Branched and cyclic alkyl groups in imidazolium-based ionic liquids: Molecular organization and physico-chemical properties.

International audienceIn the present work, three imidazolium bis{(trifluoromethyl)sulfonyl}imide ionic liquids with a linear alkyl chain (1-pentyl-3-butylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [C4C5Im][Tf2N]), with branching (1-isopentyl-3-butylimidazolium bis{(trifluoromethyl)sulfonyl}imide [C4iC5Im][Tf2N]), or with a cyclic group (1-cyclopentyl-3-butylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [C4cC5Im][Tf2N]) were synthesized for the first time. Such variation in structure could be of interest to possible applications of the present ionic liquids as task-specific ionic liquids. To understand the influence of the structure on their macroscopic properties, their density and two transport properties (viscosity and electrical conductivity), were measured as function of temperature. The introduction of branching has no effect on the density values whereas these show an increase when a cycle is substituted to the cation. Both branching of the alkyl chain and the presence of the cyclic group increase the viscosity. Surprisingly, the electrical conductivity of [C4cC5Im][Tf2N] is the highest despite its high viscosity. From the measurement of ion diffusion coefficients by Nuclear Magnetic Resonance (NMR) spectroscopy, the ionicity could be evaluated. The cycloalkane-substituted [C4cC5Im][Tf2N] was found to be more dissociated compared to the other two ionic liquids which explains the behaviour observed for the conductivity. This could mean that the cycloalkane-substituted ionic liquid might find its use in electrochemistry. The experimental viscosity, conductivity, and diffusion coefficient data were described by equations of the Vogel-Tamman-Fulcher type using a regression along a gnostic influence function. In this way, a robust global optimization algorithm could be used to obtain reliable model parameters, but also to critically evaluate the experimental data

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