Apparent and partial molar enthalpies at 298K and Gibbs free energies at 313K of aqueous solutions of the cationic surfactants 1-methyl-4-octylpyridinium chloride, bromide and iodide (OMPC, OMPB, OMPI, respectively) have been measured as a function of concentration. The changes in enthalpies upon micellization have been obtained from the experimental data by using a pseudo phase transition approach. The data reported here are a further contribution in defining the effect of the counter ion on the energetics of surfactant solutions and confirm the validity of the group contribution approach also for the counter ion. The effect of the counter ion on the trends of apparent and partial molar enthalpies is strong and inversely proportional to the radius of the hydrated counter ion. The change in structure between the compounds in which the alkyl chain is bound directly to the pyridine nitrogen and those having the tail in position 4, significantly affects the value of the cmc, showing that the latter are more hydrophobic, but scarcely affects the enthalpic properties of the solutions.
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