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Diffusion of nitroxide radical in imidazolium-based ionic liquids with different alkyl chain lengths

We applied electron paramagnetic resonance (EPR) to study dynamics of nitroxide radical in four imidazolium-based ionic liquids (ILs) with different alkyl chain lengths. The nitroxide radical was perdeuterated 2, 2, 6, 6-tetramethyl-4-oxopiperidine-1-oxyl (pDTEMPONE), which was dissolved in 1-alkyl-3-methylimidazolium tetrafluoroborate ILs ([Cnmim][BF4]) with ethyl (n=2), butyl (n=4), hexyl (n=6), and octyl (n=8) groups. The rotational diffusion of pDTEMPONE was studied by analyzing EPR spectra at low radical concentration [1], while the translational diffusion was studied by analyzing changes in EPR spectra as a function radical concentration [2]. The obtained rotational and translational diffusion coefficients exhibit different dependences on alkyl chain length (Fig. 1), which could reflect the aggregation of alkyl chains into domains for ILs with n≥4. Such an aggregation was previously indicated by X-ray diffraction measurements [3]. Figure 1 Temperature dependences of the rotational and translational diffusion coefficients of pDTEMPONE in the ionic liquids [Cnmim][BF4] with n=2, 4, 6, and 8. [1] D. Merunka, M. Peric, M. Peric, J. Phys. Chem. B 119 (2015) 3185. [2] D. Merunka, M. Peric, J. Phys. Chem. B, in press, DOI: 10.1021/acs.jpcb.7b02550. [3] A. Triolo, O. Russina, H. J. Bleif, E. Di Cola, J. Phys. Chem. B 111 (2007) 4641.

electron paramagnetic resonance ; nitroxide radical ; ionic liquids ; diffusion

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