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 thermodynamic properties of aqueous solutions 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 us, are reported here for the first time and compared with those of the corresponding N-alkylpyridinium and N-alkylnicotinamide halides. Enthalpies of dilution, densities and osmotic coefficients have been measured as a function of concentration. The results have been expressed in terms of partial molar quantities. Micellization parameters have been obtained by using the pseudophase transition approach. The differences in thermodynamic parameters of micellization between fluorinated surfactants and their protiated counterparts confirm the more pronounced hydrophobic character of the perfluoroalkane moiety, due to the positive 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 thermodynamic properties and, in particular, the micellization process is determined by a different balance between entropic and enthalpic factors; (iii) a simple scale factor, taking into account the different steric hindrances of fluorine and hydrogen atoms, is not enough to explain all the thermodynamic properties of cationic fluorinated surfactant solutions.