Novel Electrochemical Studies of Ionic Liquids

Abstract

Room-temperature ionic liquids (ILs) have been proposed as alternative solvents for organic synthesis, separations, and electrochemical applications. Here, we report studies that probe the electrochemical and solvation properties of a tetraalkylammonium (methyltributylammonium bis(trifluoromethylsulfon)imide, M3BNIm) and an imidazolium (1-butyl-3-methylimidazolium hexafluorophosphate, BMIPF 6 ) based ionic liquid. It is demonstrated that despite impurities, the cathodic limit at a Pt electrode is enhanced for the tetraalkylammonium-based IL. Electrogenerated chemiluminescence of tris(2,2‘-bipyrindinyl)ruthenium (Ru(bpy) 3 2+ ) was observed in both ionic liquids, and differences in the response were interpreted in terms of the solvent reactivity and polarity. As ILs have been proposed as alternatives to organic solvents in extraction processes, an understanding of the relative lipophilicity of the IL ions and the equilibrium potential difference established across the IL/water interface is of fundamental relevance. Here, electrochemical measurements at a conventionally polarized liquid−liquid interface (water/1,2-dichloroethane) were used to determine the relative lipophilicity of the IL constituent ions. From formal ion transfer potential values ( ) obtained, the standard ionic partition coefficients could be estimated. The polarizability of the neat ionic liquid/water interface was investigated. From these studies, it can be seen that BMIPF 6 is hydrophilic while M3BNIm is moderately hydrophobic. The significance of the potential difference established across the IL/water interface is discussed.