Why are Fluid Densities So Low in Carbon Nanotubes?

Gerald Wang, Massachusetts Institute of Technology

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The equilibrium density of fluids under nanoconfinement – in particular, fluids adsorbed in carbon nanotubes (CNTs) – can differ substantially from their bulk density. This anomalous density plays an important role in many nanoengineering applications and in the field of computational nanofluidics, but to date there has been no simple model for quantifying this phenomenon. Using a mean-field approach to describe the energetic landscape near the CNT wall, we obtain analytical results describing the length scales associated with the layering observed at the interface of a Lennard-Jones fluid and a CNT. We also show that this approach can be extended to describe the multiple-ring structure observed in larger CNTs. When combined with molecular simulation results for the fluid density in the first two rings, this approach allows us to derive a closed-form prediction for the overall equilibrium fluid density as a function of CNT radius that is in excellent agreement with molecular dynamics simulations. We show how aspects of this theory can be extended to describe several key features of water confinement within CNTs and find good agreement with results from the literature. Finally, we discuss the application of these principles to modeling simple nanoscale flow phenomena.

Abstract Author(s): Gerald J. Wang, Nicolas G. Hadjiconstantinou