Absence of a diffusion anomaly of water in the direction perpendicular to hydrophobic nanoconfining walls.

We perform molecular dynamics simulations to investigate the diffusive motion of TIP5P (the transferable intermolecular potential with five points) water in the direction perpendicular to the two hydrophobic confining walls. To calculate the diffusion constant, we use the concept of the characteristic residence time which is calculated from the exponential decay of the residence time probability density function. We find that a diffusion anomaly, increase of diffusion upon compression, is absent in the direction perpendicular to the confining walls down to the lowest temperature we simulate, 220K , whereas there is a diffusion anomaly, similar to that in bulk water, in the direction parallel to the walls. The absence of a diffusion anomaly in the direction perpendicular to the walls may arise mainly due to nanoconfinement, rather than due to the hydrophobic property of the confining walls. In addition, we find that the temperature dependence of the diffusion constant along the constant density path in the perpendicular direction shows a Vogel-Fulcher-Tammann form.