Aggregation behavior of partially fluorinated Gemini surfactants in aqueous solution: Effect of headgroups

The aggregation behavior of partially fluorinated Gemini surfactants (PFGS) with different headgroups, namely, N,N′-bis[dimethyl-(3-perfluorohexyl-2-hydroxypropyl)ammonium] hexane iodide (2C6FC3-Me), N,N′-bis(3-perfluorohexyl-2-hydroxypropyl) hexylenediamine hydrochloride (2C6FC3-Me.HCl) and N,N′-bis(3-perfluorohexyl-2-hydroxypropyl)-N,N′-dipropanesulfonylhexylenediamine (2C6FC3-Sul) in aqueous solution, were investigated through surface tension and electric conductivity measurements. The surface tension was measured as a function of the concentration at 25, 35 and 45 °C, while the electric conductivity was measured at 25, 30, 35, 40 and 45 °C. The effect of NaCl on the surface activity was also investigated. In addition, the maximum surface excess concentration (Γmax) and minimum surface area/molecule (Amin) at the air/water interface were estimated by using the Gibbs adsorption isotherm. It was found that the surface activity of 2C6FC3-Sul was superior to that of 2C6FC3-Me and 2C6FC3-Me.HCl. Moreover, thermodynamic parameters ( Δ G a g g 0 , Δ H a g g 0 , − T Δ S a g g 0 ) indicated that the aggregation formation of 2C6FC3-Me.HCl and 2C6FC3-Sul was a process of entropy driving, while the aggregation formation of 2C6FC3-Me was a process of entropy driving at low temperature and a process of enthalpy driving at high temperature. These results demonstrated that aggregation behavior of partially fluorinated Gemini surfactants were strongly affected by headgroups in aqueous solution.