  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.
