Corrosion–electrochemical behavior of Ni–Mo alloy electrodeposited from an electrolyte based on deep eutectic solvents

The need to develop new electrochemical energy storage and conversion devices requires the creation of new, available, low-cost and high-performance electrocatalytic materials, which can be produced as coatings by electrodeposition technique. The electrodeposited composite coatings based on nickel seem to be very promising in this context. We studied the corrosion resistance of electrocatalytic Ni–TiO2 composite coatings fabricated by electrodeposition method using a plating solution based on deep eutectic solvents, a new environmentally friendly and affordable type of room-temperature ionic liquids. We investigated the corrosion behavior of Ni and Ni–TiO2 coatings (5 and 10 wt.% of TiO2) in a 3% NaCl aqueous solution as a corrosive medium. The corrosion parameters were determined by linear voltammetry and electrochemical impedance spectroscopy. It was established that the inclusion of titania particles in the Ni matrix and an increase in their content in the coating leads to a shift in corrosion potential towards positive values, a decrease in corrosion current density and an increase in polarization resistance. The observed effects of improving the corrosion resistance of coatings are associated with the barrier action of particles of the dispersed phase and the formation of corrosion microcells contributing to the inhibition of local corrosion.

Deep eutectic solvents (DES) are a form of ionic liquid that can offer a larger electrochemical window than water while remaining relatively conductive as compared to other non-aqueous options. DES are made from the eutectic combination of a variety of hydrogen bond donors and acceptors. At a molar ratio of donor and acceptor specific to each DES, the melting point of the mixture is significantly depressed resulting in a room temperature liquid with a high boiling point. As a result, they are considerably less volatile and therefore a safer alternative to traditional organic solvents such as acetonitrile. The low vapor pressure associated with a deep eutectic composition allows for operation at elevated temperatures for faster kinetics, increased conductivity and lower viscosity. DES can also contain a greater concentration of supporting charge carries than many organic solvents (up to 8 M) and many redox active organic compounds are also highly soluble in DES. These factors make DES a promising electrolyte for redox flow batteries, with the potential for higher energy densities than can be achieved in aqueous electrolytes.One of the most commonly studied DES is a 1:2 molar ratio mixture of choline chloride and ethylene glycol respectively. This mixture, commonly called ‘ethaline’ is one of the most conductive, lowest viscosity DES. While these are desirable characteristics, the stable electrochemical window is only ≈2 V which is only somewhat larger than the usable window of an alkaline aqueous electrolyte. The objective of this effort is to investigate the electrochemical decomposition of ethaline at the anodic and cathodic limits.Relatively little literature exists on the decomposition of ethaline. It has been claimed that at the anode, several different chlorinated organics are created by various mechanisms and that hydrogen is evolved at the cathode from water contained in the electrolyte.1 While the oxidation of ethylene glycol (EG) is well studied2 , 3, the oxidation of ethaline and the reduction of EG and ethaline both are somewhat unknown. A series of electrolysis experiments using a divided cell were carried out using dry ethaline prepared in a glove box with a water vapor content below 5 ppm. The water content in the electrolyte was determined by Karl Fischer titration to be less than 300 ppm. The products of these experiments were analyzed using mass spectroscopy (of both the anodic and cathodic electrolytes and of the headspace above each) and FTIR. These results show that the positive decomposition involves the oxidation of ethylene glycol to glycolaldehyde, and subsequent formation of the glycolaldehyde dimer in solution. At the negative limit, hydrogen is evolved in direct proportion to the current passed. However, it can be conclusively shown that the reduction current can exceed the maximum possible current due to the reduction of water, implying that the reduction of the DES components is necessary. GC-MS analysis of the negative electrolyte showed the presence of higher molecular weight fragments consistent with the reaction scheme shown in Fig 1 below. In this scheme, the reduction of ethylene glycol results in the generation of H2 gas as observed, and the formation of ethylene oxide. The ethylene oxide produced then can react chemically with ethylene glycol to yield dimers and trimers of polyethylene glycol whose molecular masses are consistent with the peaks observed in the GC-MS analysis.The identification of these breakdown pathways will provide insight into safety concerns related to overcharge of ethaline electrolytes and will allow for the development of alternative formulations that can extend the potential limits to allow for the incorporation of higher voltage redox couples in redox flow batteries based on DES electrolytes.

