Absence of a Diffusion Anomaly in Water Perpendicular to Hydrophobic Nanoconfining Walls

We perform molecular dynamics simulations to investigate the diffusive motion of TIP5P water in the direction perpendicular to the two hydrophobic confining walls. To calculate the diffusion coefficient, we use the concept of the characteristic residence time which is calculated from the exponential decay of the residence time probability distribution function. We find that a diffusion anomaly of water, 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 may arise mainly due to nanoconfinement, rather than due to the hydrophobic property of the confining walls.