Aqueous Interfaces with Hydrophobic Room-Temperature Ionic Liquids:  A Molecular Dynamics Study

Abstract

We report a molecular dynamics study of the interface between water and (macroscopically) water-immiscible room-temperature ionic liquids "ILs", composed of PF6(-) anions and butyl- versus octyl-substituted methylimidazolium+ cations (noted BMI+ and OMI+). Because the parameters used to simulate the pure ILs were found to exaggerate the water/IL mixing, they have been modified by scaling down the atomic charges, leading to better agreement with the experiment. The comparison of [OMI][PF6] versus [BMI][PF6] ILs demonstrates the importance of the N-alkyl substituent on the extent of solvent mixing and on the nature of the interface. With the most hydrophobic [OMI][PF6] liquid, the "bulk" IL phase is dryer than with the [BMI][PF6] liquid. At the interface, the OMI+ cations retain direct contacts with the bulk IL, whereas the more hydrophilic PF6(-) anions gradually dilute in the local water micro-environment and are thus isolated from the "bulk" IL. The interfacial OMI+ cations are ordered with their imidazolium moiety pointing toward the aqueous side and their octyl chains toward the IL side of the interface. With the [BMI][PF6] liquid, the system gradually evolves from an IL-rich to a water-rich medium, leading to an ill-defined interfacial domain with high intersolvent mixing. As a result, the BMI+ cations are isotropically oriented "at the interface". Because the imidazolium cations are more hydrophobic than the PF6(-) anions, the charge distribution at the interface is heterogeneous, leading to a positive electrostatic potential at the interface with the two studied ILs. Mixing-demixing simulations on [BMI][PF6]/water mixtures are also reported, comparing Ewald versus reaction field treatments of electrostatics. Phase separation is very slow (at least 30 ns), in marked contrast with mixtures involving classical organic liquids, which separate in less than 0.5 ns at the microscopic level. The results allow us to better understand the specificity of the aqueous interfaces with hydrophobic ionic liquids, compared with classical organic solvents, which has important implications as far as the mechanism of liquid-liquid ion extraction is concerned.

Intrinsic Structure of the Interface of Partially Miscible Fluids: An Application to Ionic Liquids

“…It has also been shown that the reduced ionic character can be accounted for in an effective way by setting ionic net charges, in absolute terms, to less than unity leading to the emergence of a series of new "reduced charge models". 17,[53][54][55][56] Note that these models treat the effect of polarization in a simplified manner by fixed reduced ionic charges which makes the model unable to adapt to local 8 changes of the electric field. As the degree of polarization varies significantly from one ionic liquid to another these modified force fields are designed to accurately describe one single ionic liquid species, thus precluding general transferability.…”

The simulation of imidazolium-based ionic liquids†

“…[13] By 2004 a range of potentials based upon each of the main force field codes had appeared, some also offering improved flexibility and a more detailed representation of the cation and anion. [11,12,15,17,21, 34] At the same time "experiments" in modelling charge transfer [24,25], and including polarization effects [73] had been carried out. In some cases simplified potentials have been strategically chosen to allow for long simulation times of large volumes (864 ion pairs for 16ns).…”

“…[16] Force fields for more complex anions have appeared. [9,10] The force fields, now tested on the neat liquids, have begun to be used to explore interfacial [25,47,49] and solvation phenomena. [50,53,62] Most recently ab-initio MD methods have been used to examine the neat ionic liquid [C 1 C 1 im]Cl.…”

“…[18][19][20] The extension of the force fields to treat mixed systems has highlighted some deficiencies, [16,22] particularly in regard to charge transfer effects. [25] Comparison with ab-initio methods has also highlighted a deficiency in the description of hydrogen bonding between the cations and anions. [18][19][20] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 …”

“…In a later study a number of force field models were examined and it was concluded that charges would need scaling before accurate mixing dynamics could be obtained. [25] The effects of charge scaling on solute-solvent interactions have also been examined; only minor effects on solvation patterns were determined after the point charges of a neutral solute were halved, [39] and thus it appears that for a solute the overall charge is more important than the internal charge distribution.…”