The effect of n vs. iso isomerization on the thermophysical properties of aromatic and non-aromatic ionic liquids


This work explores the n vs. iso isomerization effects on the physicochemical properties of different families of ionic liquids (ILs) with variable aromaticity and ring size. This study comprises the experimental measurements, in a wide temperature range, of the ILs' thermal behaviour, heat capacities, densities, refractive indices, surface tensions, and viscosities. The results here reported show that the presence of the iso-alkyl group leads to an increase of the temperature of the glass transition, T-g. The isopyrrolidinium (5 atoms ring cation core) and iso-piperidinium (6 atoms ring cation core) ILs present a strong differentiation in the enthalpy and entropy of melting. Non-aromatic ILs have higher molar heat capacities due to the increase of the atomic contribution, whereas it was not found any significant differentiation between then and iso-alkyl isomers. A small increase of the surface tension was observed for the non-aromatic Its, which could be related to their higher cohesive energy of the bulk, while the lower surface entropy observed for the iso isomers indicates a structural resemblance between the IL bulk and surface. The significant differentiation between ILs with a 5 and 6 atoms ring cation in the n-alkyl series (where 5 atoms ring cations have higher surface entropy) is an indication of a more efficient arrangement of the non-polar region at the surface in ILs with smaller cation cores. The ILs constituted by non aromatic piperidinium cation, and iso-alkyl isomers were found to be the most viscous among the studied Its due to their higher energy barriers for shear stress. (C) 2016 Published by Elsevier B.V.




Thermodynamics; Chemistry; Engineering


Rodrigues, ASMC; Almeida, HFD; Freire, MG; Lopes-da-Silva, JA; 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 through the project Pest-C/QUI/UI0081/2013, and CICECO, University of Aveiro, through the project Pest-C/CTM/LA0011/2013, financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement, and COST action CM1206-EXIL - Exchange on Ionic Liquids. The authors also thank FCT for the PhD grants SFRH/BD/81261/2011 from A.S.M.C. Rodrigues and SFRH/BD/88369/2012 from Hugo F. D. Almeida. M.G. Freire acknowledges the European Research Council (ERC) for the Starting Grant ERC-2013-StG-337753.

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