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

• Published in: Fluid phase equilibria Vol. 423; pp. 190 - 202
• Main Authors: Rodrigues, Ana S.M.C., Almeida, Hugo F.D., Freire, Mara G., Lopes-da-Silva, José A., Coutinho, João A.P., Santos, Luís M.N.B.F.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 15.09.2016
• Subjects: Bis(trifluoromethylsulfonyl)imide; Cations; Density; Differential scanning calorimetry; Differentiation; Entropy; Glass transition; Heat capacity; Imidazolium; Ionic liquids; Isomerization; Isomers; Phase behaviour; Piperidinium; Pyridinium; Pyrrolidinium; Refractive index; Specific heat; Surface tension; Viscosity; Viscosity; Differential scanning calorimetry; Refractive index; Isomerization; Pyridinium; Heat capacity; Density; Glass transition; Imidazolium; Bis(trifluoromethylsulfonyl)imide; Pyrrolidinium; Phase behaviour; Ionic liquids; Surface tension; Piperidinium; Surface Tension; Glass Transition; Differential Scanning Calorimetry; Phase Behavior; Refractive Index
• ISSN: 0378-3812; 1879-0224
• DOI: 10.1016/j.fluid.2016.04.009

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, Tg. The iso-pyrrolidinium (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 the n and iso-alkyl isomers. A small increase of the surface tension was observed for the non-aromatic ILs, 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 ILs due to their higher energy barriers for shear stress. [Display omitted]