Deep eutectic solvents (DES) are a relatively new class of electrolytes with potential application in electrochemical systems. DES are mixtures of hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD) that engage in a unique hydrogen-bonding network. Like traditional organic solvents, DES have the advantage of a wide electrochemical window compared to aqueous electrolytes, but with additional advantages over organic solvents such as low volatility and high energy density due to solvation strength towards redox active species. DES have shown promise for application in redox flow battery systems currently limited by lack of nonvolatile, high energy density electrolytes. While DES offer a potential solution to these issues, high viscosity and low conductivity have thus far limited their practical application. Catechol and 1,4-benzoquinone were introduced as redox active species and hydrogen bond disruptors to ethaline DES which is a 1:2 molar mixture of choline chloride and ethylene glycol. These redox active organic molecules are of particular interest due to their high solubility in DES. Furthermore, the reduction mechanism of these molecules does not involve electrodeposition and subsequent electrode morphology changes. This addresses issues often associated with traditional redox active species such as metal halides used in redox flow batteries. The physical and electrochemical properties of ethaline with catechol and 1,4-benzoquinone were investigated. Effects of the DES hydrogen-bonding network on the redox potentials, electrochemical reversibility, and diffusivity was investigated by voltammetry. Temperature dependent viscosity, conductivity, and density were measured to investigate the impact of hydrogen bond disruptors on macroscopic properties. Finally, attenuated total reflectance infrared spectroscopy was used to investigate the molecular interactions between the redox active species and the hydrogen-bonding network in DES to relate observed macroscopic properties to molecular level features. This work provides insight into the solvation of organic redox active species in DES and its impact on physical and electrochemical properties.

Thin Ni and Cr coatings are commonly electrodeposited from aqueous electrolytes [1,2]. These often contain series of additives, whose role is to improve the leveling and adhesion of deposited films to the substrate, decrease internal stress, and limit the hydrogen evolution reaction (HER). However, certain additives, such as thiourea as well as Cr(VI) compounds are highly hazardous [1,2]. Therefore, due to the increasing environmental awareness of the society, the electrolytes containing these constituents require replacement by green alternatives, such as Deep Eutectic Solvents (DESs).DESs, usually composed of quaternary ammonium salt and hydrogen bond donor, i.e. ethylene glycol, have wider electrochemical potential window than H2O, which in principle enables obtaining metallic coatings without the HER [3]. However, due to the low conductivity and high viscosity of DESs, it is necessary to use additives, i.e. nicotinic acid, improving these physico-chemical properties and enhancing the electrodeposition process. However, due to the unavoidable presence of water (DESs are hygroscopic), H2O can be considered an additive to DESs. The addition of H2O can be beneficial since it improves the ion mobility, and therefore increases the reduction rate [4]. The current studies focus on understanding the effect of water content on the physico-chemical properties, and the structure of DESs. However, a little is known on the influence of H2O on the electrodeposition of Ni and Cr, especially on the nucleation and growth, the effect of the morphology and chemical composition of deposited films. Therefore, an in-depth study on this topic is required.The electrolytic reduction of Ni and Cr cations in choline chloride – ethylene glycol (1ChCl:2EG) on glassy carbon and steel substrates was investigated in function of water content. This study was performed by means of electrochemical techniques, such as linear sweep voltammetry and chronoamperometry. The obtained deposits were analyzed with surface sensitive methods, i.e. scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In order to bring the understanding on the influence of H2O as an additive on the early stage electrodeposition of Ni and Cr from DES, a combination of computational modelling with UV-vis spectroscopy was involved. This approach allowed correlating the observed electrochemical behavior of Ni and Cr cations with certain water concentrations. Establishing such relationship is of high relevance, as it may enable optimizing the process parameters for Ni and Cr electroreduction from DESs. [1] N. V. Mandich and D. L. Snyder. Modern Electroplating, chapter Electrodeposition of Chromium, pages 205–248. John Wiley & Sons, Inc., New Jersey, 5 edition, 2010.[2] G. DiBari. Modern Electroplating, chapter Electrodeposition of Nickel, pages 79-114. John Wiley & Sons, Inc., New Jersey, 5 edition, 2010.[3] E.L. Smith, A.P. Abbot, K.S. Ryder, Deep Eutectic Solvents (DESs) and Their Application, American Chemical Society Publications, 2014, pp. 11060 – 11082.[4] M. Lukaczynska, E.A. Mernissi Cherigui, A. Ceglia, K. Van den Bergh, J. De Strycker, H. Terryn, and J. Ustarroz, Influence of water content and applied potential on the electrodeposition of Ni coatings from deep eutectic solvents, Electrochimica Acta, 319: 690-704, 2019.

