Pore-size dependence and characteristics of water diffusion in slitlike micropores.

Abstract

The temperature dependence of the dynamics of water inside microporous activated carbon fibers (ACF) is investigated by means of incoherent elastic and quasielastic neutron-scattering techniques. The aim is to evaluate the effect of increasing pore size on the water dynamics in these primarily hydrophobic slit-shaped channels. Using two different micropore sizes (∼12 and 18 Å, denoted, respectively, ACF-10 and ACF-20), a clear suppression of the mobility of the water molecules is observed as the pore gap or temperature decreases. This suppression is accompanied by a systematic dependence of the average translational diffusion coefficient D(r) and relaxation time 〈τ(0)〉 of the restricted water on pore size and temperature. The observed D(r) values are tested against a proposed scaling law, in which the translational diffusion coefficient D(r) of water within a porous matrix was found to depend solely on two single parameters, a temperature-independent translational diffusion coefficient D(c) associated with the water bound to the pore walls and the ratio θ of this strictly confined water to the total water inside the pore, yielding unique characteristic parameters for water transport in these carbon channels across the investigated temperature range.