The surface activity and micellization of sodium perfluorooctanoate, ammounium perfluoro- octanoate and tetraalkylammonium perfluorooctanoate [C₇F₁₅COON(C_nH_2n+1)₄, n＝1, 2, 3, 4] have been studied by surface tension and fluorescence methods. The effects of counterions, especially counterion size, on those fluorinated surfactants have been discussed. It is shown that, different from the nature of common hydrocarbon surfactants, the counterions has a striking effect on these fluorinated surfactants. The cmc of the fluorinated surfactants decreased with the increase of the counterion size, while the surface area per surfactant molecule in the air-water interface increased generally with the counterion size. However, the surface tension is more complicated. The surface tension at cmc first increases from C₇F₁₅COONH₄ to C₇F₁₅COON(C₂H₅)₄ and then decreases with the increase of counterion size. It was considered that the steric hindrance, hydrophobicity, electric screen, and penetration of the counterions may be responsible for the results.

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