  We compared the diffusion of water confined in armchair and zigzag carbon
nanotubes for rigid and flexible water models. Using one rigid model,
TIP4P/2005, and two flexible models, SPC/Fw and SPC/FH, we found that the
number of the number of hydrogen bonds that water forms depends on the
structure of the nanotube, directly affecting the diffusion of water. The
simulation results reveal that due to the hydrophobic nature of carbon
nanotubes and the degrees of freedom imposed by the water force fields, water
molecules tend to avoid the surface of the carbon nanotube. This junction of
variables plays a central role in the diffusion of water, mainly in narrow
and/or deformed nanotubes, governing the mobility of confined water in a
non-trivial way, where the greater the degree of freedom of the water force
field, the smaller it will be mobility in confinement, as we limit the
competition between area/volume, and it no longer plays the unique role in
changing water diffusivity.
