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-pentadecylp yridinium bromides and tetradecyl- and hexadecyltrimethyl-ammonium bromides in salt-free aqueous solutions at 25 degrees C. For this purpose a model e quation for the calorimetric titration curve based on the mass action model of the micellization process was derived. As adjustable parameters it cont ains the aggregation number n, the degree of micelle ionization p/n, and th e enthalpy of micellization Delta H-M; Critical micelle concentrations cme were determined by applying the Philips criterion to the experimental titra tion curves and so obtained cmc values are in excellent agreement with the literature data. Fitting of the model titration curves to those obtained ex perimentally shows that, this procedure gives reliable values only for Delt a H-M while it fails to provide any useful information on the p/n data. Tn addition, for surfactants with short alkyl chains and thus with low aggrega tion numbers (n < approximate to 50) this fitting method provides also aggr egation numbers that are close to those reported in The literature.