The surface tension of the aqueous solutions of ammonium perfluorooctanoate (APFO), trimethylammonium perfluorooctanoate (TMHPFO) and triethylammonium perfluorooctanoate (TEHPFO) with the addition of different concentration of NaCl (0.1, 0.3, 0.5 mol•L^－1) was measured to investigate the surface activity change of fluorinated surfactants with different counterions upon the addition of inorganic salt. The results showed that, NaCl significantly facilitated the micellization of APFO. While for TMHPFO and TEHPFO, slight promotion on their micellization was found when the concentration of NaCl was low, and when the concentration of NaCl was large, the critical micelle concentration (cmc) increased due to the ion exchange between Na^＋and N(CH₃) (or N(CH₂CH₃) ). The minimum surface tensions (γ_cmc) of APFO and TMHPFO were increased with the increase of NaCl concentration. For TEHPFO, small amount of NaCl favored to lower γ_cmc, then γ_cmc of TEHPFO was increased with the increase of NaCl concentration. It showed that, there was always competition between the shielding effect and ion exchange effect in the solution containing salt, and the tendency of ion exchange became large with the increase of NaCl concentration. For the relatively hydrophobic triethylammonium ions in the presence of NaCl, the appearance of the increase of γ_cmc induced by ion exchange needed higher concentration of NaCl. Since it is well known that the addition of inorganic salt is one of the methods to enhance the surface activity of surfactants, this work showed that for certain anionic fluorinated surfactants with organic counterions, the addition of inorganic salt such as NaCl could not obviously enhance and even lower the efficiency of surfactants in the case of large concentration of salt. Therefore, this kind of surfactants should avoid the environment of high salinity in application.

Key words: fluorocarbon surfactant, counterion, salt effect, surface tension, ion exchange