Deep eutectic solvents are the new emerging solvents formed by the combination of hydrogen bond acceptor and hydrogen bond donor. This type of solvent is still under development for possible industrial application including in chemistry, biotechnology, and pharmaceutical processes. The deep eutectic solvents have attracted much attention because they are characterized as greener solvents when compared to the currently used volatile organic solvents. Deep eutectic solvents are to replace the ionic liquids, which are posed as green solvents, however, their toxic nature has turned to be its drawbacks. The deep eutectic solvents attracted great interest due to their unique properties such as biodegradability, thermal stability, less toxicity, easy and cheap to prepare. This work explores the activity coefficients at infinite dilution of deep eutectic solvents. The deep eutectic solvents under investigation were carefully synthesized using hydrogen bond donors (HBD) and hydrogen acceptors (HBA) at a specific ratio. The analysis of these prepared deep eutectic solvents were analysed using spectroscopic techniques (FTIR and NMR) to confirm the formation of deep eutectic solvents and the type of interaction occurring between the HBD and the HBA. Additionally, thermal stability of the prepared deep eutectic solvents was investigated. The DESs were then used to measure the activity coefficients at infinite dilution for volatile organic compounds (alkanes, alkene, alkynes, aromatic hydrocarbons, ketones, acetonitrile, tetrahydrofuran, alcohols and thiophene) using the chromatography technique. The activity coefficients at infinite dilutions were conducted over a range of temperature (313.15 - 353.15) K. The prepared deep eutectic solvents for this include.  DES1 {1- butyl-2,3-dimethylimidazolium chloride + ethylene glycol (1:3)}  DES2 {1-butyl-2,3-dimethylimidazolium chloride + diethylene glycol (1:2)}  DES3 {1-butyl-2,3-dimethylimidazolium tetrafluoroborate + ethylene glycol (1:3)}  DES4 {1 -butyl-2,3-dimethylImidazolium tetrafluoroborate + diethylene glycol (1:3)) The study also focuses on understanding the behaviour of these DES through a comprehensive analysis of their thermophysical characteristics and their ability to dissolve solutes at infinite dilution. The investigation revealed intriguing trends in the solvation behaviour of different classes of solutes within the DES. Alkanes exhibited higher activity coefficients, with a clear dependence on the alkyl chain length. Cyclic hydrocarbons showed distinct behaviour due to stronger interactions with the imidazolium ring. Alkynes demonstrated the lowest activity coefficients, attributed to the presence of triple bonds influencing solute-solvent interactions. Aromatic hydrocarbons exhibited unique solvation behaviour influenced by the delocalized pi- π- electrons on the benzene ring. To use the deep eutectic solvents at an industrial level, it is highly imperative to understand the intermolecular interactions and properties of the pure deep eutectic solvents and their mixtures with volatile organic solutes. The prepared deep eutectic solvents for thermophysical properties include.  DES5 (1-butyl-1-methylpyrrolidinium bromide + ethylene glycol)  DES6 (1-butyl-3-methylimidazolium chloride + ethylene glycol) Thermophysical properties, such as densities, speed of sound, and refractive indices were measured as a function of temperature. The study investigated the binary mixtures containing (DES5 + acetic acid or ethanol) and (DES 6 + acetic acid or ethanol). These were investigated at temperatures ranging between (293.15 and 313.15) K and at atmospheric pressure (0.1MPa) over a range of mole fraction (𝑥1= 0 to 1) as a function of DES. The measured property values were used to compute the excess properties such as excess molar volumes, isentropic compressibilities, deviation in refractive indices, deviation in isentropic compressibilities and intermolecular free length. The data obtained provides insights into the molecular interactions within the DESs, shedding light on their structural arrangements and overall stability. This study contributes to the fundamental understanding of deep eutectic solvents, offering a detailed exploration of their thermophysical properties and their solvation behaviour at infinite dilution. The findings have implications for the design and optimization of DESs for various applications, including their use as green solvents in chemical processes and separations.

