Thermodynamic properties of aqueous micellar solutions of N-(1H,1H,2H,2H perfluorooctyl)pyridinium chloride and N-(1H,1H,2H,2H perfluorodecyl)pyridinium chloride

The thermodn. properties of aq. solns. of the cationic surfactants N-(1H,1H,2H,2H perfluorooctyl) pyridinium chloride (OctylPFPC) and N-(1H,1H,2H,2H perfluorodecyl) pyridinium chlorides (DecylPFPC), recently synthesized by the authors, are reported here for the 1st time and compared with those of the corresponding N-alkylpyridinium and N-alkyl nicotinamide halides. Enthalpies of diln., densities and osmotic coeffs. were measured as a function of concn. The results were expressed in terms of partial molar quantities. Micellization parameters were obtained by using the pseudophase transition approach. The differences in thermodn. parameters of micellization between fluorinated surfactants and their protiated counterparts confirm the more pronounced hydrophobic character of the perfluoroalkane moiety, due to the pos. free energy of transferring the -CF2- group from the singly dispersed to the micellar state, which is higher than that of the -CH2- group. The results reported here lead to the following suggestions: (i) the additivity rule can be applied also to a homologous series of partially fluorinated cationic surfactants; (ii) the factor 1.5 between -CH2- and -CF2- groups obtained from the micellization free energy cannot be transferred to other thermodn. properties and, in particular, the micellization process is detd. by a different balance between entropic and enthalpic factors; (iii) a simple scale factor, taking into account the different steric hindrances of F and H atoms, is not enough to explain all the thermodn. properties of cationic fluorinated surfactant solns.