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 surf actants 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 c ompared with those of the corresponding N-alkylpyridinium and N-alkyln icotinamide halides. Enthalpies of dilution, densities and osmotic coe fficients have been measured as a function of concentration. The resul ts have been expressed in terms of partial molar quantities. Micelliza tion parameters have been obtained by using the pseudophase transition approach. The differences in thermodynamic parameters of micellizatio n between fluorinated surfactants and their protiated counterparts con firm the more pronounced hydrophobic character of the perfluoroalkane moiety, due to the positive free energy of transferring the -CF2- grou p from the singly dispersed to the micellar state, which is higher tha n that of the -CH2- group. The results reported here lead to the follo wing suggestions: (i) the additivity rule can be applied also to a hom ologous series of partially fluorinated cationic surfactants; (ii) the factor 1.5 between -CH2- and -CF2- groups obtained from the micelliza tion free energy cannot be transferred to other thermodynamic properti es and, in particular, the micellization process is determined by a di fferent 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 thermody namic properties of cationic fluorinated surfactant solutions.