Functionalized Ionic Liquids at Bacterial Membrane Interfaces: A Multiscale Perspective from Tensiometry, Microscopy,...

Recently developed ionic liquids (ILs) are gaining attention for their antibacterial properties in the face of increasing resistance to conventional antibiotics. Their structural tunability enables the design of compounds that balance antimicrobial potency and acceptable toxicity. We investigated two monoterpene-based functionalized ionic liquids (FILs)─[(1R,2S,5R)-(-)-mentoxymethyl]dimethyltetradecylammonium chloride and [(1R,2S,5R)-(-)-mentoxymethyl]tetradecylimidazolium chloride─which differ in cationic core (ammonium vs imidazolium) but share an identical hydrophobic alkyl chain. Using a multiscale approach combining Langmuir monolayers, Brewster angle microscopy, atomic force microscopy, and molecular dynamics (MD) simulations, we examined their interactions with model bacterial membrane interfaces. Both FILs integrated readily into lipid monolayers, disrupted lipid packing, and altered interfacial organization, with pronounced effects in systems enriched in anionic lipids such as cardiolipin and POPG. Despite structural differences, the two FILs showed comparable antibacterial activity, notable given the typically higher toxicity of ammonium-based FILs. MD simulations revealed that electrostatic interactions dominate FIL-lipid association, with cationic headgroups binding preferentially to phosphate oxygens, while hydrogen bonding contributes minimally. The cations preferentially bind to phosphate oxygens of anionic lipids, explaining the enhanced activity against both Gram-positive and Gram-negative membrane models. The combined experimental-computational approach highlights key design principles for developing next-generation ILs with targeted activity at biological surfaces.