Deep eutectic solvents (DESs) have been used as electrolytes in the redox flow battery due to their unique advantages including non-toxicity, biodegradability, and low cost. The most commonly used DESs are the reline DES system (a DES formed from a molar 1:2 mixture of choline chloride and urea) and the ethaline DES system (a DES formed from a molar 1:2 mixture of choline chloride and ethylene glycol). The difference in the component of DES will result in different electrochemical characteristics of the redox couples dissolved therein. In this work, the electrochemical properties of iron ions in ethaline and reline DESs, as well as those of iron ions with FeCl3 and FeCl3·6H2O dissolved in ethaline DES, were experimentally studied. The cyclic voltammetry (CV) results showed that the diffusion coefficient of iron ions in the ethaline DES is greater than that of the reline DES; the redox reversibility of iron ions with FeCl3·6H2O dissolved in ethaline is higher than that of FeCl3 in the same DES, and the reaction rate constant is larger than that of the former, while the diffusion coefficient of the former is slightly smaller than that of the latter. These differences stem from the intrinsic valence discrepancy of FeCl3·6H2O and FeCl3, which is confirmed by the FT-IR spectra analysis. Electrochemical impedance spectroscopy (EIS) also showed that the total resistance in the ethaline solution with FeCl3·6H2O was significantly lower than that with FeCl3. The results of this work can provide key parameters for electrochemical process research and numerical simulation studies on DES.

The electrocatalytic behavior of electrodeposited Ni and Ni–Mo alloy coatings in the hydrogen evolution reaction in a 1 M NaOH aqueous solution was investigated by means of the electrochemical impedance spectroscopy method. The electrochemical deposition of electrocatalytic coatings was carried out using electrolytes based on deep eutectic solvents (eutectic mixtures of choline chloride with ethylene glycol or urea). To simulate the recorded Nyquist plots reflecting the electrocatalytic performance of deposited coatings, a modified Armstrong-Henderson equivalent circuit was employed, which accounts for the involvement of adsorbed intermediates in the reaction. The equivalent circuit included three polarization resistances and three constant phase elements, allowing for the consideration of the localization of the electrochemical process on different surface microdomains. It was found that the electrocatalytic activity of nickel coatings deposited from deep eutectic solvents exceeded the activity of nickel fabricated in an aqueous electrolyte. The increase in molybdenum content in the coating was shown to enhance electrocatalytic activity. It was established that the main reasons for improving the electrocatalytic properties of the Ni–Mo alloy coatings are structural-morphological factors (increase in the degree of microheterogeneity of the surface and the development of the surface area available for electrochemical reaction) and the formation of a favorable electronic structure of the metal, leading to the acceleration of the rate-determining Volmer step.

