Synthesis and characterization of physicochemical properties of new ether-functionalized amino acid ionic liquids

Abstract

Abstract Since the effect of ether functionalities on physicochemical properties varies strongly, So the experimental study of in properties is still primarily important for ether-functionalized ionic liquids to be properly applied, as well as for providing reference data for more reliable predictions. In this work, two novel ether-functionalized amino acid ionic liquids 1-(2-methoxyethyl)-3-methylimidazolium threonine and 1-(2-ethoxyethyl)-3-methylimidazolium threonine were synthesized and characterized. Physicochemical properties, including density, surface tension, viscosity and refractive index were measured at different temperatures and atmospheric pressure. According to the volumetric properties of ether-functionalized ionic liquids, the contribution of an ether O atom in the alkoxy side chain to molecular volume, standard entropy and lattice energy were estimated to be 0.0102 nm3, 13 J·K−1·mol−1 and 3 kJ·mol−1, respectively. Combined the molar surface Gibbs energy with Lorentz-Lorenz relationship, an equation for predicting surface tensions of ionic liquids was obtained, and the predicted values agree well with the experimental data. Moreover, an equation of the polarity coefficient P was proposed and the polarity of other ionic liquids was estimated. The polarity of the same series of ionic liquids decreases with the length of the alkoxy chain, the hydrophilicity becomes relatively weak, and the hydrophobicity is relatively enhanced. Using Eyring's viscosity equation and thermodynamic formula, an expression for calculating the viscous flow activation energy of ionic liquids was obtained, and the values were in good agreement with the values calculated by the Arrhenius equation.