Determination of the vaporisation enthalpies for amino acid ionic liquids [Cnmim][Ala](n = 5, 6) and applications of the molar surface Gibbs energy

Abstract

Amino acid ionic liquids (ILs) 1-alkyl-3-methylimidazolium alanine [Cnmim][Ala] (n = 5,6) were prepared and characterized. The enthalpy of vaporisation at the average temperature, Δ1gHmo(Tav), for the two ILs were determined by means of isothermogravimetrical analysis, and in terms of Δ1gCp,mo, the value of Δ1gHmo(Tav) can be adjusted to Δ1gHmo(2 9 8); then, the polarity, δμ, of the two ILs were estimated, in which the order of the δμ is in consistent with our experiences. In terms of molar surface Gibbs energy, gs, the traditional Eötvös equation was improved, the parameters of new Eötvös one have clear physical meanings: Its slope and intercept are the molar surface entropy, s, and the molar surface enthalpy, h, respectively. In addition, combining gs with Lorentz–Lorenz equation, an expression for predicting surface tension, γ, of the ILs was obtained, and the predicted values of γ are in good agreement with the experimental ones.