Aggregation behavior of N-alkyl perfluorooctanesulfonamides in dimethyl sulfoxide solution.

N-alkyl perfluorooctanesulfonamides (C8F17SO2NHCnH2n+1, FC8-HCn, n = 2, 4, 6, 8) were shown to form aggregates in dimethyl sulfoxide (DMSO). Surface tension results revealed that the dissolution of FC8-HCn reduced the surface tension of DMSO in a manner analogous to common surfactants in aqueous solutions. Maximum surface excess amount (Gamma(max)) and minimum surface area per molecule (Amin) at the air-liquid interface were estimated. Gamma(max) decreases and Amin increases with an increase of the hydrocarbon chain length of FC8-HCn. Steady-state fluorescence and NMR measurements demonstrated that both fluorocarbon and hydrocarbon chains of FC8-HCn molecules were incorporated inside the aggregates. UV-vis spectroscopy confirmed the formation of aggregates and determined the critical micelle concentration (cmc) of FC8-HC6 and FC8-HC8 solutions. The thermodynamic parameters DeltaG(0)(agg), DeltaH(0)(agg), and DeltaS(0)(agg) for the aggregate formation of FC8-HCn in DMSO derived from the temperature dependence of the cmc revealed that the aggregate formation is an enthalpy-driven process, which was further confirmed by isothermal titration calorimetry (ITC) measurements. Moreover, the absolute values of DeltaG(0)(agg) and DeltaH(0)(agg) increase with an increase of the hydrocarbon chain length of FC8-HCn at 298 K.
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