Abstract
Bio-based ionic liquids (ILs) are being increasingly sought after, as they are more sustainable and eco-friendly. Purines are the most widely distributed, naturally occurring N-heterocycles, but their low water-solubility limits their application. In this work, four purines (theobromine, theophylline, xanthine, and uric acid) were combined with the cation tetrabutylammonium to synthesize bio-based ILs. The physico–chemical properties of the purine-based ILs were characterized, including their melting and decomposition temperatures and water-solubility. The ecotoxicity against the microalgae Raphidocelis subcapitata was also determined. The ILs show good thermal stability (>457 K) and an aqueous solubility enhancement ranging from 53- to 870-fold, in comparison to their respective purine percursors, unlocking new prospects for their application where aqueous solutions are demanded. The ecotoxicity of these ILs seems to be dominated by the cation, and it is similar to chloride-based IL, emphasizing that the use of natural anions does not necessarily translate to more benign ILs. The application of the novel ILs in the formation of aqueous biphasic systems (ABS), and as solubility enhancers, was also evaluated. The ILs were able to form ABS with sodium sulfate and tripotassium citrate salts. The development of thermoresponsive ABS, using sodium sulfate as a salting-out agent, was accomplished, with the ILs having different thermosensitivities. In addition, the purine-based ILs acted as solubility enhancers of ferulic acid in aqueous solution.
Highlights
IntroductionIonic liquids (ILs) are alternative solvents, composed of a large organic cation (e.g., tetraalkylammonium, tetraalkylphosphonium, imidazolium, cholinium, pyridinium) and an organic or inorganic anion [1]
Ionic liquids (ILs) are alternative solvents, composed of a large organic cation and an organic or inorganic anion [1]
Since [N4444][Ur] starts decomposing before melting, this compound is not categorized as an IL, according to the recent definition by Mariani et al [3]
Summary
Ionic liquids (ILs) are alternative solvents, composed of a large organic cation (e.g., tetraalkylammonium, tetraalkylphosphonium, imidazolium, cholinium, pyridinium) and an organic or inorganic anion [1]. By combining purines with different salts (e.g., sodium, lithium, phosphate, and mesylate, among others) it is possible to improve their water-solubility [20,21,22] This unlocks new prospects for the application of purines, including their use as building blocks for the synthesis of ILs. By using purines as feedstock, it may be possible to design novel, bio-based ILs, while simultaneously increasing the purine solubility. Three novel purine-based ILs and one salt were synthesized, using tetrabutylammonium as a cation and theophylline, theobromine, xanthine, and uric acid as anions (Table 1) They were further characterized, regarding melting and decomposition temperatures, water-solubility, and ecotoxicity against the microalgae Raphidocelis subcapitata.
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