Influence of molecular shape on self-diffusion under severe confinement: A molecular dynamics study

TL;DR

This study uses molecular dynamics simulations to explore how molecular shape and charge asymmetry influence the diffusion behavior of hydrocarbons confined in ZSM-5 pores, revealing shape-dependent restrictions and rotational-translational coupling.

Contribution

It provides new insights into the impact of molecular shape and charge asymmetry on diffusion dynamics under severe confinement in ZSM-5, a topic less explored in prior research.

Findings

Most molecules exhibit sub-diffusive motion at >1 ps

Rotational-translational coupling is significant due to geometrical restrictions

Molecular shape and charge asymmetry impose varying restrictions inside ZSM-5

Abstract

We have investigated the effect of molecular shape and charge asymmetry on the translation dynamics of confined hydrocarbon molecules having different shapes but similar kinetic diameters, inside ZSM-5 pores using molecular dynamics simulations. The mean square displacement of propane, acetonitrile, acetaldehyde, and acetone in ZSM-5 exhibit two different regimes - ballistic and diffusive/sub-diffusive. All the molecules except propane exhibit sub-diffusive motion at time scales greater than 1 ps. The intermediate scattering functions reveal that there is a considerable rotational- translational coupling in the motion of all the molecules, due to the strong geometrical restriction imposed by ZSM-5. Overall the difference in shape and asymmetry in charge imposes severe restriction inside the ZSM-5 channels for all the molecules to different extents. Further, the behavior of molecules confined in ZSM-5 in the present study, quantified wherever possible, is compared to their behavior in bulk or in other porous media reported in literature.