Like-charge attraction in Ionic Liquids: The influence of pressure, polarity and molecular mimics

• Applicant: Professor Dr. Ralf Ludwig
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2016 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 286149019

In the first funding period, we showed that the interaction strength of the anion, the polarizability of the cation, and the length of hydroxy alkyl groups of the cation control the delicate balance between hydrogen-bonded cation-anion and cation-cation clusters in bulk ionic liquids. We investigated the kinetic and thermodynamic stabilities of clusters of like-charged ions. In collaboration with the Johnson group at Yale University, we started initial gas phase experiments, showing cationic complexes with cooperative hydrogen bonding between the cations despite the strong repulsive forces between them. The experimental results also shed light onto the discussion about the stability of so-called “Anti electrostatic hydrogen bonds”. The plethora of exciting results about the attractive interaction between ions of like charge provokes further questions, which we want to address in the renewal proposal: How sensitive is the cationic cluster formation to pressure? Can we control the delicate balance between cation-anion ion-pairs and the cation-cation clusters by changing the dielectric medium using solvents of different polarity? How important are specific interactions with solvent molecules? Can we enhance hydrogen bonding and cluster formation by replacing the cations by their molecular mimics? Here, the smooth transfer from ionic to molecular clusters is studied. In collaboration with the Johnson group, we also plan to investigate whether the structural motifs of cationic and molecular gas phase clusters are also present in the bulk liquid phase. We will tackle these challenges with a suitable combination of synthesis of well-selected ionic liquids, bulk and gas phase infrared spectroscopy, and quantum chemical methods.

• DFG Programme: Research Grants