Physicochemical properties of [cnmim][thr] (n = 3, 5, 6) amino acid ionic liquids

Abstract

A series of ionic liquids based on amino acid, threonine, [Cnmim][Thr] (n=3, 5, 6) (1-alkyl-3-methylimidazolium threonine salt), was designed and synthesized. The density, surface tension and refractive index were measured over the range of T=(288.15–328.15)K and the atmospheric pressure. The molar surface Gibbs energy, g, have been calculated. Using the molar surface Gibbs energy, a new Eötvös equation was obtained in which each parameter has a clear physical meaning. In terms of the thermodynamic relationship, the molar surface entropy, s, enthalpy, h, and the ratio of molar surface internal energy to molar internal energy of vaporization, Er were calculated and the molar entropy of vaporization of surface, ΔSvs was estimated. For [Cnmim][Thr] and other ILs, low values of s and high values for ΔSvs, and low values for Erψ were obtained and these data indicate the surface layer of the ILs is almost the same as that of the bulk liquid, except for a few neighbours in the direction of the gas phase. From the refractive index and the molar surface Gibbs energy, an equation to predict the surface tension of ILs was derived and the predicted values were highly correlated with the corresponding experimental ones.