Deep eutectic systems based on tetrabutylammonium bromide and imidazole derivatives with different alkyl chain lengths

This study investigates the occurrence of deep eutectic compositions in the systems composed of tetrabutylammonium bromide with imidazole, 2-methylimidazole, and 2-ethylimidazole aiming to determine the effect of the alkyl chain length on the phase diagrams of the mixtures. Differential scanning calorimetry measurements indicated that all three systems have eutectic composition with melting points about 20 °C lower than those expected for the ideal mixtures. Infrared spectroscopy evidenced that the mixing mostly affects the NH group of the neutral molecules, due to the disruption of the hydrogen bond of the starting materials. In addition, the CH stretching bands of the cation are shifted as a consequence of the reduced tetrabutylammonium–anion interaction in the mixtures. DFT calculations revealed that the strong hydrogen bonds of the imidazolic molecules can indeed be replaced by weaker interactions involving Br− and either the cation or the neutral molecules. Classical molecular dynamics indicated that, in addition to the cation, each anion coordinates the stoichiometric amount of neutral molecules present in the eutectic mixtures, forming directional bonds. Moreover, only in the case of 2-ethylimidazole, only a small concentration of hydrogen bonds among the neutral molecules is retained in the mixtures.