Issue 12, 2021

Determination of the dispersion forces in the gas phase structures of ionic liquids using exclusively thermodynamic methods

Abstract

Ionic liquids are described by a delicate balance of Coulomb interaction, hydrogen bonding and dispersion forces. Dissecting the different types of interactions from thermodynamic properties is still a challenge. Here, we show that comparison of vaporization enthalpies of tetra-alkyl-ammonium ionic liquids with bis(trifluoromethylsulfonyl)imide [NTf2] anions and the related molecular liquids, trialkylamines, allows for determining dispersion interactions in the gas phase ion-pairs. For this purpose, we measured vapor pressures and vaporization enthalpies of these ionic and molecular liquids by using a quartz-crystal microbalance. For supporting these data, we determined the vaporization enthalpies additionally from experimental activity coefficients at infinite dilution. Characteristic alkyl chain length dependences of the vaporization enthalpies have been established and were used for quantifying the dispersion forces in the gas phase species. The dissected dispersion contributions suggest that the alkyl chains do not show star-like topologies but embrace the anion maximizing the dispersion interactions. For the longest alkyl chains with eight carbon atoms, the dispersion interaction is as strong as two and a half hydrogen bonds. The proportion of dispersion in the gas phase species depending on the number of methylene groups in the ammonium cations is strongly supported by quantum chemical calculations.

Graphical abstract: Determination of the dispersion forces in the gas phase structures of ionic liquids using exclusively thermodynamic methods

Supplementary files

Article information

Article type
Paper
Submitted
16 okt 2020
Accepted
04 mrt 2021
First published
09 mrt 2021

Phys. Chem. Chem. Phys., 2021,23, 7398-7406

Determination of the dispersion forces in the gas phase structures of ionic liquids using exclusively thermodynamic methods

D. H. Zaitsau, R. Ludwig and S. P. Verevkin, Phys. Chem. Chem. Phys., 2021, 23, 7398 DOI: 10.1039/D0CP05439A

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