Properties of Micelles

It has been demonstrated that p increases and then decreases with increasing sodium chloride concentration for sodium nonanoate solutions although the reverse is true of the viscosity. The limiting equivalent conductance A2 permits estimation of the kinetic charge of micelles.  

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Micellization is a second-order or continuous type phase transition. Therefore, one observes continuous changes over the course of micelle fonnation. Many experimental teclmiques are particularly well suited for examining properties of micelles and micellar solutions. Important micellar properties include micelle size and aggregation number, self-diffusion coefficient, molecular packing of surfactant in the micelle, extent of surfactant ionization and counterion binding affinity, micelle collision rates, and many others. 

DR. GUILLERMO FERRAUDI (University of Notre Dame) From your talk it appears that an important aspect of micelles is the modified reactivity imparted to excited states or chemical intermediates. If micelles are to be used to exploit this phenomenon, the structure of the micelle should be carefully defined. Can you tell us something more about the structural properties of micelles For example, if triton X-100 is used to form micelles, a variation in conditions yields micelles with different shapes, different dimensions, etc. 

It appears from a survey of the literature that the essential properties of micelles in nonpolar solvents are understood, namely their stability and variations of size, the dissociation behavior, and their solubilizing capacities. Reverse micelles can dissolve relatively large amounts of water (1-10% w/v depending on emulsion formula) as well as polar solutes and, of course, water-soluble compounds. Consequently, they can be used as media for a number of reactions, including enzyme-catalyzed reactions. Very few attempts to investigate such reverse micelles at subzero temperatures are known, in spite of the fact that hydrocarbon solutions present very low freezing points. 

In the present work, we have synthesized two betaines and three sulfobetaines in very pure form and have determined their surface and thermodynamic properties of micellization and adsorption. From these data on the two classes of zwitterionics, energetics of micellization and adsorption of the hydrophilic head groups have been estimated and compared to those of nonionic surfactants. 

The mechanical properties of Micelle-Templated Silicas (MTS) are very sensitive items for industrial process applications which might submit catalysts or adsorbents to relevant pressure levels, either in the shaping of the solid or in the working conditions of catalysis or separation vessels. First studies about compression of these highly porous materials have shown a very low stability against pressure. These results concern these specific materials tested. In this study, we show very stable MTS with only a loss of 25% of the pore volume at 3 kbar. The effects of several synthesis parameters on the mechanical strength are discussed. 

In addition to the properties of micelles described above, vesicles, which are bilayer structures and can be considered to be model membranes, separate two distinct aqueous phases an entrapped or inner water pool and the bulk aqueous phase. In principle, therefore, electron transfer may be possible across the bilayer and the sites of hydrogen and oxygen production in a water splitting system can be separated spatially. 

Fluorescent probes in reversed micellar aggregates proved, moreover, appropriate to study dynamic properties of micelles in nonpolar solvents42,44 The particularly suitable fluorescent label was the highly water sensitive terbium ion (Tb3+) (Fig. 26). 

The presence of chemical guest species in the water pool of soft-core RMs can modify the organization of the micellar components. The chemical guest species may compete with the surfactant for water molecules to build its own hydration shell. Ions may be specifically bound to the charged groups of the micellar wall resulting in dramatically changed properties of micelles. 

Gohy et al. studied the solution properties of micelles formed by two polystyrene-frZock-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP- -PEO) copolymers in water by dynamic light scattering and transmission electron microscopy [92]. Spherical micelles were observed that consist of a PS core, a P2VP shell and a PEO corona. The characteristic sizes of core, shell and corona were found to depend on the copolymer composition. The micellar size increased at pH<5 due to P2VP block protonation (Fig. 19). 

Typically, in gradient elution liquid chromatography, electrochemical detection has been difficult due to base-line shifts that result as a consequence of the altered mobile phase composition. However, a unique property of micelles allows for much improved compatibility of gradients (i.e. gradient in terms of micellar concentration or variation of small amount of additive such as pentanol) with electrochemical detectors. This has been demonstrated by the separation and electrochemical detection of phenols using micellar gradient LC (488). A surfactant (apparently non-micellar) gradient elution with electrochemical detection has also been successfully applied for the assay of some thyroid hormones by LC (491). 

II. Summary of the Physical and Chemical Properties of Micelles and Micellar Solutions. ... 

The most important property of micelles in aqueous or nonaqueous solvents is their ability to dissolve substances that are insoluble in the pure solvent. In aqueous systems, nonpolar substances are solubilized in the interior of the micelles, whereas polar substances are solubilized in the micellar core in nonaqueous systems. This process is called solubilization. It can be defined as the formation of a thermodynamically stable isotropic solution with reduced activity of the solubilized material (8). It is useful to further differentiate between primary and secondary solubilization. The solubilization of water in tetrachloroethylene containing a surfactant is an example of primary solubilization. Secondary solubilization can be considered as an extension of primary solubilization because it refers to the solution of a substance in the primary solubilizate. 

An extremely useful property of micelles is the ability to obtain high local concentrations of bound organic substrates at low macroscopic concentrations. Therefore, it is expected that there will be an increase in the quantum efficiencies of inter-molecular cycloaddition reactions in micellar solutions over homogeneous solutions. Estimated rate enhancements up to 103 over homogeneous rates are expected so that reactions that are relatively inefficient in homogeneous solutions may become quite efficient in the presence of micelles. In addition, orientational effects of solutes by the micelle may also lead to the observation of unique products. 

J. Aguado, D. P. Serrano, R. van Grieken, J. M. Escola, and E. Garagorri, Catalytic properties of micelle templated microporous and mesoporous materials for the conversion of low-density polyethylene. Stud. Surf Sci. Catal, 135, 3915-3922 (2001). 

We cannot enter here into details of the properties of micelles. The reader is referred for these to recent reviews on this subject. 

One particular property of micelles stands out above all others their ability to solubilize organic compounds in water. Benzene, for example, dissolves in SDS to the extent of 0.90 mol/mol surfactant, resulting in around 40 benzene molecules per micelle . NMR chemical shift data situate most of the benzene at the micelle-water interface, but the localization of small solubilizates in micelles is never uniform. 

Critical micelle concentration and physical properties of micelles 149... 

This chapter will be concerned with the characterization of micellization processes, physical properties of micelles, the structure of self-aggregated... 

Imae, T., Electrostatic and electrokinetic properties of micelles, in Electrical Phenomena at Interfaces Fundamentals, Measurements, and Applications, 2nded., Ohshima, H. and Furusawa, K., Eds., Marcel Dekker, New York, 1998, chap. 28. 

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