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Fatty acid-based ionic liquids (FAILs) are an emerging class of biodegradable ILs. Although long-chained FAILs in particular offer a promising alternative to problematic hydrophobic ILs in lubrication and extraction applications, recent studies have revealed the possibility of an anionic "side-chain length effect", which would adversely impact their ecofriendliness. This study therefore investigated the toxicity of FAILs with sidechain lengths between C 1 and C 18 and benign cations by applying a tripartite biological test system, including enzyme inhibition test, determination of virucidal activity against three model viruses, and bactericidal activity against 12 clinically relevant bacteria. Results do not confirm a general anionic "sidechain effect" with FAILs but rather demonstrate a diverse outcome depending on the biological test system employed. While Gram-negative bacteria and viruses were not affected by increasing FAIL anion alkyl chain length, medium-and long-chained FAILs exhibited pronounced effects at the enzyme level and on Gram-positive bacteria. While this study reinforces the necessity to establish a multilevel approach for the (eco)toxicological evaluation of FAILs, it also demonstrates a specific effectiveness of FAILs against enzymes and Gram-positive bacteria, which should be considered for possible future applications.

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