Micelles general discussion

Let us now discuss the structure of a reverse micelle. As the name suggests it has a structural arrangement exactly opposite to that of a micelle. Reverse micelles generally refer to aggregates of surficants (e.g., dioctyl sulfosuccinate, AOT) formed in a non-polar solvent. In this situation, the polar headgroups of the surficants point inward (core) and the hydrocarbon chains project outward into the non-polar solvent [3]. The solvent one uses for reverse micelle formation is usually liquid hydrocarbons. Recently the formation of reverse micelles in supercritical fluids such as ethane, propane, and carbon dioxide has been observed. 

We have noted that micellization (and surfactant self-assembly in general) is some intermediate between phase separation and simple complex formation and this is illustrated in the ways that micellization has been modelled in thermodynamic analyses. Micelle formation is generally discussed in terms of one of the following models. 

Goodeve, C.F. General discussion on soap micelles. Trans. Faraday Soc. 1935, 31, 197-198. 

The general principles which govern the effects of normal, aqueous, micelles upon reaction rates and equilibria are considered first, and then we discuss some specific reactions and the relation of micellar effects to mechanism. We also briefly consider some non-micellar species generated by amphiphiles which can also mediate reactivity. 

These reactions are slowed by addition of chemically inert organic solvents to water and are generally slower in micelles than in water, although the effect is often small. The micellar inhibition has been discussed in terms of water activity, or more likely polarity, being somewhat lower at the micellar surface than in water (Menger et al., 1981). 

The impact of salt concentration on the formation of micelles has been reported and is in apparent accord with the interfacial tension model discussed in Sect. 4.1, where the CMC is lowered by the addition of simple electrolytes [ 19,65, 280,282]. The existence of a micellar phase in solution is important not only insofar as it describes the behavior of amphipathic organic chemicals in solution, but the existence of a nonpolar pseudophase can enhance the solubility of other hydrophobic chemicals in solution as they partition into the hydrophobic interior of the micelle. A general expression for the solubility enhancement of a solute by surfactants has been given by Kile and Chiou [253] in terms of the concentrations of monomers and micelles and the corresponding solute partition coefficients, giving... 

In the past several years, we have used the SAAP method to prepare different long multiblock copolymers. Generalities about the preparation and characterization of different end-functionalized triblock copolymers are first outlined. Then, the micellization of triblock copolymer as well as coupling efficiency with and without self-assembly method are discussed. 

N h is zero at the start of interval I, since h — Q.N decreases, h increases, and the product N h increases with time during interval I. At the start of interval II, N has reached its steady-state value N. h may or may not reach an absolutely constant value. Behavior D in interval II usually involves a steady-state h value, while behavior E usually involves a slow increase in h with conversion, h will remain approximately constant or increase in interval III although a decrease will occur if the initiation rate decreases sharply on exhaustion of the initiator concentration. Most texts show Eq. 4-5 for the polymerization rate instead of the more general Eq. 4-4. Equation 4-5 applies to intervals II and III where only polymer particles exist (no micelles). It is during intervals II and III that the overwhelming percent of monomer conversion to polymer takes place. In the remainder of Sec. 4-2, the discussions will be concerned only with these intervals. 

The purpose of this article is to review studies carried out on hemes incorporated inside the micellar cavity, and examine the effect of micellar interaction on the electronic and structural properties of the heme. A comparison of these results with those on the metalloproteins is clearly in order to assess their suitability as models. The article begins with a general introduction to micellar properties, the incorporation of hemes in the micellar cavity, and then discusses results on hemes inside the micelles with different oxidation and spin states, and stereochemistry. The experimental techniques used in the studies on these aqueous detergent micelles are mostly NMR and optical spectroscopy. The present article has therefore a strong emphasis on NMR spectroscopy, since this technique has been used very extensively and purposefully for studies on hemes inside micellar cavities. 

