Toxicity and antimicrobial activity of imidazolium and pyridinium ionic liquids

Ionic liquids (ILs) are novel organic salts with a wide liquid range that have enormous potential for industrial use as “green” chemicals. Varying the cationic and anionic components can alter IL properties and toxicities. Before the likely industrial release of ILs into the environment, it is necessary to determine their toxic and antimicrobial properties. As a measure of microbial toxicity of imidazolium and pyridinium ILs with varying alkyl chain lengths, we investigated Vibrio fischeri using the Microtox method. An increase in alkyl group chain length as well as an increase in the number of alkyl groups substituted on the cation ring corresponded with an increase in toxicity. Varying the anion identity did not significantly alter toxicity. We then examined the antimicrobial effects of 1000 ppm of butyl-, hexyl- and octyl- imidazolium and pyridinium bromide ILs on the growth of a group of microorganisms representing a variety of physiological and respiratory capabilities. In general, hexyl- and octyl- imidazolium and pyridinium bromides had significant antimicrobial activity to pure cultures of Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Pseudomonas fluorescens and Saccharomyces cerevisiae. Butyl-imidazolium and pyridinium bromides were less antimicrobial than ILs with longer alkyl chain lengths to all microorganisms examined. However, the most significant antimicrobial activity was observed in tests with B. subtilis. This research provides toxicity and antimicrobial information about ILs, prior to their widespread use and release. This type of proactive approach can aid in the prevention of pollution, and avoid costs of future clean-up, and provide information about the “green” nature of practical industrial solvents.