| Abstract
| - Novel surfactants of perfluorinated double long-chain salts with divalent counterion of separate electric charge,1,1-(1,ω-alkanediyl)bispyridinium diperfluorononanoate (CnBP(FC9)2, n = 2, 4, 6, 8) were newly synthesized. Theirsolution properties were investigated by surface tension measurement over the temperature range from 298.2 to 313.2K, where magnesium diperfluorononanoate (Mg(FC9)2) was employed as a reference surfactant with divalent counterionof concentrated electric charge. From change of surface tension with concentration, the critical micelle concentration(CMC), surface excess (Γ), apparent molecular surface area (A), and −log(concentration to reduce surface tensionof water by 20 mN m-1) (pC20) were determined. The CMC values of CnBP(FC9)2 decreased with increasing chargeseparation and with increasing temperature, where the values of CnBP(FC9)2 were much smaller than those of Mg(FC9)2. In addition, the pC20 values of the former were also much larger than those of the latter. These results indicatea strong influence of the extent of charge separation or the spacer length of the counterions upon surface activity ofthe fluorinated surfactants. The surface excess or the corresponding apparent molecular surface area monotonouslychanged with the spacer length (n ≤ 6), whereas the behavior for n = 8 was much different from the other CnBP(FC9)2due to conformational change in the in-between alkanediyl chain. The entropy changes (Δs) for the surface adsorptionor condensation were found to be mostly negative for CnBP(FC9)2, where the changes approached zero with an increasein the charge separation. On the other hand, the changes for Mg(FC9)2 were positive over the whole concentrationbelow the CMC. In addition, Brewster angle microscopy indicated no condensation of the present surfactants just atthe air/solution interface.
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