Water transport through interlayer nanochannels of layered materials plays important roles in many natural and industrial systems. For smectite, i.e., the expansive clay material, experimental data suggest that expansion is accompanied by the adsorption of a large amount of water into its interlayers. However, experiments often probe equilibrium hydration states in an averaged manner, and have not been able to directly capture the temporal nature of water transport through interlayers during hydration/dehydration. Here, we combine molecular dynamics (MD) simulations, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) experiments to probe the atomic resolution dynamics of the dehydration process. Our results indicate that dehydration is a two-stage process, in which the water transport mechanism (advection vs. diffusion) is strongly coupled with the interlayer collapse and the evacuation of water from hydrophobic/hydrophilic sites of clay layers. In the first stage of dehydration, water transport follows the advection mechanism with fast evacuation of water molecules from the hydrophobic sites and fast removal of the first few water molecules from the hydration shell of Na<sup>+</sup> ions. The second stage is associated with slow desorption of the last 1-2 water molecules from Na<sup>+</sup> ions. Water movement through interlayers in the second state follows the diffusion mechanism. Our result provides a molecular picture of the coupling nature of water transport mechanism and pathway, and the collapse of hydrated clays.

Transport-mechanical-chemical coupling effects during clay dehydration

ACS Spring 2022

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