Bio-ILs: Sustainable Biosurfactant-Derived Ionic Liquids Based on Rhamnolipid against Listeria monocytogenes

Surfactants (surface-active agents) have dual characteristics of hydrophilicity and hydrophobicity, which lead them to concentrate at interfaces, as well as to aggregate into several supramolecular architectures, thereby conferring unique physicochemical properties. The possibility of exploring these properties in combination with ionic liquids (ILs) has gained significant attention in recent years. In this work, a biosurfactant produced by a microbial strain, rhamnolipid (RL), a glycolipid with l-rhamnose and β-hydroxy fatty acids, isolated from Pseudomonas aeruginosa, was paired in its anionic form with cations of interest to synthesize ILs for antimicrobial applications. The new RL-based ILs (RL-ILs) were prepared using sustainable processes and then tested on several bacteria, yielding a promising minimal inhibitory concentration (MIC) of 0.21 mg/mL for [EMIM][RL] on Listeria monocytogenes. The use of RL-ILs represents a bacteriostatic approach, which is less likely to promote resistant bacterial populations, compared with strategies based on traditional drugs. The resulting RL-ILs were comprehensively characterized using established analytical techniques, including nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), among others. To the best of our knowledge, this represents the first demonstration of biosurfactants serving as ionic liquid anions. This establishes a new class of bio-derived ILs (bio-ILs) with unique amphiphilic and supramolecular properties, expanding the design principles of ionic liquids beyond conventional synthetic salts.