Aqueous solutions of binary ionic liquids: insight into structure, dynamics, and interface properties by molecular...

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Ionic liquids (ILs) exhibit amphiphilic properties, making their aqueous solutions crucial for fundamental understanding and applications.
• Understanding the influence of water on IL mixtures is essential for designing new materials and processes.

Purpose of the Study:

• To investigate the effect of water on intermolecular interactions within three binary ionic liquid mixtures: [C4mim]/[Cl]/[BF4], [C4mim]/[Cl]/[PF6], and [C4mim]/[BF4]/[PF6].
• To analyze structural perturbations in both bulk and interfacial regions of IL-water systems.

Main Methods:

• Molecular dynamics (MD) simulations were employed to model the behavior of IL-water mixtures.
• Density functional theory (DFT) calculations were used to determine the electronic structures and energies of model IL systems.
• Analysis included distribution functions, hydrogen-bond statistics, density profiles, and bivariate distribution calculations.

Main Results:

• Water addition drastically alters anion-cation interactions, with anion-water interactions becoming dominant, primarily through hydrogen bonding.
• Chloride-containing systems exhibit strong electrostatic interactions, while the hydrophobic [C4mim]/[BF4]/[PF6] system stabilizes via dispersive forces, leading to significant water segregation.
• Hydrophobic anions, particularly hexafluorophosphate ([PF6]-), were observed to segregate towards the liquid/vacuum interface.

Conclusions:

• Anion-water interactions are key drivers in aqueous IL mixtures, surpassing anion-cation interactions.
• The hydrophobic nature of certain anions dictates their interfacial behavior and influences the overall structure of the IL-water system.
• DFT results consistently support the findings from MD simulations regarding bulk properties and interactions.