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 .1. 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) pyr idinium halides and N-octyl pyridinium iodide have been measured at 29 8 K as a function of concentration. The changes in thermodynamic prope rties upon micellization have been obtained from the experimental data by using a pseudo phase transition approach. The data here reported a llow one to evaluate the effect of the counterion on the energetics of fluorocarbon surfactants solution. This effect, however strong and in versely proportional to the radius of the hydrated counterion, seems t o be reduced with respect to the hydrogenated surfactants, at least as far as enthalpic properties of solutions are concerned. In the case o f enthalpies, it qualitatively agrees with that found for the protiate d counterparts, but it differs quantitatively. In fact, it is not poss ible to apply directly the group contribution for counterion obtained from the study of protiated analogs. In the case of volume, the substi tution of the counterion simply shifts the absolute scale of volume, b ut does not affect the change in volume upon micellization. (C) 1996 A cademic Press, lnc.