Green profiling of aprotic versus protic ionic liquids : Synthesis and microbial toxicity of analogous structures

How does the variation in the ionic nature of ionic liquids (ILs) affect their antimicrobial properties? To answer this question with a direct connection to the molecular structure of ILs is integral for the design of new task specific ILs. The effect of ionic nature can be investigated through a comparison between analogous aprotic and protic ILs. However, while there have been extensive studies on the toxicology of both aprotic and protic ILs, the number of different structures and procedures employed makes quantitative comparison impossible. To address this, a series of analogous N,N,N-trimethylethanolammonium (cholinium) derived aprotic ILs (AILs) and N,N-dimethylethanolammonium derived protic ILs (PILs) with acetate, hexanoate, D,L-mandelate and 3-ethoxypropionate anions were prepared and characterised. All ILs were subsequently screened for antimicrobial activity against eight bacterial and twelve fungi strains. From the antimicrobial activity screening, little difference was found between the toxicities of AILs and PILs with shorter chains terminating in hydroxyl functional groups (e.g cholinium hexanoate and N,N-dimethylethanolammonium hexanoate). Variations between anion structure demonstrated slightly higher toxicities for more lipophilic anions. Antimicrobial activities were found to significantly increase for ILs with a long ether chain functional groups in the cation, due to the enhanced surfactant properties of these long chain cations. The importance of toxicity screening of analogous series of AILs and PILs as part of a future comprehensive biodegradation analysis has also been proposed based on postulated IL breakdown pathways.