Micelles - Dynamic Supramolecular Assemblies

The simplest kind of supramolecular assembly formed by amphiphiles is the micelle (Fig. 4.19). Amphiphiles that form micelles usually have low hydropho-bicity and are sometimes called surfactants or detergents. Such molecules show relatively high solubility and easily disperse in water up to a certain concentration level, above which they form micelles. This concentration is called the critical micelle concentration (CMC). The micelle structure depicted in 

A similar structure, but with the roles of the hydrophilic part and the hydrophobic part exchanged, can occur in nonpolar media, such as organic solvents. This structure is called a reversed micelle. The hydrophobic part of the amphiphile is exposed to the outer medium, shielding the polar head inside the assembly. Reversed micelles can trap small amounts of water inside them. Water-soluble molecules such as enzymes can therefore be dissolved in the 

4 Molecular Self-Assembly - How to Build the Large Supermolecules 

Because mesoporous silica has an oriented regular pore structure with large surface area, it can be used as a catalyst support. 

Designing the surfactant appropriately can result in the mesoporous silica having an interior surface that is uniquely modified. In the example shown in Fig. 4.21, a surfactant with an aUcoxysilane head is covalently connected to the inner surface of a mesporous sihca through a sol-gel reaction. Selective removal of the alkyl tail by acid hydrolysis leaves open pores densely populated with functional groups. Since the surfactant bites the sihca wall and removes its tail, this method is called lizard templating . 

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

Fig. 4.19 is a cross-section the actual micelle structure is three-dimensional. The assembled structure is completely nonregular rapid exchange between micelle molecules and monomeric soluble molecules occurs. Therefore, a micelle can be regarded as a disordered dynamic supramolecular assembly. In a micellar structure, the hydrophilic part of the component molecule is located on the outer surface of the micelle, in contact with the aqueous phase, which minimizes the unfavorable contact of the hydrophobic part with water. Micelles can trap organic materials hke oils in the inner hydrophobic core, so micelle formation is used in many cleaning agents. 

We will restrict the focus of this chapter, as there are a large variety of supramolecular structures and photophysical probe molecules. Our choice is biased by our interest in systems with biological relevance and to systems which had a fundamental impact on the development of dynamic studies in organized structures. We will deal exclusively with systems in solution, mainly in the aqueous phase. The self-assembled systems being covered are micelles, due to... 

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... 

This chapter covered the use of excited states as probes for dynamic studies involving supramolecular stmctures. It was shown that excited singlet and triplet states provide complementary capabilities for these studies. Micelles are the best characterized structures, and most of the methodology currently in use was developed when studying these supramolecular systems. Other self-assemblies, such as vesicles and bile salt aggregates, have been studied to some extent, but the dynamics of guest complexation has not been fully described and will undoubtedly be explored in the future. Cyclodextrins are widely employed as... 

One particular asset of structured self-assemblies is their ability to create nano- to microsized domains, snch as cavities, that could be exploited for chemical synthesis and catalysis. Many kinds of organized self-assemblies have been proved to act as efficient nanoreactors, and several chapters of this book discnss some of them such as small discrete supramolecular vessels (Chapter Reactivity In Nanoscale Vessels, Supramolecular Reactivity), dendrimers (Chapter Supramolecular Dendrlmer Chemistry, Soft Matter), or protein cages and virus capsids (Chapter Viruses as Self-Assembled Templates, Self-Processes). In this chapter, we focus on larger and softer self-assembled structures such as micelles, vesicles, liquid crystals (LCs), or gels, which are made of surfactants, block copolymers, or amphiphilic peptides. In addition, only the systems that present a high kinetic lability (i.e., dynamic) of their aggregated building blocks are considered more static objects such as most of polymersomes and molecularly imprinted polymers are discussed elsewhere (Chapters Assembly of Block Copolymers and Molecularly Imprinted Polymers, Soft Matter, respectively). Finally, for each of these dynamic systems, we describe their functional properties with respect to their potential for the promotion and catalysis of molecular and biomolecu-lar transformations, polymerization, self-replication, metal colloid formation, and mineralization processes. 

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