Calorimetric Study of the Micellization of Alkylpyridinium and Alkyltrimethylammonium Bromides in Water

Titration calorimetry was employed to study the thermodynamic parameters of micellization of N -undecyl-, N -tridecyl-, N -tetradecyl-, and N -pentadecylpyridinium bromides and tetradecyl- and hexadecyltrimethylammonium bromides in salt-free aqueous solutions at 25 °C. For this purpose a model equation for the calorimetric titration curve based on the mass action model of the micellization process was derived. As adjustable parameters it contains the aggregation number n, the degree of micelle ionization p / n, and the enthalpy of micellization Δ H M . Critical micelle concentrations cmc were determined by applying the Philips criterion to the experimental titration curves and so obtained cmc values are in excellent agreement with the literature data. Fitting of the model titration curves to those obtained experimentally shows that this procedure gives reliable values only for Δ H M while it fails to provide any useful information on the p / n data. In addition, for surfactants with short alkyl chains and thus with low aggregation numbers ( n < ≈50) this fitting method provides also aggregation numbers that are close to those reported in the literature.