How molar ratio between two anions affects physicochemical properties and biological activity of double salt ionic liquids (DSILs)?

The strategy of mixing ionic liquids opens up new possibilities for obtaining liquid chemical systems (i.e., double salt ionic liquids, DSILs) with well-defined, beneficial properties, which is particularly important for the design of new biologically active substances. Here, we describe the application of the DSIL strategy for obtaining new systems containing a common amphiphilic cation with a hexadecyl substituent and 2 anions derived from synthetic auxins at the different molar ratio – MCPA and dicamba, both suitable for use as novel herbicides with enhanced efficiency. Detailed analysis of the NMR spectra indicates the presence of new interactions in the structure of DSILs, which affect the chemical environment of the ions and are strictly dependent on the molar ratio of the parent ionic liquids. Moreover, a non-additive effect of combining two surface-inactive anions on the surface activity of DSILs was discovered. DSILs with molar ratios from 8:2 to 2:8 had better surface activity compared to their starting components containing only one anion. Moreover, in greenhouse experiments DSIL containing molar fraction of dicamba equal to 0.2 showed significantly better efficiency than the other tested systems. This enhancement can be explained by the occurrence of a synergistic effect between both utilized anions. However, insertion into DSIL greater content of the dicamba anion nullified this beneficial effect. The initial ecotoxicity studies also pointed out potential risks associated with the increased toxicity of the new DSIL systems to non-target organisms, which warrants further research on the effects of mixing ionic liquids on environmental loading.