EPJ E Review - Water and ionic liquids. Two very different solvents, two intriguing behaviours when nanoconfined

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Published on 28 February 2017

Confinement of liquids at the nanoscale gives rise to intriguing new chemical and physical behaviours and structures. Scientists are studying the phenomenon also because of its relevance to molecular biology (permeability of ion channels and protein stability), chemical engineering (nano-fluidic devices and molecular sieves) and geology (transport through porous rocks).

In this topical review in EPJ E, Masumeh Foroutan and colleagues review and elucidate through molecular dynamics (MD) simulations some salient aspects of water and ionic liquids, nanoconfined within carbon nanotubes, between sheets of graphene or graphene oxide, between hydrophobic self-assembled monolayers or in porous silica matrices.

Depending on the confinement dimensions, the cavity surface and also pressure and temperature, the state of the confined water and its physical properties have been proven to be varied and interesting. Computer simulations are a convenient tool to study the water and ILs in nano-confinement because they can give a microscopic picture of the process. They avoid some of the experimental difficulties associated with the observation of the structures and ultimately afford a theoretical understanding of the effects that play a role in the water and ILs in nano-confinement.

In a water system, the authors explain that the microscopic static structure of water inside nanosized cavities changes because of a change in the hydrogen bonding network of water. Confined water has a heterogeneous structure with water molecules having different orientations at various distances from the confining surfaces.

In the case of ionic liquids in nanoscale confinement, by comparing several recent MD simulations, some general features of ion arrangements and dynamics emerge. The results indicate a layering behavior of the anion and cation for the ionic liquid [emim][BF4] at different distances between two graphene sheets. Also, cations and anions tended to form aggregates around each other. The diffusion values of the cations and anions increased as the temperature and distances between the two graphene sheets increased.