Effect of Counterions on Micellization of Pyrrolidinium Based Silicone Ionic Liquids in Aqueous Solutions

The micellization of two silicone ionic liquids, 1-methyl-1-[tri(trimethylsiloxy)]- silylpropylpyrrolidinium nitrate (Si 4 pyNO 3 ) and 1-methyl-1-[tri(trimethylsiloxy)]- silylpropylpyrrolidinium acetate (Si 4 pyAc), with different counterions was systematically investigated by surface tension and electrical conductivity. Surface tension of water can be reduced almost to 20 mN·m –1 with the addition of the silicone ionic liquids, indicating that the silicone ionic liquids exhibit excellent surface activity. The critical micelle concentration (CMC) values of Si 4 pyNO 3 are less than those of Si 4 pyAc. Electrical conductivity measurements show that the degree of counterion binding (β) for Si 4 pyNO 3 is twice as large as that for Si 4 pyAc. Thermodynamic parameters (Δ H m 0, Δ S m 0, and Δ G m 0 ) of micellization derived from electrical conductivities indicate that the micellization for both Si 4 pyNO 3 and Si 4 pyAc is enthalpy-driven process. The heat capacities, Δ c m, p 0, are negative for the two silicone ionic liquids relating to the removal of water accessible nonpolar surfaces. The addition of sodium halides in the aqueous solution decreases CMC remarkably; however, the surface tension at CMC stays the same with the salt-free system.