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 vols. of aq. solns. of the cationic surfactants 1-(3,3,4,4,5,5,6,6,6-nonafluorohexyl) pyridinium halides and N-octyl pyridinium iodide were measured at 298 K as a function of concn. The changes in thermodn. properties upon micellization were obtained from the exptl. data by using a pseudo-phase transition approach. The exptl. data allow one to evaluate the effect of the counterion on the energetics of fluorocarbon surfactants soln. 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 solns. are concerned. In the case of enthalpies, it qual. agrees with that found for the protonated counterparts, but it differs quant. In fact, it is not possible to apply directly the group contribution for counterion obtained from the study of protonated analogs. In the case of vol., the substitution of the counterion simply shifts the abs. scale of vol., but does not affect the change in vol. upon micellization.