Evaluation of thermodynamic properties for non-crystallizable ionic liquids

• Heat capacity in the temperature range (5–370) K was measured for three imidazolium ionic liquids.
• Tg/Tfus quotient for [Cnmim]An is close to 2/3.
• Heat capacity change at the glass transition for these liquids rise with the alkyl chain length increase.
• Trends in residual entropy and enthalpy for [Cnmim]An are determined.

Heat capacity of three glass-forming imidazolium ionic liquids (ILs) was studied by adiabatic calorimetry in the temperature range (5–370) K. Using the experimental data obtained in this work and those available in literature it was demonstrated that for all the studied glass-forming imidazolium ILs except [C8mim]BF4 the Tg/Tfus quotient is close to 2/3. The anion symmetry and the alkyl chain length were found to be the most important factors affecting heat capacity changes at the glass transition. The values of the residual entropy for the [C4mim]An ILs can be explained by conformational disorder of the ions. However, since no statistically significant dependence of this quantity of the alkyl chain length was observed it is possible that the conformational disorder of the alkyl chains in ILs is lower than in molecular liquids or gases.

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