Crystallization and Glass-Forming Ability of Ionic Liquids: Novel Insights into Their Thermal Behavior

Abstract

This work presents an extended study of the thermal behavior of the alkyl and dialkylpyridinium derivatives of the bis­(trifluoromethylsulfonyl)­imide ionic liquid series, using high resolution power compensation differential scanning calorimetry (DSC) equipment. Temperatures, enthalpies, entropies, and the heat capacity change associated with the glass transition, as well as cold crystallization, solid–solid transitions, and melting, are used to evaluate their ability to form a glass and crystallize. The effects of the cation isomerization and the alkyl chain length increase were used to investigate the nature of the irregular thermal behavior of ionic liquids in general and to establish the link between their thermal properties and nanostructuration. The observed V-shape profile of the melting temperatures versus the alkyl side-chain length is interpreted as a consequence of the balance between the initial decrease of the magnitude of the electrostatic interaction and the regular increase of the dispersive van der Waals forces. The observed differentiation between the thermal behavior below and above the critical alkyl size, CAS, is analyzed and compared with the regular behavior of both the n-alkanes and the n-alkanols. Above the CAS, the entropy and enthalpy profiles present trends similar to those observed in the alkane and alcohol series, contrasting with those observed in the molten salts-like region. These results and evidence are a strong support to the interpretation of the effect of the ILs nanostructuration in the physical-chemical properties. We found a great similarity between the thermal behavior of the imidazolium and pyridinium cation core, highlighting the strong ability of ILs to form a nanostructured network with distinguishable polar and nonpolar domains in the liquid phase.

Influence of Alkyl Chain Length on Thermal Properties, Structure, and Self-Diffusion Coefficients of Alkyltriethylammonium-Based Ionic Liquids

“…The thermal properties of ionic liquids are essential when it comes to their application, e.g., in supercapacitors, solar cells, batteries, impurities extraction, chemical reactions, and catalysis media [ 16 ]. The literature reports data about the thermal behavior of imidazolium and/or pyridinium ILs, usually obtained with the use of various methodologies (with various scanning rates, annealing periods, sample size, thermal history, and sample purity) [ 17 ]. To the best of our knowledge a complete series of alkyltriethylammonium bis(trifluoromethanesulfonyl)imides (C 4 –C 16 alkyl chain length) has never before been examined thoroughly in terms of their molecular dynamics and thermal behavior.…”

“…The literature also describes a “2/3 golden rule”, which means that for all compounds, that ratio of the temperatures of glass transition and melting (T g /T m ) is around 0.66 [ 20 ]. Lately, it was shown that for aprotic ionic liquids, the usual value of the ratio is 0.75 [ 17 ]. In the case of the [TEA-R] series, such values could be determined only for two ILs—[TEAC 10 ][TFSI] and [TEAC 12 ][TFSI]—because they were the only ones for which both melting point and glass transition temperatures were exhibited.…”

Anomalous and Not-So-Common Behavior in Common Ionic Liquids and Ionic Liquid-Containing Systems

“…Xu and Angell (2003) have discussed the Tg dependence on the molar volume (Figure 7B). Recently, Ferreira et al (2019) and Serra et al (2017) have shown that many of the aprotic ILs show high Tg/Tm values that are close to the 34 value.…”

“…Some works have tackled the prediction of the Tg of ILs (Mirkhani et al, 2012; Valderrama et al, 2017). Other studies have identified difficulties behind observing IL's crystallization (Serra et al, 2017; Ferreira et al, 2019).…”

Effect of alkyl-group flexibility on the melting point of imidazolium-based ionic liquids

“…The melting point initially decreases with the increase of n C until a critical number after which there is a change of tendency and the melting point starts to increase with n C . [7][8][9] The initial decrease is attributed to a dominant effect of the increase of conformational entropy in the liquid phase as the chain gets longer, while the increase for longer chains is explained as a predominance of the enthalpy resulting from van der Waals and hydrophobic interactions between chains. 8,9 The possibility of different conformers results in the existence of different polymorphs in the solid phase that differ from each other mainly due to the conformation of some dihedral angle.…”