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

Subject Area Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term from 2020 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 450088079
 
Final Report Year 2024

Final Report Abstract

In this project, we wanted 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 earlier experimental studies, we could show for hydroxy-functionalized ionic liquids (ILs), that two distinct types of hydrogen bonds (H-bonds) coexist: The conventional H-bonds between cation and anion (c-a) and elusive H-bonds between two cations (c-c) 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 had measured the distances for both type hydrogen bonds, showing that the (c-c) H-bonds are 10 ppm shorter than the (c-a) H-bonds despite the repulsive interaction between the cations. The double-faced nature of hydrogen bonding in hydroxyl-functionalized ILs allowed tackling fundamental questions, which we addressed in this project. Firstly, mixing OH- functionalized with non-functionalized ILs provides information about the influence of OH-defects on the size and distribution of (c-a) and (c-c) clusters in ILs (P1). 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 H-bond distribution by an OH-catcher (P2). 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 H- bond strengths in hydroxyl-functionalized ionic and molecular liquids, was only accessible through ND experiments and support of MD simulations. Overall, we could give suitable answers to most of the questions. In particular, we were successful in characterizing the cationic clusters in terms of size, shape and H-bond motifs in the mixtures depending on the OH density (P1) and the addition of the OH-catcher dimethylsulfoxide (DMSO) (P2). We could study the equilibria between (c-a) and (c-c) clusters over the entire mixture ranges. With supporting molecular dynamics (MD) simulations, we additionally demonstrated the different kinetics and life times of the two type of ionic H-bonds. Surprisingly, the H-bond lifetimes in the cationic clusters are longer than that in the cation-anion pairs despite the repulsive Coulomb interaction in the first case and the attractive Coulomb interaction in the latter case (P3, P4). Currently, we are calculating the life times of the (c-a) and (c-c) H-bonds in all the three types of mixtures by means of MD simulations. The lifetimes depend on the addition of non-hydroxy-functionalized ILs, OH-functionalized molecular mimics of the IL cation and the OH-catcher DMSO. In related studies, we could show that ion mobility in hydroxy-functionalized ILs depends on cationic clustering. Deuteron NMR relaxation measurements allowed to study the alkyl chain length behaviour for the dynamics and distributions of (c-a) and (c-c) H-bonded clusters (P5, P6). We could not realize the second part of this project, namely mixing the hydroxyl-functionalized IL with alcohol molecules for studying the liquid nanostructures upon charge-defects by changing from the ionic to the molecular liquid. As indicated already in the proposal, the synthesis of differently deuterated alcohols 4-Pentyl-1-butanol was a too big challenge. The deuterated precursor material was too expensive and the yields too low compared to the amount of material needed for providing the eleven neutron contrasts for the ND studies. Thus, we decided to focus on studying the influence of OH-defects and OH-catchers on the size and distribution of cationic clusters by means of ND experiments. Nevertheless, we performed MD simulations for all three kinds of mixtures and additional IR measurements for characterizing the (c-a) and (c-c) cluster distributions as a function of temperature and determining their H-bond life times. The successfully synthesized ILs in this project are currently used in IR and solid-state NMR measurements for getting additional information about the strength of (c-a) and (c-c) H-bonds as well as the distribution in the suggested mixtures as a function of temperature and since recently of pressure (P7).

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