Volatility study of [C1C1im][NTf2] and [C2C3im][NTf2] ionic liquids

• Vapor pressures of [C1C1im][NTf2] and [C2C3im][NTf2] ionic liquids are reported.
• [C1C1im][NTf2] presents higher enthalpy and entropy of vaporization than expected.
• The high volatility of [C2C3im][NTf2] is a result from its asymmetric character.

Vapor pressures of 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, ([C1C1im][NTf2]) and 1-ethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide, ([C2C3im][NTf2]) ionic liquids were measured as a function of temperature using a Knudsen effusion apparatus combined with a quartz crystal microbalance. Enthalpies and entropies of vaporization were derived from the fitting of vapor pressure and temperature results to the Clarke and Glew equation. [C1C1im][NTf2] presents a higher enthalpy and entropy of vaporization than the neighboring members of the series. The enthalpy of vaporization of [C2C3im][NTf2] lies in between the asymmetric and symmetric ionic liquid series, reflecting a decrease in the electrostatic interactions due to a decrease of the charge accessibility between the ionic pairs when the methyl group is replaced by an ethyl group. The obtained higher volatility of [C2C3im][NTf2] arises from its asymmetric character, leading to an higher entropic contribution that compensates the enthalpic penalty. The border conditions ([C1C1im][NTf2], [C2C1im][NTf2] and [C2C2im][NTf2]), topology ([C2C3im][NTf2]) and symmetry/asymmetry of the ILs effect were evaluated and rationalized based on a comparative analysis of the thermodynamic properties, enthalpies and entropies of vaporization.