Contact Angle, Liquid Film, and Liquid-Liquid and Liquid-Solid Interfaces in Model Oil-Brine-Substrate Systems

Oil-water-substrate wettability is of prime importance in most branches of science and technology, from biology and nanomaterials to geology and petroleum science. Wetting is a three-phase interaction phenomenon as expressed in Young's equation. Microscopically wetting is from the fluid-substrate interactions and surfaces are designated as lyophilic and lyophobic (fluid-wet and nonwet). Here we investigate the microscopic mechanisms of wettability changes by salt concentration in oil-water-mineral substrate systems. A model oil droplet (n-decane) placed in an aqueous electrolytic solution next to a solid substrate surface (muscovite mica) is simulated. A thin water layer between oil and the substrate regulates the oil-substrate interaction. We find that at zero and low salt concentrations, the oil adsorption on the hydrophilic mineral substrate is stabilized by a thin layer of water giving rise to a nonzero contact angle (partial oil wetting). As the salt concentration increases ionic adsorption and the water layer thickness increase reducing the oil-substrate wettability. Ions adsorb unsymmetrically on the substrate and promote water adsorption into the water layer. Ionic adsorption is higher away from the droplet than under the droplet. Our contact angles by molecular dynamics simulations are in agreement with experimental measurements.