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 .2. APPARENT AND PARTIAL MOLAR ENTHALPIES AND OSMOTIC COEFFICIENTSAT 313 K

We report a thermodynamic study of the aqueous solutions of 1-(3,3,4,4 ,5,5 ,6,6,6-nonafluorohexyl)pyridinium chloride, bromide, and iodide a nd N-octyl pyridinium iodide, Dilution enthalpies and osmotic coeffici ents of the aqueous solutions of these cationic surfactants have been measured at 313 K as a function of the concentration. The experimental data are expressed in terms of apparent and partial molar quantities, The changes in thermodynamic properties upon micellization have been obtained from the experimental data by using a pseudo phase transition approach. The cmc at 313 K have been evaluated from the plot of the m illiosmolality, the measured quantity, vs molality. From the compariso n with the trends of the enthalpies at 298 K of the same set of compou nds, the effect of temperature on the energetics of their solutions ca n be derived, The trends of thermodynamic properties vs molality and t he micellization parameters confirm that the effect of the counterions , however strong and inversely proportional to the radius of the hydra ted counterion, seems to be reduced with respect to the hydrogenated a nalogs, The curves of the apparent and partial molar enthalpies vs m f or the bromide and the iodide are lowered, with respect to the curve o f the chloride, by an amount comparable to that at 298 K, This observa tion suggests that the changes in the absolute trends of the curves an d in the micellization enthalpies are due to the modification of the m ore mobile hydrophobic hydration shell of the perfluoroalkyl chain, wh ereas the hydration sphere of the counterions is practically unaffecte d, The heat capacity data and the comparison with the behaviour of hyd rogenated analogs is in agreement with the above observation, The tren ds of the free energies confirm that the degree of counterion binding, beta, and the aggregation number, n, increase with the increasing of the radius of the hydrated counterion, (C) 1996 Academic Press, Inc.