Solid-liquid equilibrium and heat capacity trend in the alkylimidazolium PF6 series

Abstract

The heat capacity and thermal behavior trend along the 1-alkyl-3-methylimidazolium hexafluorophosphate [CnC1im][PF6] (with n=2–10, 12) ionic liquids series, is used to explore the effect of the alkyl chain length in the nanostructuration. The heat capacities of the studied ILs were measured with an uncertainty better than ±0.15% and are in excellent agreement with the available data in the literature. An odd-even effect for the specific and volumic heat capacities of the [CnC1im][PF6] series was found. The observed odd-even effect in the liquid heat capacity was rationalized considering the preferential orientation of the terminal CH3 group. The higher specific/volumic heat capacities shown for the [C6C1im][PF6] and [C8C1im][PF6] are an indication of an additional conformational disorder increase in the liquid phase that could be related with a weaker alkyl chain interdigitation capability of the even number chain ILs. The melting temperatures and consequent ∆slHmo and ∆slSmo trend along the alkyl series present a V-shape profile that is explained based on the analysis of the balance between the initial decrease of the electrostatic interaction potential and the increase of the van der Waals interactions with the increasing size of the alkyl side chain of the cation. The inhibition of crystallization for intermediate alkyl chain size (from [C5C1im][PF6] to [C8C1im][PF6]) seems to arise from the overlapping of the hypothetical cold crystallization temperature by the melting temperature. Above the critical alkyl size, CAS, a regular increase in the entropy and enthalpy profiles presents a similar shape than the observed in other alkane series and is a strong support of the intensification of the ILs nanostructuration.