How does the molar ratio between two anions affect the properties of surface active double salt ionic liquids (DSILs)?

The strategy of mixing ionic liquids enables the formation of double salt ionic liquids (DSILs)—liquid systems consisting of three or more ions with tailored, beneficial properties that are crucial for designing new active chemical ingredients. In this study, we utilized this approach to obtain new DSILs containing a common surface active cation with a hexadecyl substituent and two anions derived from commonly used synthetic auxin herbicides—4-chloro-2-methylphenoxyacetic acid (MCPA) and 3,6-dichloro-2-methoxybenzoic acid (dicamba)—at varying molar ratios. Nuclear magnetic resonance analysis revealed significant chemical shift changes (up to 0.256 ppm for the methylene group in MCPA anion) which were linearly or exponentially dependent on the molar ratio of the DSIL counterparts, indicating specific, competitive interactions between the ions. The performed studies of physicochemical properties, including density, refractive index, and phase transition temperatures in most cases indicated a linear dependence of these properties on the molar ratio of the DSIL counterparts. However, combining two surface-inactive anions unexpectedly enhanced surface activity of the analyzed systems. DSILs with molar ratios from 8:2 to 2:8 exhibited nearly 50 % lower critical micelle concentrations than their single-anion counterparts. This increase in surface activity was responsible for an almost twofold increase in the aquatic toxicity toward Chlorella vulgaris.