Common solvents used for aromatic extraction from aliphatics typically degrade into toxic compounds, while green alternatives perform poorly compared to the state-of-the-art solvents. Deep eutectic solvents (DES) are a novel solvent type made of hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA). DES have been applied in various applications, including advanced separations. In this study, DES were studied experimentally and using the Conductor-like Screening Model (COSMO) to separate benzene from cyclohexane as model compounds for an aromatic:aliphatic system. Both equilibrium and kinetic studies were performed to determine the liquid liquid equilibrium (LLE) and mass transfer rate for the DES-based separation. Selected HBAs including tetrabutylammonium bromide (N4444Br), tetrahexylammonium bromide (N6666Br), choline chloride (ChCl), and methyltriphenylphosphonium bromide (METPB) were paired with HBDs including ethylene glycol (EG) and glycerol (Gly). COSMO was used, with adjustments to reflect DES specific interactions, to predict the liquid-liquid equilibrium (LLE). COSMO results showed that ChCl and N6666Br-based DES extracted too little benzene or too much cyclohexane, respectively, to be considered for experimental evaluation. Overall, the COSMO model predictions for LLE of EG-based DES were very accurate, with root-mean-square deviations (RMSD) below 1% for both N4444Br:EG and METPB:EG. The glycerol systems were less accurately modeled, with RMSD’s of 4% for N4444Br:Gly and 6% for METPB:Gly. The lower accuracy of glycerol system predictions fmay be due to limitations in COSMO for handling glycerol’s influence on polarizability in the DES that is not seen in EG-based DES. Mass transfer kinetics were determined experimentally for DES and the results were fit to a first order kinetics model. METPB:Gly had the highest mass transfer coefficient at 0.180 min−1, followed by N4444Br:EG at 0.143 min−1. N4444Br:Gly and METPB:EG had the lowest mass transfer coefficients at 0.096 min−1 and 0.084 min−1, respectively. It was found that mass transfer rate was not directly related to maximum benzene solubility, as N4444Br:EG and METPB:Gly had the highest and lowest benzene removal, respectively, but had similar mass transfer coefficients.

The main challenge of large-scale biofuel production is related to the extraction of its undesired impurities including glycerol, water, methanol, soap/catalyst, free fatty acids, glycerides, and others. There are many ways to remove glycerol, and herein, the one alternative is the extraction of glycerol from biodiesel by deep eutectic solvents. In this regard, the mixture of a choline chloride (ChCl) and urea, methyltriphenylphosphonium chloride (MTPPCl), and ethylene glycol (EGL), as a deep eutectic solvent (DES), is effective in removing glycerol from biofuel. In this work, we have investigated the formation mechanism of ChCl and urea, and then MTPPCl and EGL, as a DES, and then extraction of glycerol from biofuel via DES implementing density functional theory (DFT)by Gaussian09 software, B3LYP basis set, and classical all-atom molecular dynamics (MD) simulationsby Gromacs software, GROMOS force field. DFT approximation demonstrates that Cl ion plays an important binding role in the formation of complexes ChCl/urea-based DES + biofuel and in MTPPCl/EGL-based DES + biofuel. We have also considered the formation and change of hydrogen bonds upon the formation of these systems using the DFT method. Large HOMO-LUMO gaps in ChCl/urea-based DES + biofuel and in MTPPCl/urea-based DES + biofuel demonstrate the stability of the complexes. The results of MD work have stated that the chloride ion formed bonding with the choline/ethylene glycol EGL, while still weakly intermolecular interacting with the urea/methyltriphenylphosphonium in ChCl/urea- and MTPPCl/EGL-based DESs. Further results of MD simulations stated that the DESs had a higher intermolecular interaction with glycerol in comparison with biofuel, thereby favoring the extraction process of glycerol from model biofuel. • Intermolecular interactions of choline chloride and urea, methyl triphenyl phosphonium chloride, and ethylene glycol-based DESs and their applications in the extraction of glycerol from biofuel studied by DFT calculations and classical all-atom molecular dynamics simulations. • Calculated outputs of DFT calculations and classical all-atom molecular dynamics simulations for DESs and their applications in the extraction of glycerol from biofuel were discussed in detail. • The molecular formation mechanism of choline and methyl triphenyl phosphonium-based DESs and their application in the extraction process of glycerol from biofuel were summarized.

