Some anomalies in the self-diffusion of water in disordered carbons

Abstract

The unusual dynamics of water under confinement is important to a number of conventional and emerging industrial processes, and to life in general; nevertheless, our understanding of this critical area is still at an early stage. Nanoporous carbons have provided a key material through which to investigate this dynamics, however, existing simulations have been based on the use of idealised slit pore carbons or carbon nanotubes with smooth energy landscapes, and fail to capture the influence of structural disorder inherent to real carbons. We show here that the irregular structure of such carbons critically influences the dynamics and the mode of diffusion (single file, sub-diffusion or Fickian). Our molecular dynamics simulations, using a realistic hydrophobic carbon model based on hybrid reverse Monte Carlo simulation of the structure of an activated carbon fibre, show the existence of a single file diffusion mode between the ballistic and Fickian modes in the narrowest pore regions of this material, not seen in simulations using model 2-dimensional slit pores. A rich variety of behaviour is found in this sub-diffusion regime, with the fits of time-dependent mean square displacement (MSD) to the power law of time ( ) revealing that the exponent a varies significantly with temperature, especially at low temperatures (273K-350K). It reaches a minimum value of 0.5 at 298 K corresponding to the single file diffusion regime, and approaches unity at 610 K corresponding to the Fickian mode. It is demonstrated that confinement effects lead to the experimentally observed non-Arrhenius behaviour of the water dynamics at low temperatures (< 350 K), for both the realistic carbon model and an idealised 2-dimensional slit pore model, due to the transport of the adsorbed water as a large cluster in a water monolayer.