Effect of the Counterion on Thermodynamic Properties of Aqueous Micellar Solutions of 1-(3,3,4,4,5,5,6,6,6-Nonafluorohexyl)pyridinium Halides. I. Apparent and Partial Molar Enthalpies and Volumes at 298 K

Apparent and partial molar enthalpies and volumes of aqueous solutions of the cationic surfactants 1-(3,3,4,4,5,5,6,6,6-nonafluorohexyl) pyridinium halides and N-octyl pyridinium iodide have been measured at 298 K as a function of concentration. The changes in thermodynamic properties upon micellization have been obtained from the experimental data by using a pseudo phase transition approach. The data here reported allow one to evaluate the effect of the counterion on the energetics of fluorocarbon surfactants solution. This effect, however strong and inversely proportional to the radius of the hydrated counterion, seems to be reduced with respect to the hydrogenated surfactants, at least as far as enthalpic properties of solutions are concerned. In the case of enthalpies, it qualitatively agrees with that found for the protiated counterparts, but it differs quantitatively. In fact, it is not possible to apply directly the group contribution for counterion obtained from the study of protiated analogs. In the case of volume, the substitution of the counterion simply shifts the absolute scale of volume, but does not affect the change in volume upon micellization.