Micellar surface electrical potential

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. 

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... 

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 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 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. 

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 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. 

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