Solubility and Thermodynamic Properties of Ammonium-Based Gemini Ionic Liquids in Pure Solvents

Abstract

Asymmetrical ammonium-based gemini ionic liquids, 1-trimethylammonium-3-(1-methylpiperidinium)propane dibis[(trifluoromethyl)sulfonyl]imide ([N111C3MPip][NTf2]2) and 1-trimethylammonium-3-(4-methylmorpholinium)propane dibis[(trifluoromethyl)sulfonyl]imide ([N111C3MMor][NTf2]2), were synthesized and structurally characterized by 1H NMR, 13C NMR, and MS. The melting temperatures and associated enthalpies and entropies of fusion of these gemini ionic liquids have been determined through differential scanning calorimetry (DSC). Thermal stabilities of [N111C3MPip][NTf2]2 and [N111C3MMor][NTf2]2 have been investigated by thermogravimetric analysis technology under pure nitrogen atmosphere. The solubility data of [N111C3MPip][NTf2]2 and [N111C3MMor][NTf2]2 in water, ethanol, benzyl alcohol, 2-phenylethanol and n-heptane were determined by a synthetic method using a laser monitoring observation technique in the temperature range 298.15–333.15 K under atmospheric pressure. The solubility in all pure solvents increased with the increasing temperature and the greatest solubilities of both gemini ionic liquids were observed in benzyl alcohol. The modified Apelblat, Buchowski–Ksiazaczak λh and NRTL models were employed to correlate the experimental solubility data. The calculated results show good agreement with the experimental data and modified Apelblat and λh equations are more accurate than the NRTL model. The standard state enthalpy, entropy, and Gibbs energy of dissolution of the GILs in the studied solvents were evaluated through the van′t Hoff equation using the experimental solubility data. The excess enthalpies of the solutions were also determined by the λh model. The dissolution behavior can serve for the synthesis and purification process of GILs.