Aggregation numbers of ionic

Table V. Aggregation Numbers of Ionic Micelles in Water + Acetone Mixtures at 298.15 K°...

The size and shape of micelles are determined by a delicate balance between various factors, such as chemical constitution, electrical repulsion of head groups, amphiphile and solute concentration, and temperature. The addition of electrolytes will in general raise aggregation numbers of ionic micelles and may even induce sphere-rod transitions. Temperature has an enormous influence on aggregation numbers of nonionic micelles, but only a little effect on those of ionic and amphoteric micelles. There is a vast literature covering the subject (24,25,36). 

Detailed examination of m and n values over a wide temperature range have made it clear that both variables change with temperature. These changes should be taken into account when considering changes in thermodynamic variables of micelle formation. The micelle aggregation number of ionic surfactants decreases almost linearly with increasing tem-... 

Fluorescence spectroscopy is also particularly well-suited to clarify many aspects of polymer/surfactant interactions on a molecular scale. The technique provides information on the mean aggregation numbers of the complexes formed and measures of the polarity and internal fluidity of these structures. Such interactions may be monitored by fluorescence not only with extrinsic probes or labelled polymers, but also by using fluorescent surfactants. Schild and Tirrell [154] have reported the use of sodium 2-(V-dodecylamino) naphthalene-6-sulfonate (SDN6S) to study the interactions between ionic surfactants and poly(V-isopropylacrylamide). 

In the case of ionic surfactants, a reduction in aggregation number will result when the counter-ion radius is increased (Kon-no and Kitahara, 1971a). The change in the size of aggregates as a function of the structure of ionic surfactants was studied (Eicke, 1980 Kon-no and Kitahara, 1971a Kon-no et al., 1983 Matijevic, 1993 Ward and C. du Reau, 1993). The aggregation numbers of anionic surfactants (sodium bis(2-ethylhexyl)sulfosuccinate and sodium l,3-(2-... 

The size and shape of micelles have been a subject of several debates. It is now generally accepted that three main shapes of micelles are present, depending on the surfactant structure and the environment in which they are dissolved, e.g., electrolyte concentration and type, pH, and presence of nonelectrolytes. The most common shape of micelles is a sphere with the following properties (i) an association unit with a radius approximately equal to the length of the hydrocarbon chain (for ionic micelles) (ii) an aggregation number of 50-100 surfactant monomers (iii) bound counterions for ionic surfactants (iv) a narrow range of concentrations at which micellization occurs and (v) a liquid interior of the micelle core. 

Figure 7. Variations of aggregation numbers for ionic and non-ionic micelles in aqueous binary mixtures at 298.15 K...

Some essential discoveries concerning the organization of the adsorbed layer derive from the various spectroscopic measurements [38-46]. Here considerable experimental evidence is consistent with the postulate that ionic surfactants form localized aggregates on the solid surface. Microscopic properties like polarity and viscosity as well as aggregation number of such adsorbate microstructures for different regions in the adsorption isotherm of the sodium dedecyl sulfate/water/alumina system were determined by fluorescence decay (FDS) and electron spin resonance (ESR) spectroscopic methods. Two types of molecular probes incorporated in the solid-liquid interface under in situ equilibrium conditions... 

At higher ionic surfactant concentrations. Van der Waals interaction between hydrocarbon chains and hydrophobic bonding results in aggregation, forming clusters called hemi-micelles (12.17-21). The aggregation numbers of hemi-micelles are not well established estimates range from 2 to -250 (15.22.23). Surface charge is reduced more rapidly than at lower solution concentrations and is ultimately reversed as solution concentration and adsorption increase so the adsorption bond includes a chemical or specific contribution. 

A typical spherical micelle in aqueous solution has an average number of surfactant molecules, or aggregation number, of 40-100. The diameter of a spherical or cylindrical micelle is around 4-6 nm for typical surfactants. The shape of the micelle depends on surfactant molecular structure, solution ionic strength, temperature, and presence of organic solutes in solution, among other factors. 

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