Ionic micellar surface potential

The counterion binding with ionic micelles is generally described in terms of two alternative approaches the first one is the widely used pseudophase ion-exchange model (Chapter 3, Subsection 3.3.7) and the second one, less commonly used, is to write the counterion binding constant in terms of an ionic micellar surface potential (Q) (Chapter 3, Section 3.4). The value of K in Equation 6.16 is expected to remain independent of [CTACllj as long as the degree of association... 

MEUF is also used to remove multivalent heavy metal ions with ionic surfactants. The ionic micellar surface has a high charge density and a high absolute electrical potential. Therefore, the heavy metal cations electrostatically adsorb onto or near the micellar surface formed by anionic surfactants such as SDS and sodium aUcylbenzene suphonate [55, 57]. Similarly, cationic surfactants, e.g., cetylpyridinium chloride has been shown to be effective in removing multivalent hazardous anions [55]. Non-ionic surfactant micelles are larger and hence more effective [58], as detailed in Chapter 6. 

In Equation 3.63, F is the Faraday constant Zr, an ionic valence state of ion R and RT, the multiple of gas constant and absolute temperature. An excellent agreonent between the calculated values of Kqh from Equation 3.11 as well as Equation 3.62 and Equation 3.63 has been reported, which validates the use of Equation 3.11 for ionic nticeUar-mediated semiionic reactions. Distribution of reactive counterions, discussed in terms of micellar surface potentials, has led to equations similar to those based on the ion-exchange model. 

Studies of the adsorption of surface active electrolytes at the oil-water interface provide a convenient method for testing electrical double layer theory and for determining the state of water and ions in the neighborhood of an interface. The change in the surface amount of the large ions modifies the surface charge density. For instance, the surface ionic area of 100 per ion corresponds to 16, /rC/cm. The measurement of the concentration dependence of the changes of surface potential were also applied to find the critical concentration of formation of the micellar solution [18]. 

The ionic groups on the micellar surface and the counterions will give rise to a nonuniform electrostatic potential according to the Poisson equation. If furthermore the electrostatic effects dominate the counterion distribution the ion concentration is determined by following a Boltzmann distribution. These approximations lead to the Poisson-Boltzmann equation. 

Surfactants play a crucial role in emulsification and emulsion stability. A first step in any quantitative study on emulsions should be to determine the equilibrium and dynamic properties of the oil-water interface, such as interfacial tension, Gibbs elasticity, sinfactant adsorption, counterion binding, siuface electric potential, adsorption relaxation time, etc. Useful theoretical concepts and expressions, which are applicable to ionic, nonionic, and micellar surfac-... 

The electrostatic surface potential at the micellar surface can attract or repel ionic reaction species, and a strong hydrophobic interaction can bring about the incorporation into micelles even of reagents that bear the same charge as ionic micelles. The number of reagent molecules per micelle can often be controlled by adjusting the surfactant concentrations and thus a chemical reaction can be induced to yield specific products by selecting the proper combination of reactants and surfactants. In fact, the research... 

Photochemistry in micellar systems is a type of micellar catalysis in the sense that the photochemical process takes place in the micellar domain. The outstanding recent progress in micellar photochemistry is well laid out in a number of review articles and papers on photochemical and photophysical processes in micellar assemblies. " As described in previous chapters, hydrophobic organic solutes solubilize well in the micellar core, whereas the micellar surface controls the concentration of hydrophilic solutes. The electrostatic potential of up to a few hundred millivolts at the surface of ionic micelles is especially effective in attracting or repelling ionic species. Thus, micelles are microscopically heterogeneous and well suited as surfaces for reactions of appropriate reactants. 

One of the most important characteristics of ionic micelles is their electrostatic potential (up to hundreds of millivolts at the micellar surface) and their resulting ability to select specific counterion species. The potential also depends on the counterion, so the above two quantities are not necessarily independent. They have a crucial effect on electron transfer reactions in micellar systems. 

Special examples of mixture adsorption are competitive adsorption of the different forms of the same substance, such as pH-dependent ionic and undissociated molecular forms, monomers, and associates of the same substance, as well as potential-dependent adsorption of the same compound in two different orientations in the adsorbed layer. Different orientations on the electrode surface—for example, flat and vertical—are characterized with different adsorption constants, lateral interactions, and surface concentrations at saturation. If there are strong attractive interactions between the adsorbed molecules, associates and micellar forms can be formed in the adsorbed layer even when bulk concentrations are below the critical micellar concentration (CMC). These phenomena were observed also at mineral oxide surfaces for isomerically pure anionic surfactants and their mixtures and for mixtures of nonionic and anionic surfactants (Scamehorn et al., 1982a-c). 

The micellar charge was also corroborated by the Guoy—Chapman diffused, double-layer model. At equilibrium, the surface charge of the micelle alters the ionic composition of the interface with respect to the bulk concentration. The difference between the actual proton concentration on the interface and the one measured at the bulk by pH electrode is observed as a pK shift of the indicator and is related with the Gouy-Chapman potential (Goldstein, 1972). 

A detailed physicochemical model of the micelle-monomer equilibria was proposed [136], which is based on a full system of equations that express (1) chemical equilibria between micelles and monomers, (2) mass balances with respect to each component, and (3) the mechanical balance equation by Mitchell and Ninham [137], which states that the electrostatic repulsion between the headgroups of the ionic surfactant is counterbalanced by attractive forces between the surfactant molecules in the micelle. Because of this balance between repulsion and attraction, the equilibrium micelles are in tension free state (relative to the surface of charges), like the phospholipid bilayers [136,138]. The model is applicable to ionic and nonionic surfactants and to their mixtures and agrees very well with the experiment. It predicts various properties of single-component and mixed micellar solutions, such as the compositions of the monomers and the micelles, concentration of counterions, micelle aggregation number, surface electric charge and potential, effect of added salt on the CMC of ionic surfactant solutions, electrolytic conductivity of micellar solutions, etc. [136,139]. 

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