Ionic liquids at nonane-water interfaces: molecular dynamics studies

Molecular Dynamics Study of the Effect of Water on Hydrophilic and Hydrophobic Ionic Liquids

We performed molecular dynamics (MD) simulations of ionic liquid (IL)-water mixtures to investigate the effects of water addition. The IL cation 1-butyl-3-methylimidazolium ([C₄mim]) and the four anions, nitrate (NO₃), tetrafluoroborate (BF₄), hexafluorophosphate (PF₆), and bis(trifluoromethanesulfonyl)imide (NTf₂), were used to examine the effects of differences in hydrophobicity and anion size. The radial distribution functions of water-water have two different water content dependences. In NO₃ and BF₄ systems, the effect of water-anion-water structures connected by hydrogen bonds due to the strong interaction of anion-water is large. The growth of water clusters changes the peak heights of the radial distribution functions in PF₆ and NTf₂ systems. The increase in the diffusion constant is small in the NO₃ system but is not small in the other systems. The relaxation time of the anion-water hydrogen bonds in the NO₃ system is much longer than those in the other systems. It is the reason for the low water content dependency of the diffusion constants of the NO₃ system. The water constant dependences of structures in NO₃ and BF₄ systems are similar, but that in the diffusion constant is not. The BF₄ system shows hydrophilic features in the structural change and hydrophobic features in the water content dependences of the diffusion constant. The addition of water molecules provides various hydrophobicity/hydrophilicity of anions.

Formation of Aqueous Biphasic Systems with an Ionic Liquid Induced by Metallic Salts: Nanoscopic Views from Molecular Dynamics Simulations

The formation of aqueous biphasic systems (ABSs) based on aqueous ionic liquid (IL)/salt mixtures has been investigated via molecular dynamics simulations (with IL butyl-methyl-imidazolium triflate; salts NaCl, CsCl, SrCl₂, and EuCl₃). The analysis of ion distributions, solvation, and mutual interactions during the dynamics reveals the heterogeneity of all solutions due to ion segregation into mutually exclusive IL and salt domains, even in monophasic solutions ("ionic sociology"). Ion segregation and ABS formation are found to increase with (i) the salt content and (ii) the IL content, (iii) in the order Na⁺ < Sr²⁺ < Eu³⁺, and (iv) when the IL ion "polarity" is diminished, following experimental trends. The structuration of the solution is rationalized as a synergistic water transfer from the best donating ion pair (first hydration shell of hydrophobic moieties of IL ions) to the best accepting pair (M ⁿ⁺ and Cl^- ions, beyond their first shell). In ABSs, the IL- and salt-containing phases are linked by a well-defined "interface" that decreases in width when MCl _n becomes more hydrophilic and/or more concentrated. In the IL-rich phase of ABSs, the hydration of IL ions and their mutual interactions are shown to be similar to those displayed at aqueous interfaces.