Ionic liquid confined in silica nanopores: molecular dynamics in the isobaric–isothermal ensemble

Abstract

Molecular dynamics simulations in the isobaric–isothermal ensemble are used to investigate the structure and dynamics of an ionic liquid confined at ambient temperature and pressure in hydroxylated amorphous silica nanopores. The use of the isobaric–isothermal ensemble allows estimating the effect of confinement and surface chemistry on the density of the confined ionic liquid. The structure of the confined ionic liquid is investigated using density profiles and structural order parameters while its dynamics is assessed by determining the mobility and ionic conductivity of the confined phase. Despite the important screening of the electrostatic interactions (owing to the small Debye length in ionic liquids), the local structure of the confined ionic liquid is found to be mostly driven by electrostatic interactions. We show that both the structure and dynamics of the confined ionic liquid can be described as the sum of a surface contribution arising from the ions in contact with the surface and a bulk-like contribution arising from the ions located in the pore centre; as a result, most properties of the confined ionic liquid are a simple function of the surface-to-volume ratio of the host porous material. In contrast, the ionic conductivity of the confined ionic liquid, which is a collective dynamical property, is found to be similar to the bulk. This study sheds light on the complex behaviour of hybrid materials made up of ionic liquid confined in inorganic porous materials.

Keywords:

• amorphous silica
• surface
• ionic liquids
• molecular dynamics

Acknowledgements

We are pleased to contribute this paper in honour of Pierre Turq on the occasion of his 70th birthday. Among his numerous scientific works, Pierre Turq has made significant contributions to the field of ionic solutions and adsorption and transport in clays.

Additional information

Funding

This work was supported by the French Research Agency ANR within the framework of the project Nanochalco [grant number ANR-11-BS08-015]; the center Informatique National de l’Enseignement Supérieur (CINES) for a generous allocation on the supercomputer YODA [grant number c2012096964].