Hydroxy-Functionalized Ionic Liquids under Pressure: The Influence on Hydrogen Bonding between Ions of Opposite and Like Charges

Hydroxy functionalization of cations in ionic liquids (ILs) can lead to formation of hydrogen bonds between their OH groups, resulting in so-called (c–c) H-bonds. Thereby, the (c–c) H-bonds compete with regular H-bonds (c–a) between the OH groups and the anions. Polarizable cations, weakly interacting anions, and long alkyl chains at the cation support the propensity for the formation of (c–c) H-bonds. At low temperatures, the equilibrium between (c–c) and (c–a) H-bonds is strongly shifted in favor of the cation–cation interaction. Herein, we clarify the pressure dependence on (c–c) and (c–a) H-bond distributions in the IL 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate [HOC2C1Im][PF6], in mixtures of [HOC2C1Im][PF6] with the nonhydroxy-functionalized IL 1-propyl-3-methylimidazolium hexafluorophosphate [C3C1Im][PF6] and in [HOC2C1Im][PF6] including trace amounts of water. The infrared (IR) spectra provide clear evidence that the (c–c) H-bonds diminish with increasing pressure in favor of the (c–a) H-bonds. Adding trace amounts of water results in enhanced (c–c) clustering due to cooperative effects. At ambient pressure, the water molecules are involved in the (c–c) H-bond motifs. Increasing pressure leads to squeezing them out of H-bond clusters, finally resulting in demixing of water and the IL at the microscopic level.