Insights on Capillary Adsorption of Aqueous Sodium Chloride Solution in the Nanometer Calcium Silicate Channel: A Molecular Dynamics Study

Molecular dynamics was utilized to investigate the capillary transport of Na+, Cl- ions and water molecules in the nanometer channel constructed by calcium silicate hydrate. The early stage of water imbibition depth as a function time, presenting constant-velocity and visco-inertia regimes, agrees well with classic capillary adsorption theory. With increasing pore size from 3.5 to 6 nm, the initial constant velocity reduces, and the penetration depth increases for both ions and water molecules. Structurally, water molecules confined in the nanopore show advancing meniscus, orientation preference, and disturbed hydration shell in the vicinity of the hydrophilic C-S-H surface. On the other hand, the water and ions have different capillary transport behavior: the ions migrate slower than water molecules in the C-S-H gel pore. The transport discrepancy between water and ions is more pronounced with the decrease of nanopore size due to the following reasons: The ionic pairs such as Ca2+ and Cl- accumulate to form a cluster, blocking further penetration of other ions, and the immobilization effect is much stronger from the surface calcium ions and nonbridging oxygen atoms to elongate the residence time of ions.