Effects of van der Waals interaction on the N$_2$ adsorption on carbon nanotubes: proposal of new force field parameters

TL;DR

This study investigates the impact of van der Waals interactions on N$_2$ adsorption in carbon nanotubes and introduces new force field parameters to improve simulation accuracy.

Contribution

The paper develops and validates a new Lennard-Jones potential based on DFT data, surpassing Lorentz-Berthelot rules in modeling N$_2$-carbon interactions.

Findings

LB underestimates interaction energies

New force field improves agreement with ab-initio calculations

LB affects predictions of nitrogen adsorption isotherms

Abstract

The separation of carbon dioxide CO$_2$ from nitrogen gas (N$_2$), the main component of flue gas, has become an emerging action to mitigate climate change. Feasible and efficient approaches to exploring the separation properties of materials are molecular dynamics (MD) and Monte Carlo (MC) simulations. In these approaches, a careful choice of force fields is required to avoid unrealistic predictions of thermodynamic properties. However, most studies use Lorentz-Berthelot combining rules (LB) to obtain the interaction between different species, an approximation that could not capture the essence of interfacial interactions. In this context, we verified how accurate LB is in describing the interaction of N$_2$ molecules and carbon nanostructures by comparing the interaction energies from LB with those from density functional theory (DFT) calculations. We selected carbon nanomaterials because they are considered promising materials to perform N$_2$/CO$_2$ separation. The results show that the LB underestimates the interaction energies and affects the prediction of fundamental properties of solid-fluid interfacial interactions. To overcome this limitation, we parametrized a Lennard-Jones potential using energies and forces from DFT, obtained through the van der Waals functional KBM. The proposed potential show good transferability and agreement to ab-initio calculations. Grand Canonical Monte Carlo simulations were performed to verify the effects of employing LB in predicting the amount of nitrogen gas adsorbed inside different CNTs. LB predicts a lower density inside them. Moreover, our results suggest that LB leads to a different characterization of the adsorption properties of carbon nanotubes, by changing significantly the adsorption isotherm.