Micellar electrical potential

In the second group the variation of the CMC of the pure ionic surfactant with the micellar composition was taken into account. However, other effects, such as changes in the micellar electrical potential, are included in the micellar activity coefficients in a manner similar to the first group of theories [45, 46]. 

Theories, considering changes of the micellar electrical potential and degree of counterion binding form a third group [29, 30, 47, 48]. 

The effects of dodecyltrimethylammonium bromide and chloride, tetradecyltrimethylammonium bromide, CTAB, and NaLS on the dissociation constants of 20a and 20c were investigated by Mukerjee and Banerjee (1964), and the differences between the bulk and the micellar surface pK s of the indicators were interpreted in terms of the electrical potential difference and changes in the pX. Thus, the higher pK at the surface of the cationic micelles as compared to that in the bulk solution can be attributed to a lower effective dielectric constant at the micelle surface. 

Electrokinetic chromatography is conducted in capillary tubes and mobility is generated by the application of electrical potential but separation arises as a result of interactions with a micellar stationary phase formed by surfactants added to the mobile phase. Therefore electrokinetic chromatography contains elements of both liquid chromatography and capillary electrophoresis. 

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-... 

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. 

Micellar Effects on Chemical Equflibria.—A few studies have been made of acid-base equilibria in micelles. Hydronium ion activity in anionic micelles has been measured conductimetrically using hydrophilic indicators, it being found that a plot of mn+ versus [H ]-t-[Na ] is linear with a slope of 0.82. The quantity mH+ is defined as the number of micellized hydrogen ions per surfactant head group, namely mH = [H ]tot—[H ]w/ [D]tot c.m.c., where [DJtot is the total catalyst concentration. The use of fluorescent indicators (21a) and (21b) in anionic, neutral, and cationic surfactantspermitted the evaluation of the electrical potential at the micellar surface as a function of added electrolytes. Indicator pK values for mixed micelles and pK values of weak... 

Gel electrophoresis (GE) was developed in the 1940s, while capillary electrophoresis appeared 40 years later. Then chromatography with electric potential-driven liquid flow also developed into micellar electroldnetic chromatography (MEKC) and electrochromatography (EC), both with capillary columns. Electrophoresis, thus, is not a chromatographic technique, since there is no stationary phase, except in MEKC and EC. 

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. 

The fluctuations of the electrical potential between two electrodes placed in two cells filled with a micellar solution and connected by a capillary tube are expected to be affected by the micelle lifetime. Indeed, the fluctuations should differ... 

The amount of i bound to the surface °Uj depends on the amount of micellized surfactant, °n2, on the concentration of i in the bulk b, on the way in which the shape of micellar aggregates develops with the concentration of surfactant, o ih nature of both phases, on their electrical potentials, and on the surface energy. 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

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]. 

One potential application of the work on oriented nematic phases of rodlike molecules is to solutions containing cylindrical micelles. Orientation could be achieved by a shear field or perhaps by an electric field. Gotz and Heckman (9) confirmed the existence of anisotropic electrical conductivity for a concentrated surfactant solution in a shear field. They used their results to show that the solution contained cylindrical rather than platelike micelles. Of course, the magnitude of the electrical conductivity in an aqueous micellar solution should be quite different from that in the nematic phase of an organic material. So the conditions for and types of electrohydrodynamic instabilities could be different as well. 

The energy of the electric double layer is directly dependent on the square of the surface potential (Equation 4) and the observed increase of the potassium oleate alcohol ratio should enhance the stability of the inverse micelle. The stability of the inverse micelle is not the only determining factor. Its solution with a maximal amount of water is in equilibrium with a lamellar liquid crystalline phase (7) and the extent of the solubility region of the inverse micellar structure depends on the stability of the liquid crystalline phase. 

A reduction of the stability of the liquid crystalline phase means a reduced region where it is stable and a corresponding increase of the region for the inverse micellar solution. The present results agree with these predictions, and it is justifiable to relate the changes in stability areas mainly to modifications of the potential distribution within the electric double layers. 

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