Tuning Water Networks via Ionic Liquid/Water Mixtures.

Archana Verma,John P Stoppelman,Jesse G Mcdaniel

doi:10.3390/ijms21020403

Archana Verma, John P Stoppelman + Show 1 more

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https://doi.org/10.3390/ijms21020403

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Abstract

Water in nanoconfinement is ubiquitous in biological systems and membrane materials, with altered properties that significantly influence the surrounding system. In this work, we show how ionic liquid (IL)/water mixtures can be tuned to create water environments that resemble nanoconfined systems. We utilize molecular dynamics simulations employing ab initio force fields to extensively characterize the water structure within five different IL/water mixtures: [BMIM][BF], [BMIM][PF], [BMIM][OTf], [BMIM][NO] and [BMIM][TFSI] ILs at varying water fraction. We characterize water clustering, hydrogen bonding, water orientation, pairwise correlation functions and percolation networks as a function of water content and IL type. The nature of the water nanostructure is significantly tuned by changing the hydrophobicity of the IL and sensitively depends on water content. In hydrophobic ILs such as [BMIM][PF], significant water clustering leads to dynamic formation of water pockets that can appear similar to those formed within reverse micelles. Furthermore, rotational relaxation times of water molecules in supersaturated hydrophobic IL/water mixtures indicate the close-connection with nanoconfined systems, as they are quantitatively similar to water relaxation in previously characterized lyotropic liquid crystals. We expect that this physical insight will lead to better design principles for incorporation of ILs into membrane materials to tune water nanostructure.

Highlights

Water is a key component of numerous chemical, biological and geological systems and materials
We use the terminology “hydrophobic” to refer to the [BMIM+][PF6−] and [BMIM+][TFSI−] ionic liquid (IL), which phase separate at sufficient water content and “hydrophilic” to refer to [BMIM+][BF4−], [BMIM+][NO3−] and [BMIM+][OTf−] which are fully miscible with water [28]
The behavior of water in nanoconfined environments has long been a subject of interest to chemists and biochemists. Due to their amphiphilic character and strong Coulombic interactions, ionic liquids form mixtures with water that exhibit a variety of water networks, with water structures often resembling those in nanoconfined systems

Summary

Introduction

Water is a key component of numerous chemical, biological and geological systems and materials. The properties of water-containing systems are strongly influenced by water’s unique hydrogen bonding structure and large cohesive energy relative to its molecular weight, with these attributes giving rise to the universally important hydrophobic effect [1]. The surrounding chemical environment may perturb the structure and dynamic properties of water itself, which is significant when water exists in nanoconfinement. Examples of water in nanoconfinement are ubiquitous, including water in biological and/or artificial membranes, lyotropic liquid crystals, cavities in proteins and enzymes, carbon nanotubes, zeolites and metal organic frameworks [2,3,4,5,6,7]. Beyond academic interest, understanding the behavior of water in strongly perturbed environments has technological importance in membrane science, catalysis, electrochemistry, separations and medicine

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