The possibility to carry out conformational studies of peptides at low concentrations and in the presence of complex biological systems represents a major advantage of fluorescence spectroscopy over other techniques. Fluorescence quantum yield or lifetime determinations, anisotropy measurements and singlet-singlet resonance energy transfer experiments can be used to study the interaction of peptides with lipid micelles, membranes, proteins, or receptors. These fluorescence techniques can be used to determine binding parameters and to elucidate conformational aspects of the interaction of the peptide with a particular macro-molecular system. The limited scope of this chapter does not permit a comprehensive review of the numerous studies of this kind that have been carried and only a few general aspects are briefly discussed here. Fluorescence studies of peptide interactions with macromolecular systems published prior to 1984 have been reviewed. 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

In general, the mechanism of self-reproduction of micelles and vesicles can be considered an autopoietic mechanism, since growth and eventually division comes from within the structure itself. This point will be considered again in Chapter 8, on autopoiesis, where the mechanism of the self-reproduction process will also be discussed. 

Micelles and vesicles can be formed above a certain concentration. For instance, small micelles are formed above critical micellar concentration, cmc. (The latter abbreviation is often used for critical vesicle concentration, too. However, sometimes a more general term critical aggregate concentration, cac is also applied.) Bilayers of specific amphiphiles with two tails are typical of the central part of cell membranes discussed in some detail in the next chapter. Studying artificial mono- and bilayers (uniform or with built in pores) is indispensable for gaining information about the structure and functioning of cell membranes involving the transport through them. 

In discussing Reaction (F), we remarked that other anions are observed to compete with OH " in the Stern layer. This sort of electrolyte inhibition is widely observed, and the dependence of the inhibition on both the size and charge of the ions generally corresponds to expectations. For example, in the base-catalyzed hydrolysis of carboxylic esters in the cationic micelles, anions inhibit the reaction in the order N03 > Br " > Cl > F. For acid-catalyzed ester... 

An early review of micellization in block copolymers was presented by Tuzar and Kratochvfl (1976), and these authors have recently provided an excellent review of the literature up to 1992 (Tuzar and Kratochvfl 1993). Micellar properties of block copolymers were also reviewed by Price (1982). A discussion of micellization was included in the general reviews on block copolymers by Riess et al. (1985) and Brown et al. (1989). Excellent reviews focussed on the solution properties of a particular class of copolymer, i.e. copolymers of polyoxyethylene) with poly(oxypropylene) have been presented by Alexandridis and Hatton (1995) and by Chu (1995) and Chu and Zhou (1996). Micellization and micellar association in related poly(oxyethylene)/poly(oxybutylene) copolymers have been summarized by Booth et al (1997). 

The widespread interest in transport across membranes of living cells has led to studies of diffusion in lyotropic liquid crystals. Biological membranes are generally thought to contain single bimolecular leaflets of phospholipid material, leaflets which are like the large, flat micelles of lamellar liquid crystals. No effort is made here to review the literature on transport either across actual cell membranes or across single bimolecular leaflets (black lipid films) which have often been used recently as model systems for membrane studies. Instead, experiments where lamellar liquid crystals have been used as model systems are discussed. 

Florence (1983) provide a comprehensive reference for the use of surfactants in drug formulation development. The treatment by Florence (1981) of drug solubilization in surfactant systems is more focused on the question at hand and provides a clear description of surfactant behavior and solubilization in conventional hydrocarbon-based surfactants, especially nonionic surfactants. This chapter will discuss the conventional surfactant micelles in general as well as update the reader on recent practical/commercial solubilization applications utilizing surfactants. Other uses of surfactants as wetting agents, emulsiLers, and surface modiLers, and for other pharmaceutical applications are nc emphasized. Readers can refer to other chapters in this book for details on these uses of surfactant Polymeric surfactant micelles will be discussed in Chapter 13, Micellization and Drug Solubility Enhancement Part II Polymeric Micelles. 

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