N-donor functionalised ionic liquids: their thermo-physical, electrochemical and transport properties

Two functionalised room temperature ionic liquids 1-ethyl-1,4-dimethyl piperazinium bis{(trifluoromethyl)sulfonyl}amide (1) and 1-(2-dimethylaminoethyl)dimethylethylammonium bis{(trifluoromethyl)sulfonyl}amide (2) were synthesised. Properties measured were the electrochemical window, melting point, glass and decomposition temperatures, density, thermal expansivity, viscosity, conductivity and ion self-diffusion coefficients. These were compared with those of 1-butyl-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl}amide (3), as a benchmark salt. A marked difference in thermal stability was observed between the two tertiary amine functionalised salts and the non-functionalised benchmark system, the former being up to 170 K less stable than the latter. The compounds of this study also have narrower electrochemical windows than structural counterparts lacking a tertiary amine function, by about 3 V. The ion self-diffusion coefficients and conductivities are higher for the open chain (2) than for the cyclic (1) at each temperature, and the viscosity less. The transport properties are examined using fractional Stokes – Einstein and Nernst – Einstein relations and the velocity correlation coefficients (VCC) calculated. The fractional Stokes – Einstein exponents range from (0.95 to 0.98). The Nernst – Einstein equation deviation parameters (Δ) are such that the mean of the like-ion VCCs is greater than that for the unlike ions: Δ = (0.36 and 0.42) respectively, values typical for ionic liquids. The VCCs are analysed in terms of the anti-correlated motion of ions, a feature of one-component molten salts, that contrasts with the correlated motions of ions in electrolyte solutions. According to the VCC analysis, the distinct diffusion coefficients follow the order D--