The unusual dynamics of water under confinement were considered. The use of idealized slit pore carbons or carbon nanotubes having smooth energy landscapes failed to capture the influence of structural disorder inherent in real carbons. It was shown here that the irregular structure of such carbons critically influenced the dynamics and diffusion mode (single file, sub-diffusion, Fickian). The present molecular dynamics simulations, using a realistic hydrophobic carbon model based upon hybrid reverse Monte Carlo simulation of the structure of an activated carbon fiber, revealed the existence of a single-file diffusion mode between ballistic and Fickian modes in the narrowest pore regions of the material. This was not seen in simulations using model 2D slit pores. A rich variety of behaviors was found in this sub-diffusion regime, with the mean square displacement being proportional to tα; revealing that α varied significantly with temperature; especially at 273 to 350K. It reached a minimum value of 0.5 at 298K, corresponding to the single-file diffusion regime, and approached unity at 610K; corresponding to the Fickian mode. It was demonstrated that confinement effects led to the experimentally observed non-Arrhenius behavior of the water dynamics below 350K for both the realistic carbon model and an idealized 2-D slit pore model. This was attributed to the transport of the adsorbed water as a large cluster in a water monolayer.

Some Anomalies in the Self-Diffusion of Water in Disordered Carbons. T.X.Nguyen, S.K.Bhatia: Journal of Physical Chemistry C, 2012, 116[5], 3667-76