Physicochemical properties of choline chloride-based deep eutectic solvents and excess properties of their pseudo-binary mixtures with 1-butanol

Because of increasing atmospheric anthropogenic ammonia (NH3) emission, researchers are devising new techniques to capture NH3. Deep eutectic solvents (DESs) are found as potential media for NH3 mitigation. In the present study, we have carried out ab initio molecular dynamics (AIMD) simulations to decipher the solvation shell structures of an ammonia solute in reline (1:2 mixture of choline chloride and urea) and ethaline (1:2 mixture of choline chloride and ethylene glycol) DESs. We aim to resolve the fundamental interactions which help stabilize NH3 in these DESs, focusing on the structural arrangement of the DES species in the nearest solvation shell around NH3 solute. In reline, the hydrogen atoms of NH3 are preferentially solvated by chloride anions and the carbonyl oxygen atoms of urea. The nitrogen atom of NH3 renders hydrogen bonding with hydroxyl hydrogen of the choline cation. The positively charged head groups of the choline cations prefer to stay away from NH3 solute. In ethaline, strong hydrogen bonding interaction exists between the nitrogen atom of NH3 and hydroxyl hydrogen atoms of ethylene glycol. The hydrogen atoms of NH3 are found to be solvated by hydroxyl oxygen atoms of ethylene glycol and choline cation. While ethylene glycol molecules play a crucial role in solvating NH3, the chloride anions remain passive in deciding the first solvation shell. In both the DESs, choline cations approach NH3 from their hydroxyl group side. We observe slightly stronger solute-solvent charge transfer and hydrogen bonding interaction in ethaline than those in reline.

Deep eutectic solvents (DESs) and ternary deep eutectic solvents (TDESs) and are derived from two or more salts as the hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs). In this work, the several DESs and a TDES were prepared. Allyltriphenyl phosphonium bromide and potassium carbonated were selected as HBAs to mix with various HBDs such as glycerol (GL), ethylene glycol (EG), diethylene glycol (DEG) and triethylene glycol (TEG) into different molar ratios. Two different groups of bacteria were selected for investigation of toxicity of species, namely Escherichia coli (EC) as a Gram negative bacterium and Listeria monocytogenes (LM) as a Gram positive bacterium. The results revealed that by increasing alkyl chain length on HBD, the toxicity of DESs increases towards EC bacterium. However, there appears to have no direct relationship between effect of alkyl chain length and toxicity DESs towards LM bacterium. Moreover, by studying the effect of molar ratio the toxicity of DESs, it was observed that there is no direct relationship between them. Furthermore, it was found that the type of HBD has a dominant effect on the toxicity of DESs compared to type of salt. By comparing the toxicity of DESs in this work with that of ILs in literature, it was found that DESs are less toxic than ILs. The last interesting result of this work is that TDES exhibited the lowest toxicity on EC and LM bacteria compared with DESs.

