Pore-size dependence and characteristics of water diffusion in slit-like micropores

TL;DR

This study investigates how pore size influences water diffusion in slit-like micropores of activated carbon fibers using neutron scattering, revealing systematic dependence of water mobility on pore size and temperature.

Contribution

It introduces a scaling law linking water diffusion in micropores to a temperature-independent parameter and pore size, advancing understanding of confined water dynamics.

Findings

Water mobility decreases with smaller pore size and lower temperature.

A scaling law accurately describes water diffusion based on confined water fraction.

Distinct diffusion parameters are identified for different pore sizes.

Abstract

The temperature dependence of the dynamics of water inside microporous activated carbon fibers (ACF) is investigated by means of incoherent elastic and quasi- elastic 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 (\sim 12 and 18 {\AA}, 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 Dr and relaxation time <{\tau}_0> of the restricted water on pore size and temperature. The observed Dr values are tested against a proposed scaling law, in which the translational diffusion coefficient Dr of water within a nanoporous matrix was found to depend solely on two single parameters, a temperature independent translational diffusion coefficient Dc associated with the water bound to the pore walls and the ratio {\theta} 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.