Physicochemical properties and theoretical studies of novel fragile ionic liquids based on N-allyl-N,N-dimethylethylammonium cation

Abstract

A series of novel ionic liquids (ILs) based on N-allyl-N,N-dimethylethylammonium (N112A+) are synthesized and characterized. Their physical properties and electrochemical stabilities are investigated and discussed toward the anion constituents. Herein, four anions are studied including one planar non-fluorinated anion (DCA−) and three fluorinated anions (TFSI−, OTf−, TFA−). The experimental properties are compared to those predicted by density functional theory (DFT) calculations. All salts are found to be low melting compounds while being liquid at room temperature. N112A-TFSI is the most thermally stable (Td = 329 °C) and N112A-TFA is the least stable (Td = 158 °C). The temperature effect on ILs transport properties is determined and discussed using the ionicity and Angell's fragility concepts. Among the good and fragile prepared ILs, N112A –DCA exhibits outstanding transport properties thanks to the low dynamic viscosity (19.97 mPa·s) and high ionic conductivity (19.20 mS·cm−1) reached at 298 K. These suitable properties are explained, from theoretical calculations, by the lower intermolecular interactions of ion-pairs in N112A –DCA and quantified by the decreased value of dispersion ion-pairs interaction energy (−6 kJ·mol−1). The electrochemical stability window (ESW) of selected ILs is strongly affected by the anion structure. The ESW values are found to decrease in the following order N112A-TFSI (4.40 V) > N112A-OTf (3.80 V) > N112A-DCA (3.00 V) > N112A-TFA (2.18 V). Taking into account the most likely oxidation and reduction reactions, the ESWs are discussed and validated by the density-based solvent model (SMD). The experimental values of ESWs are very close to the predicted ones.