Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids

Abstract

In this paper, the morphology and bulk physical properties of 1,3-dialkylimidazolium bis{(trifluoromethane)sulfonyl}amide ([(C(N/2))(2)im][NTf(2)]) are compared to that of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C(N-1)C(1)im][NTf(2)]) for N = 4, 6, 8, and 10. For a given pair of ionic liquids (ILs) with the same N, the ILs differ only in the symmetry of the alkyl substitution on the imidazolium ring of the cation. Small-wide-angle X-ray scattering measurements indicate that, for a given symmetric/asymmetric IL pair, the structural heterogeneities are larger in the asymmetric IL than in the symmetric IL. The correlation length of structural heterogeneities for the symmetric and asymmetric salts, however, is described by the same linear equation when plotted versus the single alkyl chain length. Symmetric ILs with N = 4 and 6 easily crystallize, whereas longer alkyl chains and asymmetry hinder crystallization. Interestingly, the glass transition temperature is found to vary inversely with the correlation length of structural heterogeneities and with the length of the longest alkyl chain. Whereas the densities for a symmetric/asymmetric IL pair with a given N are nearly the same, the viscosity of the asymmetric IL is greater than that of the symmetric IL. Also, an even-odd effect previously observed in molecular dynamics simulations is confirmed by viscosity measurements. We discuss in this paper how the structural heterogeneities and physical properties of these ILs are consistent with alkyl tail segregation.

A Systematic Study on the Mesomorphic Behavior of Asymmetrical 1-Alkyl-3-dodecylimidazolium Bromides

“…6 Our observation could be a reflection of the ion dynamics in ILs, which has also been observed for viscosity with a similar even− odd effect. 26 A high local order in systems with even chain lengths observed by MD simulations was reported by Wang and Voth; they showed the diffusion constants of compounds with odd alkyl chain lengths are smaller than the diffusion constants of compounds with even chain lengths. 27 In contrast, the clearing points increase rapidly with rising n, for n ≥ 7, and no even−odd effect is observed.…”

Vapor pressures of 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids with long alkyl chains

“…The symmetry of the ionic liquid cation, changes dramatically the thermophysical properties, as it was already shown in the literature. 30,33,34,43,64 In fact, only recently the extended series of [C N/2 C N/2 im][NTf 2 ] ionic liquids was fully studied concerning the heat capacities, viscosities, densities and the liquid structure using SWAX 33,34,64 and more recently using molecular dynamics simulations, 9 and two regions were identified, with a shift around [C 6 , is in agreement with the expected effect arising from the nanostructuration in ionic liquids. 9 It is well established that in the liquid phase of ionic liquids, the polar regions tend to organize themselves in order to enhance the electrostatic interactions between the cation and anion and the alkyl chains of the cation for a non-polar segregated network.…”

Insights into the Mechanism of Extraction of Uranium (VI) from Nitric Acid Solution into an Ionic Liquid by using Tri‐n‐butyl phosphate

“…[1][2][3][4] Compared with molecular organic solvents, imidazolium-, tetraalkylammonium-, or phosphonium-based RTILs exhibit advantageous physicochemical properties, [5][6][7] including low vapor pressure, nonflammability, stabilityt owards water and acids, aw ide electrochemical window,a nd ability to solvate ions or neutral molecules, and also tunability of their structure and thus of their properties.…”