Why are Fluid Densities So Low in Carbon Nanotubes?

TL;DR

This paper develops an analytical model combined with simulations to explain why fluid densities are significantly lower in carbon nanotubes compared to bulk fluids, focusing on nanoconfinement effects.

Contribution

It introduces a mean-field analytical approach to predict fluid densities in CNTs, validated by molecular simulations and extendable to water confinement.

Findings

Analytical predictions match molecular dynamics results.

Layering effects influence fluid density in CNTs.

Model extends to water confinement scenarios.

Abstract

The equilibrium density of fluids under nanoconfinement can differ substantially from their bulk density. Using a mean-field approach to describe the energetic landscape near the carbon nanotube (CNT) wall, we obtain analytical results describing the lengthscales associated with the layering observed at the fluid-CNT interface. When combined with molecular simulation results for the fluid density in the layered region, this approach allows us to derive a closed-form prediction for the overall equilibrium fluid density as a function of the CNT radius that is in excellent agreement with molecular dynamics simulations. We also show how aspects of this theory can be extended to describe water confined within CNTs and find good agreement with results from the literature.