Dynamic molecular ordering in multiphasic nanoconfined ionic liquids detected with time-resolved diffusion NMR

Abstract

Molecular motion in nanosized channels can be highly complicated. For example, water molecules in ultranarrow hydrophobic nanopores move rapidly and coherently in a single file, whereas by increasing the pore size they organize into coaxial tubes, displaying stratified diffusion. Interestingly, an analogous complex motion is predicted in viscous charged fluids, such as room temperature ionic liquids (RTILs) confined in nanoporous carbon or silica; however, experimental evidence is still pending. Here, by combining ¹H NMR diffusion experiments in different relaxation windows with molecular dynamics simulations, we show that the imidazolium-based RTIL [BMIM]⁺[TCM]⁻, entrapped in the MCM-41 silica nanopores, exhibits an intricate dynamic molecular ordering; adsorbed RTIL molecules form a fluctuating charged layer near the pore walls, while in the bulk pore space they diffuse discretely in coaxial tubular shells, with molecular mean square displacement following a nearly ∼τ^0.5 time dependence, characteristic of single file diffusion.

Molecular motion in nanosized pores can be extremely complex. Here, NMR diffusion experiments in different relaxation windows and molecular dynamics simulations suggest an unusual dynamic molecular ordering when an ionic liquid is confined in nanoporous silica.