Beyond Green: Solvent-Dependent Antimicrobial Effects of Textile-Relevant Ionic Liquids on Key Bacterial Pathogens

The rising demand for sustainable industrial practices necessitates the development of environmentally benign alternatives to traditional volatile organic solvents. Ionic liquids (ILs), characterized by their unique properties, represent a promising class of such alternatives. This study conducted a comprehensive ecotoxicological assessment of four textile-relevant ILs: 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), 1-allyl-3-methylimidazolium chloride ([AMIM][Cl]), choline chloride ([Ch][Cl]), and tetraoctylammonium bromide ([TOA][Br]) or [N8888][Br]. The antimicrobial activity of these imidazolium- and ammonium-based ILs was investigated against Escherichia coli DH5a and Staphylococcus aureus MTCC 3103, serving as model Gram-negative and Gram-positive bacteria, respectively, utilizing the agar well diffusion method. Key findings revealed that while the positive control, tetracycline, established strong inhibition zones (approximately 25 mm for S. aureus, 22 mm for E. coli), IL toxicity was highly dependent on both chemical structure and solvent. In aqueous solution, the imidazolium-based [BMIM][Cl] showed significant activity, producing inhibition zones of approximately 18 mm against both bacterial strains. [TOA][Br] exhibited a dramatic increase in toxicity when dissolved in ethanol, generating inhibition zones exceeding 25 mm, a potency surpassing that of tetracycline. Instead, the hydrophilic [Ch][Cl] showed minimal activity even in ethanol. Findings also highlight the complexity and major influence of solvent interactions on the ecotoxicity of ILs. This research provides essential data for the informed design and sustainable application of ILs in industrial processes, emphasizing the critical need for thorough environmental risk assessment that extends beyond their inherent non-volatility.