Atomic-level insights into the mechanism of saline-regulated montmorillonite (001)-salt droplet interface wetting: A molecular dynamics study

Salinity regulation of the wettability of clay minerals plays an important role in many research fields, but the mechanism influencing the process is not fully understood. Molecular dynamics simulations were used with a NaCl solution and the montmorillonite (Mt) to explore the process by which salinity changes surface wettability. Additionally, the effects of salinity on ionic water cooperation, hydration layer formation, and the hydrogen bond network were analyzed. The results showed that the droplet contact angle increased almost linearly with increasing salt concentration. In an ionic droplet, the oxygen atom of a water molecule moves toward the Na ion and the hydrogen atom moves toward the Cl ion, and the Cl ion has more influence on the droplet structure. There are multiple hydration layers in the droplet, and the ion concentration impacts the hydrogen bond network density more than the water structure density. The main ionic component in the first hydration layer of the ionic droplet is the Cl ion, which changes the direction of the OH bond and leads to a decrease in the hydrogen bond density between the first layer of the water molecule and the hydration layer above, which reduces the adsorption of the Mt. (001) surface charge on the upper water molecule. With an increase in salt ion concentration, the spread range of the first layer of water molecules on the mineral surface is reduced, and ionic hydration results in destruction of the hydrogen bond network of the droplet. Specifically, the connection between hydration layers is weakened, so the adsorption of the upper water on the surface is shielded, leading to an increase in hydrophobicity. This research helps to reveal the mechanism underlying many physical processes or phenomena in nature.