Cation Symmetry effect on the Volatility of Ionic Liquids


This work reports the first data for the vapor pressures at several temperatures of the ionic liquids, [C(N/2)C(N/2)im][NTf2] (N = 4, 6, 8, 10, 12) measured using a Knudsen effusion apparatus combined with a quartz crystal microbalance. The morphology and the thermodynamic parameters of vaporization derived from the vapor pressures, are compared with those for the 1-alkyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide series, [C(N-1)C(1)im][NTf2] (N = 3 - 9, 11, and 13). It was found that the volatility of [C(N/2)C(N/2)im][NTf2] series is significantly higher than the asymmetric cation ILs with the same total number of carbons in the alkyl side chains, [C(N-1)C(1)im][NTf2]. The observed higher volatility is related with the lower enthalpy of vaporization. The symmetric cation, [C(N/2)C(N/2)im][NTf2], presents lower entropies of vaporization compared with the asymmetric [C(N-1)C(1)im][NTf2], indicating an increase of the absolute liquid entropy in the symmetric cation ILs, being a reflection of a change of the ion dynamics in the IL liquid phase. Moreover both the enthalpy and entropy of vaporization of the [C(N/2)C(N/2)im][NTf2] ILs, present a clear odd-even effect with higher enthalpies/entropies of vaporization for the odd number of carbons in each alkyl chain ([C(3)C(3)im][NTf2] and [C(5)C(5)im][NTf2]).






Rocha, MAA; Coutinho, JAP; Santos, LMNBF

nossos autores


Thanks are due to Fundacao para a Ciencia e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to Centro de Investigacao em Quimica, University of Porto. Marisa A. A. Rocha acknowledges the financial support from FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of a Research Grants SFRH/BD/60513/2009. Thanks are also due to FCT for financial support for the project PTDC/CTM/103664/2008.

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