Standard states and equilibrium in ionic micellar systems

A theoretical treatment of ionic micelle formation based on the Mass Action Law (i.e. on the Molecular Concept) is presented. In the expression for the thermodynamic equilibrium constant micelles were considered as molecular entities. Therefore, the activities, i.e. the concentrations and activity coefficients, of all the interacting solute species, including the micellar molecules, were introduced. The standard state of the charged micellar molecules was taken to correspond to that of ions dissolved in a hypothetical solution containing one mole per litre of entities at the ideal condition of zero ionic strength. Experimental approach to the ideal state was not feasible, because extrapolation to infinite dilution would have changed the composition of micellar species (aggregation and association numbers) and would even have led to their disappearance below the critical micellization concentration. The application of the Debye–Hückel and Gouy–Chapman theories for evaluation of the activity coefficients of ionic micellar species was discussed. The equilibrium of micelle formation was examined in dodecyltrimethylammoniumbromide, cetyltrimethylammoniumbromide, and sodiumdodecylsulphate solutions.