abstract
This work presents the vapor pressure at several temperatures for the 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide series, [C(N/2)C(N/2)im][NTf2] (N = 14, 16, 18, and 20), measured by a Knudsen effusion method combined with a quartz crystal microbalance. The thermodynamic properties of vaporization of the ionic liquids under study are analysed together with the results obtained previously for the shorter alkyl chain length [C(N/)2C(N/)2im][NTf2] (N = 2, 4, 6, 8, 10, and 12), in order to evaluate the effect of the alkyl side chains of the cation and to get additional insights concerning the nanostructuration of ionic liquids. The symmetry effect is explored, based on the comparison with the asymmetric imidazolium based ionic liquids, [C(N-1)C(1)im][NTf2]. A trend shift on the thermodynamic properties of vaporization along the alkyl side chains of the extended symmetric ionic liquids, around [C(6)C(6)im][NTf2], was detected. An intensification of the odd-even effect was observed starting from [C(6)C(6)im][NTf2], with higher enthalpies and entropies of vaporization for the odd numbered ionic liquids, [C(7)C(7)im][NTf2] and [C(9)C(9)im][NTf2]. Similar, but less pronounced, odd-even effect was found for the symmetric ionic liquids with lower alkyl side chains length, [C(N/2)C(N/2)im][NTf2] (with N = 4, 6, 8, 10, and 12). This effect is related with the predominant orientation of the terminal methyl group of the alkyl chain to the imidazolium ring and their influence in the cation-anion interaction. The same Critical Alkyl length at the hexyl, (C6C1 and C6C6) was found for both asymmetric and symmetric series indicating that the nanostructuration of the ionic liquids is related with alkyl chain length. (C) 2014 AIP Publishing LLC.
keywords
AB-INITIO CALCULATIONS; VAPORIZATION ENTHALPIES; THERMODYNAMIC PROPERTIES; PHYSICOCHEMICAL PROPERTIES; CATION SYMMETRY; GAS-PHASE; LENGTH DEPENDENCE; SELF-AGGREGATION; CONDENSED STATE; VOLATILITY
subject category
Chemistry; Physics
authors
Rocha, MAA; Coutinho, JAP; Santos, LMNBF
our authors
acknowledgements
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 through the project Pest-C/QUI/UI0081/2013, and CICECO, University of Aveiro, through the project Pest-C/CTM/LA0011/2013. M. 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 Ph.D. Research Grant, SFRH/BD/60513/2009.