Edward QUITEVIS1, Sophia SAGALA1, Jagdeep KAUR1, Noah WILLIAMS1
1Texas Tech University, Lubbock, United States
The rotational dynamics of both the cation and anion in the ionic liquid (IL) 1-ethyl-3-methylimidazolium methyl phosphonate ([C2C1im][(OMe)(H)PO2]) and its mixtures with methylimidazole (MIM) were studied using NMR relaxation techniques. The spin-lattice relaxation rate 1/T1 and the spin-spin relaxation rate 1/T2 of the C2-hydrogen on the cation and the hydrogen on the phosphorous atom of the anion, as well as the viscosity of the neat IL and that of the IL/MIM mixtures were measured as a function of temperature T. The NMR relaxation rates do not vary linearly with the ratio , indicating NMR relaxation is not in the extreme line-narrowing regime. The rotational correlation time was obtained by fitting the ratio T1/T2 versus by an equation with the relaxation rates given by the Solomon equations for dipolar relaxation. Good fits were obtained when is assumed to follow a power law for both the cation and anion with the exponent less than one in the neat IL and approaching one with increasing dilution in the IL/MIM mixtures. This suggests that the rotational motion of the ions is governed by dynamic heterogeneity in the neat IL which then approaches Debye-Stokes-Einstein behavior with increasing dilution in the IL/MIM mixtures. To obtain a molecular level understanding of the increase of with increasing dilution, ion-ion and ion-MIM interactions were studied by performing 2D NOE measurements (HOESY/NOESY) on the IL and IL/MIM mixtures.