Character and influence of hydrogen bonding on the microstructures of ionic liquids by means of neutron diffraction and molecular dynamics simulations

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In this project, we want to study the double-faced nature of hydrogen bonding in hydroxyl-functionalized ionic liquids (ILs) by means of neutron diffraction (ND) and molecular dynamics (MD) simulations. In recent experimental studies, we have shown that in this IL family, two distinct types of hydrogen bonds (HBs) coexist: The conventional HBs between cation and anion (ca) and elusive HBs between two cations (cc) that are supposed to be much weaker due to the repulsive Coulomb force between the like-charged ions. Despite this expectation, however, we observed structural motifs involving hydrogen-bonded cationic clusters in the bulk liquid and the gas phase. In preliminary ND experiments on pure hydroxyl-functionalized ILs, we have measured the distances for both type hydrogen bonds, showing that the (cc) HBs are 10 ppm shorter than the (ca) HBs despite the repulsive interaction between the cations. The double-faced nature of hydrogen bonding in hydroxyl-functionalized ILs allows tackling fundamental questions, which we address here. Firstly, mixing OH-functionalized with non-functionalized ILs provides information about the influence of OH-defects on the size and distribution of (ca) and (cc) clusters in ILs. Secondly, mixing the IL with alcohol molecules mimicking the IL cation, allows studying the liquid nanostructures upon charge-defects by changing from the ionic to the ML. Thirdly, adding a strong polar proton acceptor molecule competing for the OH-groups in the IL opens a path for controlling the HB distribution by an OH-catcher. Varying the size and distribution of hydrogen bonded clusters upon hydroxyl- and charge-defects or introducing OH-catchers, allows tuning the properties of ILs and their mixtures, which are both attracting increasing interest in science and technology. The direct determination (H…O) and (O…O) bond distances, indicating the HB strengths in hydroxyl-functionalized ionic and molecular liquids, is only accessible through ND experiments and support of MD simulations.