When Do Ionic Liquids Stop Behaving as Ionic Liquids? Assessing the Environmental Relevance of Ion Pairing.

Ionic liquids (ILs) have attracted considerable scientific interest over nearly three decades, with thousands of studies highlighting their low volatility, thermal stability, and potential for property modification through cation and anion selection. However, their behavior under environmental conditions remains less clear, particularly when water, soil constituents, and competing ions are present. This article examines whether concepts developed for neat ILs can be directly transferred to aqueous and soil systems. Particular attention is given to ion pairing, dissociation, surfactant-like behavior, sorption, toxicity, and biodegradation. Evidence from studies on herbicidal ILs, conventional precursor salts, and their mixtures indicates that, under environmentally relevant conditions, cations and anions frequently behave as largely independent chemical species. Hydrophobic cations are often responsible for sorption and microbial toxicity, whereas herbicidal anions typically retain mobility and biodegradation patterns comparable to their conventional salt forms. These observations question the environmental significance of ion pairing and suggest that the extent to which IL properties can be controlled through cation and anion selection may be more limited than commonly assumed. Greater emphasis should therefore be placed on precise terminology, appropriate experimental controls, and validation under realistic environmental conditions when interpreting the behavior and environmental relevance of ILs.