This study was undertaken in the aim to try the limit of extraction of Zn from Zn-Ni system. The aim was realized by the addition of MoO42- ions into the galvanic bath containing Ni2+ and Zn2+ ions. Zn-Ni-Mo layers were deposited under galvanostatic conditions on (OH18N9) austenitic steel substrate. The influence of Na2MoO4 concentration in a bath on the surface morphology, chemical and phase composition and the corrosion resistance of obtained layers, was investigated. The properties of Zn-Ni-Mo layers were compared to the properties of electrolytic Zn-Ni layer. Structural investigations were performed by the X-ray diffraction (XRD) method. The surface morphology and chemical composition and surface chemical elements distribution of deposited layers were studied using a scanning electron microscope. Electrochemical corrosion resistance investigations were done by classical Stern method and electrochemical impedance spectroscopy. The potentiodynamic curves in the range of  0.05V to the potential of open circuit, were obtained. On the base of these curves the parameters like corrosion potential- Ecor, corrosion current density- icor and the polarization resistance- Rp were determined. These values served as a measure of the corrosion resistance of obtained layers. Results of impedance investigations were presented on the Nyquist Z”= f (Z’) and the Bode log Z = f (log) and  = f (log), diagrams. On the basis on this research, it was exhibited that surface morphology, chemical composition of Zn-Ni-Mo layers are dependent on Mo contents. The optimal content of Na2MoO4 in the bath for the sake of corrosion resistance in 5% NaCl, is found to be 1.2 gdm-3.

The interest on deep eutectic solvents (DES) has been increasing. However, the ecotoxicological profile of DES is scarcely known. Also, despite previous studies showed that DES components dissociate in water, none assessed DES toxicity using the classical and adequate models for mixture toxicity prediction - concentration addition (CA) and independent action (IA). This study evaluates the ecotoxicological profile of DES based on [N1111]Cl, [N2222]Cl and [N3333]Cl as hydrogen bond acceptors (HBA) combined with hydrogen-bond donors (HBD) vis. ethylene glycol and 1-propanol, through the Microtox® Acute Toxicity Test. CA and IA with deviations describing synergism/antagonism, dose-ratio and dose-level effects were fitted to the toxicity data. Neither the starting materials nor DES were found hazardous to Aliivibrio fischeri, in this specific case agreeing with the claimed “green character” of DES. Among the starting materials, ethylene glycol was the least toxic, whereas [N3333]Cl was the most toxic (30 min-EC50 = 96.49 g L−1 and 0.5456 g L−1, respectively). DES toxicity followed the same trend as observed for the salts: [N1111]Cl-based DES < [N2222]Cl-based DES < [N3333]Cl-based DES. The IA model, with specific deviations, adjusted better in 5 out of 6 DES. Antagonism was observed for [N1111]Cl-based DES, and synergism for [N3333]Cl-based DES and for 1-propanol:[N2222]Cl. The application of the mixture toxicity models represents a breakthrough in the problematic of assessing the toxicity of the countless number of DES that can be created with the same starting materials, since they provide the expected toxicity of any virtual combination between HBA and HBD.

Deep eutectic solvents (DESs) have been widely used to capture CO2 in recent years. Understanding CO2 mechanisms by DESs is crucial to the design of efficient DESs for carbon capture. In this work, we studied the CO2 absorption mechanism by DESs based on ethylene glycol (EG) and protic ionic liquid ([MEAH][Im]), formed by monoethanolamine (MEA) with imidazole (Im). The interactions between CO2 and DESs [MEAH][Im]-EG (1:3) are investigated thoroughly by applying 1H and 13 C nuclear magnetic resonance (NMR), 2-D NMR, and Fourier-transform infrared (FTIR) techniques. Surprisingly, the results indicate that CO2 not only binds to the amine group of MEA but also reacts with the deprotonated EG, yielding carbamate and carbonate species, respectively. The reaction mechanism between CO2 and DESs is proposed, which includes two pathways. One pathway is the deprotonation of the [MEAH]+ cation by the [Im]− anion, resulting in the formation of neutral molecule MEA, which then reacts with CO2 to form a carbamate species. In the other pathway, EG is deprotonated by the [Im]−, and then the deprotonated EG, HO-CH2-CH2-O−, binds with CO2 to form a carbonate species. The absorption mechanism found by this work is different from those of other DESs formed by protic ionic liquids and EG, and we believe the new insights into the interactions between CO2 and DESs will be beneficial to the design and applications of DESs for carbon capture in the future.