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

abstract

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.

keywords

ALKYL SIDE-CHAINS; THERMODYNAMIC PROPERTIES; HEAT-CAPACITY; SURFACE TENSIONS; PHYSICOCHEMICAL PROPERTIES; PHYSICAL-PROPERTIES; 1-BUTYL-1-METHYLPYRROLIDINIUM BIS(TRIFLUOROMETHYLSULFONYL)IMIDE; IMIDAZOLIUM; TEMPERATURE; CATION

subject category

Thermodynamics; Chemistry; Engineering

authors

Rodrigues, ASMC; Almeida, HFD; Freire, MG; Lopes-da-Silva, JA; Coutinho, JAP; Santos, LMNBF

Groups

G4 - Renewable Materials and Circular Economy
G5 - Biomimetic, Biological and Living Materials

Projects

Centre for Research in Ceramics and Composite Materials - CICECO (LA0011: 2013-2014) (PEst-C/CTM/LA0011/2013)

Igy Technology: A Purication Platform using Ionic-Liquid-Based Aqueous Biphasic Systems (IGYPURTECH)

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, 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.