Physicochemical properties of N-alkylpyridine trifluoroacetate ionic liquids [CnPy][TFA] (n = 2–6)

Abstract

Series of N-alkylpyridine trifluoroacetate ionic liquids [CnPy][TFA] (n = 2–6) were designed and synthesized by the neutralization method and characterized by 1H NMR and DSC. The density, surface tension, and refractive index of [CnPy][TFA] (n = 2–6) with different mass fraction of water were measured by standard addition method (SAM) at (293.15–343.15) K and the respective properties for the anhydrous [CnPy][TFA] (n = 2–6) were calculated on the basis experimental data. The molecular volume Vm, the standard entropy S0(298), the crystal energy UPOT, the surface entropy Sa, the surface energy Ea of ionic liquid [CnPy][TFA] (n = 2–6) were calculated. The enthalpy of vaporization at 298.15 K were estimated according to the Kabo’s and Rebelo’s empirical equation, and the traditional Eötvös equations were modified based on the calculated molar surface Gibbs energy, and its coefficients have clear physical meaning. The thermal expansion was estimated by the classical interstitial theory. At the same time, the classical interstitial theory has been improved and the molar surface Gibbs energy of interstice was proposed. In addition, surface tension could be estimated by the molar surface Gibbs energy, and the estimated values were highly correlated with the experimental ones. Finally, the concept of solubility parameter was introduced, which can be used to estimate the polarity of ionic liquids and it was consistent with our experience.