Designing ionic liquids: the chemical structure role in the toxicity

Abstract

Ionic liquids (ILs) are a novel class of solvents with interesting physicochemical properties. Many different applications have been reported for ILs as alternatives to organic solvents in chemical and bioprocesses. Despite the argued advantage of having low vapor pressure, even the most hydrophobic ILs show some degree of solubility in water, allowing their dispersion into aquatic systems and raising concerns on its pollutant potential. Moreover, nowadays most widespread notion concerning the ILs toxicity is that there is a direct relationship with their hydrophobicity/lipophilicity. This work aims at enlarging the currently limited knowledge on ILs toxicity by addressing negative impacts in aquatic ecosystems and investigating the possibility of designing hydrophobic ILs of low ecotoxicity, by the manipulation of their chemical structures. The impact of aromaticity on the toxicity of different cations (pyridinium, piperidinium, pyrrolidinium and imidazolium) and hydrophobic anions (bis(trifluoromethylsulfonyl)imide [NTf(2)] and hexafluorophosphate [PF(6)]) was analysed. Concomitantly, several imidazolium-based ILs of the type [C( n )C( m )C( j )im][NTf(2)] were also studied to evaluate the effects of the position of the alkyl chain on the ILs' toxicity. For that purpose, standard assays were performed using organisms of different trophic levels, Vibrio fischeri, Pseudokirchneriella subcapitata and Daphnia magna, allowing to evaluate the consistency of the structure-activity relationships across different biological targets. The results here reported suggest the possibility of designing ILs with an enhanced hydrophobic character and lower toxicity, by elimination of their aromatic nature.

Understanding the impact of the central atom on the ionic liquid behavior: Phosphonium vs ammonium cations

“…Moreover, the results of Table V also show that the [NTf 2 ] anion is the most toxic, which is in close agreement with the literature results. 7,39 Despite the scarce number of studies reporting toxicity data for these non-aromatic and acyclic families, the same tendency here described was also identified in literature. 40 Maginn and co-workers 40 It is widely accepted in the literature 41,42 that the toxicities of ionic liquids are related with their hydrophobicities.…”

“…Specific heuristics have been previously proposed covering the ILs' structural characteristics, namely the alkyl chain length and isomerism and the presence/absence of the aromaticity effect in a large range of physical, chemical, and biological properties. [4][5][6][7] It aims at understanding the impact in the ILs thermodynamic, thermophysical, and biological properties promoted by the simple substitution of a phosphorous by a nitrogen atom on the cation's core. In contrast to the ion substitution studies, the atom substitution, of the constituent ions, has been object of a limited number of studies.…”

Development of predictive QSAR models for Vibrio fischeri toxicity of ionic liquids and their true external and experimental validation tests

“…16,17,20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Table S1 of the ESI. † While the other descriptors were obtained without the need of any geometry optimization process, the QTMS parameters were derived from the ab initio based optimized geometry at the HF/6-31G(d) level of theory, and were limited to only cations.…”

Ionic liquids breakdown by Fenton oxidation

“…They are gaining increasing attention due to their unique properties, such as extremely low vapor pressure, high thermal and chemical stabilities, non-flammability and high solvent capacity [1,2]. Thus, these materials are often branded as "environmentally friendly" or "green solvents", and have been suggested as ideal replacements for volatile organic solvents [3][4][5].…”