The effect of ion shape and concentration on tunable wetting of water droplets on graphite surfaces

Abstract

We study the wetting behaviors of aqueous NaCl and NaNO3 electrolyte nano-droplets on graphite surfaces by molecular dynamics simulations. Despite the same ion charge and similar Stokes radii, while the contact angle increases with NaCl concentration up to 3 M, the contact angle decreases with NaNO3 concentration up to 3 M. We show that while the liquid/vapor surface tension increases with both NaCl and NaNO3 concentrations, the solid/liquid interfacial tension varies with NaCl and NaNO3 concentrations oppositely. The Cl− ions are depleted from the first dense water layer at the graphite/water interface, leading to an increasing interfacial tension and an increasing contact angle with ion concentration. Beyond 3 M NaCl, the ionic double layer at the interface counteracts the depletion of ions at the immediate graphite/water interface. Therefore, the contact angle saturates beyond around 3 M NaCl. By contrast, the planar NO3− ions are adsorbed in the dense water layer at the immediate graphite/water interfaces, conform to the interface, and form excessive hydrogen bonds with water at the interface. Consequently, the interfacial tension is decreased with increasing NaNO3 concentrations, leading to a decreasing contact angle up to around 3 M NaNO3. At NaNO3 concentrations beyond 3 M, the interfacial tension no longer decreases, likely due to the saturation of NO3− in the dense region; therefore, the contact angle slightly increases with NaNO3 concentration due to the increasing liquid/vapor surface tension. Our research sheds light on the mechanisms of the effect of ion shape and concentration on droplet wetting on solids.