Diffusion of molecules in ionic liquids/organic solvent mixtures. Example of the reversible reduction of O2 to...

Transport properties of molecules dissolved in room-temperature ionic liquids are highly sensitive to the charge carried by the molecule because of complex ion-ion interactions that could be tuned by addition of a cosolvent. In this connection, the one-electron reduction of oxygen was used as a probe system for studying the effects of the addition of a cosolvent such as dimethylformamide (DMF) into a pure ionic liquid (triethylbutylammonium bis(trifluoromethylsulfonyl)imide) ([Et(3)BuN][NTf(2)]) on the diffusion of charged species versus neutral species. Experimental data about the diffusion coefficients of O(2) (D(O(2))) and O(2)(*-) (D(O)((2)(*-))) and their ratios (gamma = D(O)((2)(*-))/D(O(2))) were extracted using scanning electrochemical microscopy (SECM) in transient mode as a function of the DMF concentration. The ratio gamma and both of the diffusion coefficients D(O)((2))(*-) and D(O(2)) were found to increase exponentially with the DMF volume fractions following the same general tendency described for the viscosity. However, D(O)((2))(*-) varies on a much larger range than D(O)((2)) (around 1000 times more), and O(2)(*-) retains an almost "pure ionic" behavior for higher DMF fractions. All of these results support the occurrence of a sharp transformation in the bonding character of the RTIL cation upon addition of a molecular solvent, as predicted in recent theoretical simulations.