Ionic micelles electrical surface potential

The potential x as the difference of electrical potential across the interface between the phase and gas, is not measurable. But its relative changes caused by the change of solution composition can be determined using the proper voltaic cells (see Section IV). The name surface potential is unfortunately also often used for the description the ionic double layer potential (i.e., the ionic part of the Galvani potential) at the interfaces of membranes, microemulsion droplets and micelles, measured usually by the acid-base indicator technique (Section V). 

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. 

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]. 

The latter phenomenon was well described already in the 1970s by Fernandez and Fromherz [39] who studied the pK shift of hydroxycoumarin (fluorescent in the anionic form) and aminocoumarin (fluorescent in the neutral form) amphiphilic derivatives, solubilized in ionic surfactant micelles. The study demonstrated that the attraction of the proton to the negatively charged surface of anionic micelles hinders dissociation of acids (which thus behave as weaker ones as compared to water) and promotes protonation of bases (which thus behave as stronger ones as compared to water). The positively charged surface acts oppositely. The authors showed that the potential of the electric double layer, is related to the shifted p of the indicator as... 

The electrical potential, ij/, at the interface between the micellar core and the surrounding water may be estimated by the Gouy-Chapman theory of the electrical double layer. In the classical theory, a uniform continuous interfacial surface charge is assumed, which is neutralized by a diffuse ionic layer of charges in the aqueous solution. In a detailed model of the Stern layer proposed by Stigter [35-37], this theory is refined to allow for the size and high concentration of the charge carriers at the micelle surface. 

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