  In the last decades a large effort has been devoted to the study of water
confined in hydrophobic geometries at the nanoscale (tubes, slit pores),
because of the multiple technological applications of such systems, ranging
from drugs delivery to water desalinization devices. To our knowledge, neither
numerical/theoretical nor experimental approaches have so far reached a
consensual understanding of structural and transport properties of water under
these conditions. In this work, we present molecular dynamics simulations of
TIP4P/2005 water under different hydrophobic nano-confinements (slit pores or
nanotubes, with two degrees of hydrophobicity) within a wide temperature range.
On the one side, water is more structured near the hydrophobic walls,
independently on the confining geometries. On the other side, we show that the
combined effect of confinement and curvature leads to an enhanced diffusion
coefficient of water in hydrophobic nanotubes. Finally, we propose a confined
Stokes-Einstein relation to extract viscosity from diffusivity, whose result
strongly differs from the Green-Kubo expression that has been used in previous
work. We discuss the shortcomings of both approaches, which could explain this
discrepancy.
