Abstract:

Because of the unique acid-base behavior of amino acids, we combined glycine as the anion with an ether group as the cation and prepared a novel ionic liquid (IL), 1-methoxyethyl-3-methylimidazolium glycine [MOEMIM][Gly]. The formation of this IL was confirmed by ¹H-NMR, ¹³C-NMR, differential scanning calorimetry (DSC) and thermogravimetry (TG) analyses. The density, ρ, and surface tension, γ, of the ILs were measured in the temperature range from 298.15 K to 338.15 K at intervals of 5 K using the standard addition method, because the strong hydrogen bonds between the IL [MOEMIM][Gly] and trace amounts of water made it, extremely difficult to remove water by convention methods. The calculated molar volume, V_m, of [MOEMIM][Gly] increased with increasing temperature. The thermal expansion coefficient, α, was also obtained based on the density values. In terms of the molar surface Gibbs free energy, g_s for [MOEMIM][Gly], the traditional Eötvös equation is improved so that it has physical significance, that is, the intercept C₀ represents molar surface enthalpy, h, which is a temperature-independent constant and the slop C₁ = −($\partial $g_s/$\partial $T)_p is molar surface entropy, s, it shows that modified Eötvös equation is not only an empirical equation, but also a strict thermodynamic one. In addition, by a combination of g_s and the modified Eötvös equation, the surface tension values of [MOEMIM][Gly] were estimated and compared with the experimental values; both sets of data were in good accordance with each other.

Key words: 1-Methoxyethyl-3-methy limidazolium glycine, Density, Surface tension, Molar surface Gibbs free energy