Oriele PALUMBO1, Annalisa PAOLONE1, Daniel RAUBER2, Frederik PHILIPPI3, Tom WELTON3
1Consiglio Nazionale delle Ricerche,- Istituto dei Sistemi Complessi, Roma, Italy
2Department of Chemistry, Saarland University, Saarbrücken, Germany
3Department of Chemistry, Molecular Sciences Research Hub, Imperial College, London, United Kingdom
Ionic liquids (ILs) present fascinating properties potentially useful for several applications and the most interesting point is the possibility to modify these properties to meet the requirements of the different applications by tailoring the ions composition of the liquids. Despite the large amount of research carried out in recent years on ILs, there are still some challenges to be tackled to foster their widespread use. Indeed, macroscopic properties are defined by microscopic properties and intermolecular interactions and, among various molecular features, a fundamental role is played by conformational flexibility.
Dynamic mechanical spectroscopy is a well-known technique largely used to study the structural relaxation behaviour of bulk materials. In recent years a new method has been developed [1,2] to measure the mechanical modulus of the ILs and its variation during the main phase transitions occurring by varying the temperature, in both the liquid and the solid states.
These measurements show the role of possible anion conformers on the complex relaxation dynamics of ILs, confirming the occurrence of translational motion by means of hopping processes.
We present DMA spectra of BMIM based ILs with anions having different flexibility, clearly showing the effects of flexibility on the mechanical properties. The obtained data provide more detailed information about the possible involvement of nonequivalent configurations as well as their energy separation. Their analysis reveal the possible rate-limiting steps underlying the structural relaxation [3].
Moreover, results on ILs composed of quaternary cations and two anions having different conformational flexibility [4] show that flexible ions are capable of a fast relaxation on a local level, while for other ILs having rigid ions, a fast dynamic at a local level in the liquid phase is rarely observed since a partial transition to a solid state is favored. For these liquids, the measurements suggest the formation of aggregates which can be suppressed by the multiple ether functionalization [4].
[1] O. Palumbo, F. Trequattrini, F. M. Vitucci and A. Paolone, J. Phys. Chem. B, 2015, 119, 12905–12911
[2] O. Palumbo, F. Trequattrini, G. B. Appetecchi, L. Conte and A. Paolone, J. Mol. Liq., 2017, 243, 9–13
[3] F. Philippi, D. Rauber, O. Palumbo, K. Goloviznina, J. McDaniel, D. Pugh, S. Suarez, C. C. Fraenza, A. Padua, C. W. M. Kay, T. Welton, Chemical Science, 2022, DOI: 10.1039/d2sc03074h
[4] O. Palumbo, A. Paolone, D. Rauber, C.W.M. Kay, F. Philippi and T. Welton, J. All. and Compds., 919 (2022) 165860 doi: 10.1016/j.jallcom.2022.165860