Syntheses and aggregation behavior of pyridine-based CH–CF hybrid surfactants

Abstract

Five cationic CH–CF hybrid surfactants, each of which has a hydrocarbon chain (CnH2n+1, n=8, 10, 12, 14, 16) and a fluorocarbon one (C6F13), have been synthesized. Taking the advantage of the Grignard reaction and the subsequent quaternization, the hybrid surfactants were obtained with high synthetic yields over 65%. Their structures have been unambiguously confirmed by FTIR, 1H NMR, 19F NMR, 13C NMR and MS measurements. 19F NMR revealed that the presence of the chiral carbon center induced a nonequivalent magnetic resonance of the adjacent fluorine nuclei. The aggregation behavior of the hybrid surfactants were investigated by surface tension, conductivity and steady-state fluorescence measurements. Surface tension measurements revealed that the hybrid surfactants exhibited high surface activity. With increasing hydrocarbon chain length, the critical micellar concentration (cmc) decreased while the surface tension at the cmc (γcmc) increased. Both the results of conductivity and surface tension measurements showed there exists a stepwise aggregation behavior for the hybrid surfactants with n=8 and 10. Steady-state fluorescence measurements showed fairly good agreement with the results obtained from conductivity and surface tension measurements. Evaluation of the thermodynamic parameters (ΔGm0, ΔHm0, ΔSm0) showed that the aggregate formation of the hybrid surfactants with n<16 is enthalpy-driven at low temperature, which turns to entropy-driven at high temperature. While the aggregate formation of the hybrid surfactant with n=16 is entropy-driven within all the investigated temperature ranges.