A Regulation Method of the Wettability of Solid Surfaces: Oil-Water Wettability Alteration by Replacing Adsorbed Polar Molecules via Salt Ions

Wettability of solid surfaces is generally regulated by modifying the microstructures and other properties of surfaces themselves. Here, we present that the wettability of solid surfaces exhibiting polar interactions with fluids can be substantially regulated only by adding a small amount of substances in fluids, instead of modifying the physicochemical properties of solid surfaces. We show that the adsorbed polar molecules on rock surfaces can be replaced by salt ions in the water phase, and ultimately the oil-wet surfaces are altered to water-wet. The influence of Na⁺ and Ca²⁺ ions on the wettability of rock surfaces with initially adsorbing stearic amine and pyrrole is revealed via molecular dynamics simulations. The contact angles of oil droplets are altered from 68.1° to 132.0° at most, which is very consistent with previous experimental measurements. The contact angles’ increase with increasing ionic concentration and the effect of divalent Ca²⁺ ions on the wettability alteration is more obvious compared to the monovalent Na⁺ ions. It is proved from the aspects of spatial distribution, microscopic interaction, and system energy of polar molecules and ions that the replacement of polar molecules is due to the negative substrates preferentially contacting with the free cations in the water phase. This work opens a new route for the regulation of surface wettability, especially in the field of water-flooding oil recovery technology.