Aggregation behavior of N-alkyl perfluorooctanesulfonamides in dimethyl sulfoxide solution

Abstract

N-alkyl perfluorooctanesulfonamides (C 8F 17SO 2NHC n H 2 n +1, FC 8 HC n , n = 2, 4, 6, 8) were shown to form aggregates in dimethyl sulfoxide (DMSO). Surface tension results revealed that the dissolution of FC 8 HC n reduced the surface tension of DMSO in a manner analogous to common surfactants in aqueous solutions. Maximum surface excess amount ( Γ max) and minimum surface area per molecule ( A min) at the air–liquid interface were estimated. Γ max decreases and A min increases with an increase of the hydrocarbon chain length of FC 8 HC n . Steady-state fluorescence and NMR measurements demonstrated that both fluorocarbon and hydrocarbon chains of FC 8 HC n molecules were incorporated inside the aggregates. UV–vis spectroscopy confirmed the formation of aggregates and determined the critical micelle concentration (cmc) of FC 8 HC 6 and FC 8 HC 8 solutions. The thermodynamic parameters Δ G agg 0 , Δ H agg 0 , and Δ S agg 0 for the aggregate formation of FC 8 HC n 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 Δ G agg 0 and Δ H agg 0 increase with an increase of the hydrocarbon chain length of FC 8 HC n at 298 K.