Molecular dynamics simulations of the aqueous interface with the [BMI][PF6] ionic liquid: comparison of different solvent models

Abstract

We report a Molecular Dynamics (MD) study of the interface between water and the hygroscopic room temperature Ionic Liquid "IL" [BMI][PF6] (1-butyl-3-methyl-imidazolium hexafluorophosphate), comparing the TIP3P, SPC/E and TIP5P models for water and two IL models where the ions are +/-1 or +/-0.9 charged. A recent MD study (A. Chaumont, R. Schurhammer and G. Wipff, J. Phys. Chem. B, 2005, 109, 18964) showed that using TIP3P water in conjunction with the IL(+/-1) model led to water-IL mixing without forming an interface, whereas a biphasic system could be obtained with the IL(+/-0.9) model. With the TIP5P and SPC/E models, the juxtaposed aqueous and IL phases are found to remain distinct for at least 20 ns. The resulting IL humidity, exaggerated with the IL(+/-1) model, is in better agreement with experiment using the IL(+/-0.9) model. We also report demixing simulations on the "randomly mixed" liquids, using the IL(+/-0.9) model for the ionic liquid. With the three tested water models, the phases separate very slowly ( approximately 20 ns or more) compared to "classical" chloroform-water mixtures (less than 1 ns), leading to biphasic systems similar to those obtained after equilibration of the juxtaposed liquids. The characteristics of the interface (size, polarity, ion orientation, electrostatic potential) are compared with the different models. Possible reasons why, among the three tested water models, the widely-used TIP3P model exaggerates the inter-solvent mixing, are analyzed. The difficulty in computationally and experimentally equilibrating water-IL mixtures is attributed to the slow dynamics and micro-heterogeneity of the IL and to the different states of water in the IL phase.

Application of Static Charge Transfer within an Ionic‐Liquid Force Field and Its Effect on Structure and Dynamics

“…[40,41] Herein, we systematically investigate the effects on structure and dynamics of the molecular ionic charges. Experimental structural studies on ionic liquids are uncommon.…”

Strontium Nitrate Extraction to Ionic Liquids by a Crown Ether: A Molecular Dynamics Study of Aqueous Interfaces with C4mim+- vs C8mim+-Based Ionic Liquids

“…The final views of these systems ( Figure 7) show that, in spite of the long simulated times, the final states markedly differ from those obtained from juxtaposed phases. As observed for neat water/IL mixtures, 50 For systems I and J based on the hydrophobic [C 8 mim][PF 6 ] liquid, one observes a better, but still incomplete phase separation. In the J system with [Sr⊂18C6,(NO 3 ) 2 ] complexes, 3 complexes finally sit in the IL phase and the 6 others concentrate at the interface.…”

“…The simulated IL/water biphasic strontium solutions differ from the chloroform/water analogues by a number of features. First, as for neat interfaces, 50 note the difficulty of equilibrating the IL biphasic systems. In spite of our computational efforts, simulations that started from juxtaposed liquids did not converge to the same separated phases, compared to those obtained from artificially mixed liquids (mixing-demixing simulations), due to the viscosity of the IL, the slow diffusion of its components, and to the stronger interactions of the IL with water, compared to classical organic molecules.…”

Room temperature ionic liquids: A simple model. Effect of chain length and size of intermolecular potential on critical temperature

“…17 All atom molecular simulations of RTIL [18][19][20][21][22][23] have been used to understand their relation with different solvents, specially water. The relations among the factors that affect the behavior of such substances, principally their lower melting point, are difficult to elucidate with these all atom models due to the large number of parameters involved, mainly the molecular structure and the force fields.…”