Ionic liquids based on diethylmethyl(2-methoxyethyl)ammonium cations and bis(perfluoroalkanesulfonyl)amide anions: influence of anion structure on liquid properties

Abstract

A series of diethylmethyl(2-methoxyethyl)ammonium (DEME)-based ionic liquids were prepared using bis(perfluoroalkanesulfonyl)amide (C_nF_2n+1SO₂)₂N anions with different perfluoroalkyl chain lengths (n = 0, 1, 2, 3, and 4), and the influence of the structural variation on their thermal, ion-diffusive (ionic conductivity and viscosity), ion-concentration (molar concentration and ion association), and solvatochromic (polarity and hydrogen-bond acceptor ability) properties was investigated. The elongation of the perfluoroalkyl chain causes the pronounced suppression of ionic conductivity, fluidity, and polarity. According to the crystallographic study of the corresponding (C_nF_2n+1SO₂)₂N salts formed with high-symmetrical tetramethylammonium cations, the decreased ion diffusivity must be a consequence of the increased contribution of the interionic van der Waals interactions of F⋯F type and hydrogen-bonding interactions of C–H⋯F type in addition to C–H⋯O type. The Kamlet–Taft π*-scale (polarity) is under the control of the ion concentration, associated with the perfluoroalkyl chain length in the anions. The larger Kamlet–Taft β-scale (hydrogen-bond acceptor ability) of DEME-based ionic liquids with a longer perfluoroalkyl chain appears to be responsible for the larger degree of ion association of oppositely charged ions, which was manifested in the Walden rule deviation.