(231d) Probing the Structure of Ionic (and non Ionic) Liquids with 129Xe NMR

It is now well established that room temperature ionic liquids (RTILs) are highly structured fluids, displaying both polar and apolar domains that self-assemble forming bi-continuous phases, liquid crystals, etc. This structuration, confirmed by several experimental techniques such as X-ray and neutron diffraction studies, determines the macroscopic properties of the fluid and is also observed in molecular simulation studies.

Due to its large electron cloud and high polarizability, the NMR active ¹²⁹Xe atom is very sensitive to the local molecular environment around it. Depending only on the solvent, atomic Xe presents a range of chemical shifts larger than 250 ppm. Moreover, its small size as opposed to the usual molecular probes (e.g. fluorescence, solvatochromic, etc), assures little perturbation of the liquid structure.

In this work, ¹²⁹Xe chemical shifts have been obtained, as a function of temperature, for solutions of Xe in a wide range of ionic liquids, alkanes, perfluoroalkanes and perfluoroalkylalkanes (PFAA), providing insight into the structure of these liquids and helping to detect/confirm the presence of transitions to highly organized phases.

For solutions of Xe in PFAA evidence has been obtained confirming the existence of mesophases below the melting point of some of the studied PFAA.

In the case of ILs, these were chosen varying both the cation and anion families and the length of the alkyl side-chains. The experimental data show evidence for the change of liquid structure with the increase of the alkyl chain length along a given IL family, for the mobility of Xenon in both polar and apolar domains, and for its preferential interaction with the apolar regions. Molecular dynamics simulations were performed and help to elucidate the NMR results.

Additionally, in the cases where the ILs displayed thermotropic liquid crystalline behaviour, ¹²⁹Xe spectroscopy was able to identify the isotropic liquid to liquid crystal transition, and to provide insight on the differences between the molecular vicinity of Xenon in both phases.

This work allows us to propose ¹²⁹Xe NMR spectroscopy as a simple and accessible experimental link between the molecular structure of liquids and their macroscopic behaviour.