As it is well known, ionic liquids (ILs) are compounds formed entirely by ions which have low melting points. Their applications seems countless, since they are still not fully studied as pure, in mixtures with other compounds or with active pharmaceutical ingredients (APIs) incorporated in their structures among other possibilities (Rodríguez and Brennecke 2006; Rana et al. 2010; Silva et al. 2016; Toledo-Hijo et al. 2016; Salgado et al. 2019a).
The main characteristic of ILs is the low vapor pressure, which means non-volatility, and therefore they were, firstly, addressed to substitute traditional industrial solvents, most of which are volatile organic compounds (VOCs), one of the most source of environmental pollution in chemical industry (Rogers and Seddon 2003). This fact led to designate them as green solvents, although recent studies concluded that the toxicity of some ILs is similar, or even higher, than traditional solvents (Studzinska and Buszewski 2009; Santos et al. 2014).
Furthermore the non-volatility, other characteristic properties of ILs are, for example, the high thermal and chemical stability, high viscosity, the solubility in water and other solvents, wide electrochemical window and specially the tuneability (Yasuda et al. 2013). All these characteristics make ILs good candidates to be used in high temperature applications, as lubrication [6], as well as desulfurization of fuels (Gutiérrez et al. 2018), batteries (Menne et al. 2013; Balducci 2017; Wang et al. 2020), fuel cells (Nakamoto and Watanabe 2007), fluids in refrigeration systems (Sánchez et al. 2016; Moreno et al. 2018), APIs (Shamshina and Rogers 2020), etc.
ILs can be divided into two different subclasses depending on their structural characteristics: protic (PILs) and aprotic (AILs) ionic liquids. PILs are formed by the proton transfer from acid to base, and hence, they consist of proton-donor and -acceptor sites, which are responsible for building extended three-dimensional hydrogen bond networks as in the case of water, and AILs are mainly based on bulky organic cations (i.e. pyrrolidinium, imidazolium…) with long alkyl chain substituents, and on a huge variety of anions (i.e. TFSI, FAP, halides). In recent years, significant growth in the structure − property relationships of ILs has been achieved with a better understanding of the intermolecular forces (Salgado et al. 2013; Sánchez et al. 2019).
One of the most cited applications on literature is the electrochemistry, based on mixtures of ILs and inorganic salts that improve some properties that further extend the range of application of these compounds (Salgado et al. 2019b; Yang et al. 2020). Salgado et al. (Salgado et al. 2019b) have stated that melting and glass transition temperatures decrease with increasing salt concentration, while thermal stability is not significantly affected. Kim et al. (Kim et al. 2011) have studied the mixture n-butyl-n-methylpyrrolidinium TFSI with Li TFSI salt showing slight decrease on ionic conductivity when salt concentration increases. These papers remarks also the applicability to refrigeration besides the most traditional applications on electrochemistry and other uses.
However, together with good physico-chemical properties, current European Union environmental legislation, including REACH (Regulation concerning registration, Evaluation, Authorization and Restriction of Chemicals) (Commission 2006), calls for safety materials, –highlighting the principles of Green Chemistry as prevention, economy, less hazardous chemical synthesis, efficient use of energy, use of renewable raw and biodegradable materials, monitoring of real-time technological processes, and provision of an adequate level of chemical safety.
Therefore, it is urgent to establish evaluation procedures to estimate the toxicity of ILs that can readily provide the needed information and reducing the costs. Aliivibrio fischeri (A. fischeri) is a well-known marine luminescent bacterium with short reproductive cycle, and whose toxicity inference may be extrapolated to a wide variety of aquatic organisms, and thus can be effectively applied for toxicological risk assessment (Ventura et al. 2013; Parajó et al. 2019).
It is well stablished that the structure of ILs has important influence on the physical and chemical properties and also in toxicity, although some deep studies need to be performed. Thus, the specific choice of cation and anion has an important influence on the ecotoxicity of ionic liquids. It is well known that ILs with aromatic cations are more toxic than non-aromatic ones, mainly due to their water solubility (Ventura et al. 2013). Furthermore imidazolium and pyridinium based ILs with a specific anion show the highest harmful effects among other cations, with EC50 of 130 mg/L for the 1-butyl‐3‐methylpyridinium bromide and 5525 mg/L for 1‐butyl‐1‐methylpyrrolidinium bromide as Ibrahim et al. (Ibrahim et al. 2017) underlined in their work. These researchers also highlight that the alkyl chain length has also a strong influence on the ecotoxicity, as well as in other thermophysical properties such as density and viscosity. As a consequence, EC50 is reduced by almost four orders of magnitude from 3234 mg/L (relatively harmless) for 1‐ethyl‐3‐methylimidazolium chloride to 0.58 mg/L for 1-hexadecyl‐3‐methylimidazolium chloride (highly toxic, according Passino and Smith classification (Passino and S. B. Smith 1987)). With regard to the anion, in spite of its undoubted influence, the effects depend on the cation moiety, for example for the cation [Emim]+, the EC50 are 9213 mg/L and 1631 mg/L for the [Cl]− and [TFSI]− anions, respectively. Nevertheless, for the cation [C8mim]+ the corresponding values are 2.36 mg/L for chloride and 6.44 mg/L for [TFSI]− (moderately toxic), which shows that the trend followed for the anions can change with the alkyl chain length.
With the aim to contribute to enlarge de database of toxic effects of ILs and the consequent improvement of the knowledge of relationship between toxicity and structure, the ecotoxicity of two protic ILs (ethylammonium nitrate (EAN) and ethylimidazolium nitrate (EIm NO3)) and two aprotic ILs (butylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide (C4C1pyrr TFSI) and butyldimethylimidazolium bis(trifluoromethylsulfonyl)imide (C4C1C1Im TFSI)) was tested towards changes on the bioluminescence of the bacteria A. fischeri, using the Microtox® standard toxicity test. Additionally, changes on the ecotoxicity as consequence of the doping of these ILs with different salts with electrochemical interest were also determined, firstly the doping of the corresponding lithium salt (LiNO3, for the protic ILs and LiTFSI for the aprotic IL) was evaluated, and finally effects of mono (LiNO3), di (Ca(NO3)2·4H2O, Mg(NO3)2·6H2O) and trivalent (Al(NO3)3·9H2O) salts were also estimated for EAN. The effective concentration (EC50) of these mixtures was determined over three standard periods of time, namely 5, 15 and 30 min and compared with the corresponding values to pure ILs.