Thermodynamics of long-chain 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids

Highlights: • Heat capacity and parameters of phase transitions are determined for [C_nmim]NTf₂. • Vapour pressures are measured with a Knudsen apparatus. • Thermodynamic properties are derived for the crystal, liquid, and gas phases. - Abstract: The heat capacities in the temperature range (5 to 370) K and the parameters of solid-phase transitions and fusion were determined for three [C_nmim][NTf₂] (n = 10, 14, 16) ILs. The temperature-dependent vapour pressures of [C₁₄mim][NTf₂] and [C₁₆mim][NTf₂] were measured with a Knudsen effusion apparatus combined with a quartz crystal microbalance. Thermodynamic properties of these compounds in the crystal, liquid, and gas states were derived from the obtained data. The results obtained in this work were combined to those available in the literature for the short-chain homologues to discuss regularities in thermodynamic properties of the series. While some properties demonstrate simple linear dependence on the alkyl chain length, a more complicated behaviour with strong non-linearity or a break at n = 6 is observed for others.