engleski

Equilibrium Of Formation Of Ionic Micelles And Counterion Association

Theoretical interpretation in micellar systems is commonly based on either "Mass Action Law" or on the "Pseudo Phase Separation Model".1 The "Mass Action Law" concept considers micelles as molecular entities. The problem related to this model is that the aggregation number n and the extent of counterion association m depend on the composition of the solution. The "Pseudo Phase Separation Model" is suitable for nonionic micelles of high aggregation number. However, in the case of ionic surfactants the composition of the "pseudo phase" is questionable since counterions and surfactant chains in the micellar phase are not in the equivalent proportions. The exact and consistent thermodynamic approach would require the definition of the micellar phase as composed by surfactant chains and the associated counterions as species adsorbed at the surface. When „ Mass Action Law” , i.e. the molecular model, is applied one should introduce activities of all interacting solute species, including micellar molecules, in the expression for the thermodynamic equilibrium constant. Accordingly, one should deal with the activity coefficients. The problem is to properly define standard state of charged micellar molecules. In considering this problem micellar species should be considered as dissolved ions. Their (hypothetical) standard state is, as for all other dissolved species, the solution containing one mole per liter of micellar entities, but at the ideal condition of the zero ionic strength.2 In principle the activity coefficients of micellar ionic species could be evaluated by the Debye-Hückel theory. The experimental approach is not possible, since for ionic micellar species the extrapolation to infinite dilution will be accompanied with the change in their composition (n and m values), and moreover they would simply disappear below the c.m.c. The dependency of the conductivity, and electrode potentials of Ag/AgBr and surfactant sensitive electrode on the CtaBr concentration, was measured at 30.0 &ordm ; ; C. Concentrations of Cta+ and Br ions were calculated which enabled the complete characterization of the CtaBr aqueous system. The thermodynamic micellization equilibrium constant , as well as counterion association thermodynamic equilibrium constant were obtained on the basis of the "Mass Action Law" concept3. The activity coefficients of the charged micellar molecules were taken into account.

micelle; counterion association; equillibrium; surface complexation model

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