Effects of counterion structure on the surface activities of anionic fluorinated surfactants whose counterions are organic ammonium ions

The surface tension of dialkylammonium perfluorooctanoate [C7F15COONH2(CnH2n+1)2, n = 1–4], dialkylammonium perfluorooctylsulfonate [C8F17SO3NH2(CnH2n+1)2, n = 1–4] and trialkylammonium perfluorooctylsulfonate [C8F17SO3NH(CnH2n+1)3, n = 1–4] in aqueous solution was measured by drop volume method at 25 °C. The critical micelle concentration (cmc), the surface tension at cmc (γcmc), the maximum amount adsorbed (Γmax) and the minimum molecular area (Amin) in the surface adsorption layer were obtained. Because of the limited solubility for C7F15COONH2(C4H9)2, C8F17SO3NH2(C4H9)2 and C8F17SO3NH(C4H9)3 at 25 °C, their cmc and γcmc values could not be successfully obtained. Combining with other series in previous works, an overview picture for the effects of organic ammonium counterions on anionic fluorinated surfactants could be drawn. The rules for the interactions between organic ammonium ions and surfactive anions were summarized (including headgroup type, the chain length of fluorinated hydrophobic tail, and the substituent number, the alkyl chain length and the presence of hydroxyl groups in counterion structure). The mechanism behind these rules was discussed in detail, relying on the electrostatic, hydrophobic and steric factors.