Synthetic auxin-based double salt ionic liquids as herbicides with improved physicochemical properties and biological activity

Abstract Over the past decade, numerous advantages of double salt ionic liquids have been discovered, including beneficial physicochemical properties and a synergistic increase in biological activity resulting from competitive interaction among three or more ions based on their chemical structures. Therefore, we applied the approach mentioned above in our study by designing new ionic liquids consisting of amphiphilic choline-like cations and two or three anions derived from synthetic auxins. The influence of combining three anions with herbicidal activity in one compound on its physical, chemical and biological properties is discussed here for the first time. Double salts were obtained according to a fast, reliable and sustainable synthesis method, which made it possible to obtain pure ionic liquids, reaching 99-100% efficiency. Comparison of NMR spectra of products containing one or more anions has revealed that the introduction of additional ions has a clear impact on the chemical environment of atoms, which may explain the observed changes in double salt properties. The effects of the addition of the 2nd and 3rd anions on basic physicochemical parameters (including density, refractive index, viscosity, phase transitions, solubility and surface activity) were studied. Biological experiments have confirmed that the herbicidal activity of the obtained ionic liquids with three herbicide anions is higher than that of commercial preparations and most tested ionic liquids with only one biologically active anion, which confirms numerous advantages of the “double salt ionic liquid” approach.