THERMODYNAMIC PROPERTIES OF AQUEOUS-SOLUTIONS OF PERFLUORINATED IONIC SURFACTANTS

The enthalpies of dissolution of perfluorooctanoic acid, a perfluoropolyether carboxylic acid and their sodium salts were measured as a function of temperature. The heat capacity changes (Delta C-p) were obtained from the slope of enthalpy vs. temperature plot. The values of Delta C-p for perfluorooctanoic acid and its sodium salt are higher than those of the corresponding hydrogenated compounds and are accompanied by lower critical micellization concentrations. The finding is explained on the basis of the higher hydrophobicity of perfluorinated compounds. Perfluoropolyether carboxylic acid and its sodium salt showed relatively lower Delta C-p values which may be due to the branching or the presence of oxygen in the chain. Measurements of the heat capacity change serve also for the explanation of the temperature dependency of the solubility of surfactants.