PERFLUORINATED SURFACTANTS - PROPERTIES AND APPLICATIONS .1. THERMODYNAMIC PROPERTIES

Thermodynamic properties of fluorinated surfactants are briefly reviewed. They have been principally obtained by the dependence of the c.m.c. on temperature and pressure and only a few direct measurements have been done. The differences in thermodynamic parameters of micellization between fluorinated surfactants and their protonated counterparts have been attributed to the more pronounced hydrophobic character of the perfluoroalkane moiety, confirmed by the positive free-energy of transferring the -CF2- group from the singly dispersed to the micellar state, higher than that of the -CH2- group. The behaviour of a mixed fluorocarbon-hydrocarbon surfactant micellar solution has been explained in terms of non-ideality of mixing, determining the coexistence of two different kinds of mixed micelles, one rich in hydrocarbon, the other in fluorocarbon surfactants. The studies of ternary systems, water-surfactant-solubilizate, done by means of different techniques, do not agree, probably because of kinetic reasons; nevertheless they put in evidence that the energy of transfer for a methylene group in a fluorocarbon micelle, due to the non-ideal mixing between hydrocarbon and fluorocarbon, is less negative than in a hydrocarbon micelle. The enthalpic data for sodium perfluorooctanoate aqueous solution, obtained from heats of dilution by means of an LKB flow microcalorimeter are also reported. The enthalpy of micellization, evaluated by using the pseudo-phase transition model, results in 8.70 kJ mol-1, in good agreement with the value obtained by using the one-step mass action model of Woolley and Burchfield. © 1990.