Ionic micelle structure

The cmc decreases with increasing chain length of the apolar groups, and is higher for ionic than for non-ionic or zwitterionic micelles. For ionic micelles it is reduced by addition of electrolytes, especially those having low charge density counterions (Mukerjee and Mysels, 1970). Added solutes or cosolvents which disrupt the three-dimensional structure of water break up micelles, unless the solute is sufficiently apolar to be micellar bound (Ionescu et al., 1984). 

Numerous books and reviews have been published on this subject (e.g. Fendler and Fendler, 1975 Mittal, 1977). Therefore, the structural characteristics of micelles will be presented only to the extent that is necessary for the subsequent discussions. These surfactants form micelles at concentrations above the cmc (critical micelle concentration). Such micelles have average radii of 12-30 A and contain 20-100 surfactant molecules. The hydrophobic part of the aggregate forms the core of the micelle while the polar head groups are located at the micellar surface. Micelles at concentrations close to their cmc are assumed to possess spherical and ellipsoidal structures (Tanford, 1973, 1978). A schematic representation of a spherical ionic micelle is shown in Fig. 1. 

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. 

Non-ionic surfactants either decrease or have insignificant effects on the rate constants for hydrolysis of carboxylic esters (Lach and Pauh, 1959 Riegelman, 1960 Nogami et al., 1960, 1962 Kakemi et al., 1962 Mitchell, 1963 Behme et al., 1965 Mitchell and Broadhead, 1967 Saheki et al., 1968 Ullmann et al., 1968). The available data do not warrant conclusions on the relationship between substrate or surfactant structure on the magnitude or nature of catalysis by non-ionic micelles, but it should be noted that synthetic and naturally occurring amphiphiles cause very similar retardations of the rate of alkaline hydrolysis of ethyl p-aminobenzoate (Lach and Pauli, 1969). 

Otzen, D.E. Protein unfolding in detergents Effect of micelle structure, ionic strength, pH and temperature. Biophysical Journal 2002, 83 (10), 2219-2230. 

In aqueous media, the surfactant molecules are oriented, in all these structures, with their polar heads predominantly toward the aqueous phase and their hydro-phobic groups away from it. In vesicles, there will also be an aqueous phase in the interior of the structure. In ionic micelles, the aqueous solution-micelle interfacial region contains the ionic head groups, the Stern layer of the electrical double layer with the bound counterions, and water. The remaining counterions are contained in the Gouy-Chapman portion of the double layer that extends further into the aqueous phase. For POE nonionics the structure is essentially the same, except that the outer region contains no counterions, but includes coils of hydrated POE chains. 

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