular assemblies such as surfactant-polymer complex, mixed micelle, and vesicle as well as random phenomena in physicochemical systems. 

Amphiphiles Molecular Assembly and Applications-, Nagarajan, R., Ed. ACS Symposium Series Washington DC, 2011. 

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). 

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]. 

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... 

Another type of self-assembly mode is based on looser molecular interactions, where one of the main binding forces comes from hydrophobic interactions in aqueous media. Amphiphihc molecules (amphiphiles) that have a hydrophihc part and a hydrophobic part form various assembhes in water and on water. The simplest example of this kind of assembly is a micelle, where amphiphiles seh-assemble in order to expose their hydrophilic part to water and shield the other part from water due to hydrophobic interactions. A similar mechanism also leads to the formation of other assembhes, such as hpid bilayers. These molecules form spherical assembhes and/or two-dimensional membranes that are composed of countless numbers of molecules. These assembhes are usually very flexible. When external signals are applied to them, they respond flexibly while maintaining their fundamental organization and shape. This research held was initiated by the work of Bangham in 1964. It was found that dispersions of hpid molecules extracted from cells in water spontaneously form cell-like assembhes (liposomes). In 1977, Kunitake and Okahata demonstrated the formation of similar assembhes from various arti-flcial amphiphiles. The latter finding showed that natural lipids and artificial amphiphiles are not fundamentally different. 

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. 

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