Aggregation behavior of N-alkyl perfluorooctanesulfonamides in dimethyl sulfoxide solution

N-alkyl perfluorooctanesulfonamides (C8F17SO2NHCnH2n+1, FC8-HCn = 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 (A(min)) at the air-liquid interface were estimated. Gamma(max) decreases and A(min) 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 Delta G(agg)(0), Delta H-agg(0), and Delta S-agg(0) 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 Delta G(agg)(0) and Delta H-agg(0) increase with an increase of the hydrocarbon chain length of FC8-HCn at 298 K. (C) 2009 Elsevier Inc. All rights reserved.