Solubility Of Water In Ionic Liquids Research Articles

Comprehensive study on the impact of the cation alkyl side chain length on the solubility of water in ionic liquids

A comprehensive study on the phase behaviour of two sets of ionic liquids (ILs) and their interactions with water is here presented through combining experimental and theoretical approaches. The impact of the alkyl side chain length and the cation symmetry on the water solubility in the asymmetric [CN−1C1im][NTf2] and symmetric [CN−1CN−1im][NTf2] series of ILs (N up to 22), from 288.15K to 318.15K and at atmospheric pressure, was studied. The experimental data reveal that the solubility of water in ILs with an asymmetric cation is higher than in those with the symmetric isomer. Several trend shifts on the water solubility as a function of the alkyl side chain length were identified, namely at [C6C1im][NTf2] for asymmetric ILs and at [C4C4im][NTf2] and [C7C7im][NTf2] for the symmetric ILs. To complement the experimental data and to further investigate the molecular-level mechanisms behind the dissolution process, density functional theory calculations, using the Conductor-like Screening Model for Real Solvents (COSMO-RS) and the electrostatic potential-derived CHelpG, were performed. The COSMO-RS model is able to qualitatively predict water solubility as a function of temperature and alkyl chain lengths of both symmetric and asymmetric cations. Furthermore, the model is also capable to predict the somewhat higher water solubility in the asymmetric cation, as well as the trend shift as a function of alkyl chain lengths experimentally observed. Both COSMO-RS and the electrostatic potential-derived CHelpG show that the interactions of water and the IL cation take place on the IL polar region, namely on the aromatic head and adjacent methylene groups that explains the differences in water solubility observed for cations with different chain lengths. Furthermore, the CHelpG calculations for the isolated cations in the gas phase indicates that the trend shift of water solubility as a function of alkyl chain lengths and the difference of water solubility in symmetric may also result from the partial positive charge distribution/contribution of the cation.

Impact of the cation symmetry on the mutual solubilities between water and imidazolium-based ionic liquids

Aiming at the evaluation of the impact of the ionic liquids (ILs) cation symmetry on their phase behaviour, in this work, novel mutual solubilities with water of the symmetric series of [CnCnim][NTf2] (with n=1–5) were determined and compared with their isomeric forms of the asymmetric [CnC1im][NTf2] group. While the solubility of isomeric ILs in water was found to be similar, the solubility of water in ILs follows the same trend up to a maximum cation alkyl side chain length. For n≥4 in [CnCnim][NTf2] the solubility of water in the asymmetric ILs is slightly higher than that observed in the symmetric counterparts. The thermodynamic properties of solution and solvation derived from the experimental solubility data of ILs in water at infinite dilution, namely the Gibbs energy, enthalpy and entropy were used to evaluate the cation symmetry effect on the ILs solvation. It is shown that the solubility of ILs in water is entropically driven and highly influenced by the cation size. Accordingly, it was found that the ILs solubility in water of both symmetric and asymmetric series depends on their molecular volume. Based on these findings, a linear correlation between the logarithm of the solubility of ILs in water and their molar volume is here proposed for the [NTf2]-based ILs at a fixed temperature.

Quantitative structure–activity relationship for toxicity of ionic liquids to Daphnia magna: Aromaticity vs. lipophilicity

Water solubility of ionic liquids (ILs) allows their dispersion into aquatic systems and raises concerns on their pollutant potential. Again, lipophilicity can contribute to the toxicity of ILs due to increased ability of the compounds to cross lipoidal bio-membranes. In the present work, we have performed statistical model development for toxicity of a set of ionic liquids to Daphnia magna, a widely accepted model organism for toxicity testing, using computed lipophilicity, atom-type fragment, quantum topological molecular similarity (QTMS) and extended topochemical atom (ETA) descriptors. The models have been developed and validated in accordance with the Organization for Economic Co-operation and Development (OECD) guidelines for quantitative structure–activity relationships (QSARs). The best partial least squares (PLS) model outperforms the previously reported multiple linear regression (MLR) model in statistical quality and predictive ability (R2=0.955, Q2=0.917, Rpred2=0.848). In this work, the ETA descriptors show importance of branching and aromaticity while the QTMS descriptor ellipticity efficiently shows which compounds are influential in the data set, with reference to the model. While obvious importance of lipophilicity is evident from the models, the best model clearly shows the importance of aromaticity suggesting that more lipophilic ILs with less toxicity may be designed by avoiding aromaticity, nitrogen atoms and increasing branching in the cationic structure. The developed quantitative models are in consonance with the recent hypothesis of importance of aromaticity for toxicity of ILs.

An overview of the mutual solubilities of water–imidazolium-based ionic liquids systems

Ionic liquids (ILs) are a novel class of chemical compounds with interesting properties that are driving a lot of research in several fields. For ionic liquids to be effectively used as solvents in liquid–liquid extractions, the knowledge of the mutual solubilities between ILs and the second liquid phase is fundamental. Furthermore, while ILs cannot volatilise and lead to air pollution, even the most hydrophobic ones present some miscibility with water posturing secondary aquatic environmental risks. Despite the importance of the ILs and water systems, there are few extensive reports on their phase equilibria. The aim of this work is to discuss and understand the mutual solubilities of water and imidazolium-based ILs. The results indicate that these mutual solubilities are primarily defined by the anion followed by the cation alkyl side chain length. It was also found that the solubility of the studied ILs in water is more dependent on the ILs structural modifications than the solubility of water in those ILs. The substitution of the most acidic hydrogen in the imidazolium cation by a methyl group leads to different behaviours in both rich-phases, where the solubility of water in ILs showed to be more hydrogen bonding dependent. From the solubility results it can be concluded that the hydrophobicity of the anions increases in the order [BF 4] − < [CH 3(C 2H 4O) 2SO 4] − < [C(CN) 3] − < [PF 6] − < [N(SO 2CF 3) 2] − while the hydrophobicity of the cations increases, as expected, with the alkyl chain length increase.