Water Solubility Trends in Ionic Liquids: The Quantitative Structure–Property Relationship Model versus Molecular Dynamics

Abstract

The knowledge of water solubility in ionic liquids (ILs) is an important property with an impact on the design of many physical and chemical processes, like the purification of organic compounds or the establishment of decontamination procedures. The development of methods to predict or establish solubility trends in ILs is, therefore, extremely relevant, as it may avoid expensive and time-consuming experimental procedures. In this work, we compare results of water solubility in ILs predicted by a quantitative structure–property relationship (QSPR) model with trends found using aggregation studies in molecular dynamics (MD) simulation results. This study was performed for ILs combining the cations 1-butyl-1-methylpyrrolidinium and 1-butyl-1-methylmorpholinium, with the anions bis­(pentafluoroethylsulfonyl)­imide (BETI–), trifluoromethanesulfonate (TF–), and tetrafluoroborate (BF4 –). Both methods indicated that, at 298.15 K, the water solubility in ILs was almost independent of the investigated cations. However, if the IL is composed of a hydrophobic anion, a slight increase in the mixability of the IL with water may be observed if the cation can form H-bonds. The QSPR model indicated that the hydrophobic BETI– anion leads to solubilities (x H2O ∼ 0.33), approximately half of those predicted when the cations are combined with TF– and BF4 – anions (x H2O ∼ 0.60). The MD results suggested that this difference is essentially related to the ability of the water molecules to interact with the anion. This interaction involves the formation of networks of molecules, where H2O is completely solvated by anions. These structures make the formation of interactions between water molecules difficult, which are responsible for their segregation from solution and, therefore, to liquid–liquid phase separation. For the investigated ILs, the MD data also suggest that the solubility trends are inversely proportional to the number of “isolated” anions relative to ···AN–H2O–AN–H2O··· networks.

The Surface of Ionic Liquids in Water: From an Ionic Tug of War to a Quasi‐Ordered Two‐Dimensional Layer

“…The Pádua and Canongia [56,57] force field was used for the ionic liquids, while the TIP4P‐2005 [58] model was used for water. This non‐polarizable IL force field has been shown to be consistent for pure IL and water‐IL based systems [59] . As recently published, [36] the TIP4P‐2005 is suitable for studying interface phenomena involving IL aqueous solution, showing a good compromise between results and performance.…”

“…This non-polarizable IL force field has been shown to be consistent for pure IL and water-IL based systems. [59] As recently published, [36] the TIP4P-2005 is suitable for studying interface phenomena involving IL aqueous solution, showing a good compromise between results and performance. We simulated all systems at 283 K by using a thermostat based on the v-rescale [60] algorithm with a coupling time of 0.5 ps.…”

Imidazolium based ionic liquid-phase green catalytic reactions

“…[59][60][61][62][63][64][65] The number of water clusters changes the nanostructural organisation of IL spaces and affect the characteristics of this fluid. [66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81] The confined water in some ILs is activated and can act as a base or nucleophile and react, for example, with CO 2 forming carbonates. 82 Keeping track [62][63][64][65] of water content is very important, as the common anions BF 4…”

Perspectives in the Computational Modeling of New Generation, Biocompatible Ionic Liquids

“…These statistical models allow one to trace the structure–activity relationship (SAR, QSAR, etc. ) − avoiding altogether the need to perform actual time-consuming MD and use relatively cheap ab initio calculations on isolated constituents …”