Do H-bonds explain strong ion aggregation in ethylammonium nitrate + acetonitrile mixtures?

Abstract

Binary mixtures of the protic ionic liquid ethylammonium nitrate (EAN) and acetonitrile (AN) were studied at 25 °C over the entire composition range by means of broadband dielectric spectroscopy covering 0.2 ≤ ν/GHz ≤ 89. The dielectric spectra could be decomposed into two relaxation processes, both of which proved to be composite modes. For dilute solutions the higher-frequency Debye relaxation centered at ∼60 GHz is associated with the rotational diffusion of AN molecules, whereas at higher salt concentrations ultra-fast intermolecular vibrations and librations of EAN dominate the process. For EAN-rich solutions the lower-frequency relaxation is mainly due to jump reorientation of the ethylammonium cation, whereas contact ion pairs (CIPs) dominate this mode for dilute solutions. From the relaxation amplitudes effective solvation numbers and ion-pair concentrations were determined. For vanishing EAN mole fraction, x_EAN → 0, an effective cation solvation number of ∼7 was found which steeply drops until x_EAN ≈ 0.2 but shows only moderate decrease later on. The obtained association constant for EAN, K⁰_A = 970 L mol⁻¹, exceeds that of other 1 : 1 electrolytes in AN by a factor of ∼30–50. This observation, as well as the fact that CIPs are formed despite strong cation solvation, indicates that ion pairing is mainly driven by the formation of strong hydrogen bonds between anions and cations.