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 behavior, 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 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.

Thermal Analysis of Binary Mixtures of Imidazolium, Pyridinium, Pyrrolidinium, and Piperidinium Ionic Liquids

“…The five binary mixtures showed a value of Cp which is intermediate of the values of the individual ionic liquids, therefore, these Cp values reveal that the Cp of the two pure ionic liquids mixtures exhibits a practically ideal behavior, this behavior was also observed and reported in the bibliography [8] for mixtures of other ionic liquids. Furthermore, it can be seen that the piperidinium-based ionic liquid present higher heat capacity than their analogous pyrrolidinium, pyridinium, and imidazolium-based ionic liquids, in agreement with previous publications on the topic [6].…”

“…Within this approach, the possibility of mixing ionic liquids is a viable alternative, since mixed ILs can offer a significant improvement in the phase behavior properties and extend their applicability to other processes. There are many studies on the thermal analysis of pure ILs, however, there is a lack of studies related to heat capacities and transition temperatures of binary mixtures of ionic liquids in the literature [1][2][3][4][5][6][7].…”

Facile Cellulose Dissolution and Characterization in the Newly Synthesized 1,3-Diallyl-2-ethylimidazolium Acetate Ionic Liquid

“…Following the works of Song, et al [ 30 ], Rodrigues, et al [ 31 ] and Ren, et al [ 32 ], viscosities of the newly synthesized ILs were measured by a digital rotational viscometer (NDJ-8S, Shanghai Precision & Scientific Instrument Co., Ltd., Shanghai, China) at temperatures ranging from 20 °C to 60 °C with a temperature uncertainty of ±0.1 °C. Each data point of the viscosity is the average value of three measurements.…”

Vaporization Thermodynamics of Morpholinium Based Ionic Liquids

“…For validation of this method for ionic liquids of interest in this work, we used the experimental value =607.5±0.8 J ⋅ K −1 ⋅ mol −1 for N ‐propyl‐ N ‐methyl‐piperidinium bis(trifluoromethylsulfonyl)imides reported by Rodrigues et al [19] . The estimate =603 J ⋅ K −1 ⋅ mol −1 according to Eq.…”

“…This equation was parametrized with help of 100 ILs of different structures. [18] For validation of this method for ionic liquids of interest in this work, we used the experimental value C o p;m l ð Þ = 607.5 � 0.8 J • K À 1 • mol À 1 for N-propyl-N-methyl-piperidinium bis (trifluoromethylsulfonyl)imides reported by Rodrigues et al [19] The estimate C o p;m l ð Þ = 603 J • K À 1 • mol À 1 according to Eq. ( 11) was convincing close to the experiment and we used the method by Ahamadi et al [18] for calculations liquid phase heat capacities of ILs listed in Table 2.…”