Dialkoxy functionalized quaternary ammonium ionic liquids as potential electrolytes and cellulose solvents

Abstract

A series of new ionic liquids, based on dialkoxy-functionalized quaternary ammonium cations {side chains: 1 = CH₃, 1O1 = CH₃OCH₂, 1O2 = CH₃OC₂H₄, 2O2 = C₂H₅OC₂H₄; cations: [N_11,1O1,1O2], [N_11,1O1,2O2], [N_11,1O2,1O2], [N_11,1O2,2O2] and [N_11,2O2,2O2]}, with BF₄⁻, (CF₃SO₂)₂N⁻ (NTf₂) and CH₃CO₂⁻ (OAc) as counteranions, have been prepared and characterized. Their basic properties, such as spectroscopic characteristics, melting point, glass transition temperature, thermal stability, electrochemical window, density, refractive index, viscosity and conductivity, were measured and comparatively studied. The incorporation of two flexible alkoxy chains makes the quaternary ammonium salts highly qualified to be low-viscous and high-conductive room temperature ILs, and even some of them have significantly better fluidity than the popular imidazolium ILs with a similar molecular weight, e.g.[N_11,1O1,2O2]BF₄ (151 cP and 2.11 mS cm⁻¹, M_w: 249) vs.[HMIm]BF₄ (220 cP and 1.2 mS cm⁻¹, M_w: 256) at 25 °C. The electrochemical windows of these ILs were evaluated up to 5.5 V. In addition, the dialkoxy OAc ILs were found to have excellent solvent power for cellulose under mild conditions, e.g. a solution of 18 wt% microcrystalline cellulose in [N_11,2O2,2O2]OAc at 80 °C. By precipitation with water, the dissolved cellulose (I crystal structure) was regenerated as nanosized cellulose II particles with increased surface area and decreased crystallinity, determined by FE-SEM and XRD.