THERMODYNAMICS OF MICELLE FORMATION OF 1-DODECANESULFONIC ACID

The critical micelle concentrations (cmc's) of 1-dodecanesulfonic acid in aqueous solution were precisely determined over the temperature range 278.2-323.2 K by an electrical conductivity method. The degrees of counterion binding to micelle were obtained over the above temperature range by cmc change with total counterion concentration. Molecular weights of micelles were determined by static light scattering. Activity change of the counterion H+ with total surfactant concentration was estimated by pH values of the solutions. The mass action model was applied to the micelle formation in order to enumerate the three micellization parameters from these values: micellization constant, micelle aggregation number, and the number of counterions per micelle. Thermodynamic parameters (DELTAG-degrees, DELTAH-degrees, DELTAS-degrees) of micelle formation were then calculated from the temperature change of these parameters. At lower temperatures, the micellization was found to be entropy-driven, while at higher temperatures it was enthalpy-driven. The micellization parameters also made it possible to evaluate the concentration change of every chemical species with total surfactant concentration, and the total conductivity was divided into the contribution of each chemical species. The activity coefficient of the counterion started to increase above the cmc due to its excess concentration in the intermicellar bulk phase.