Tunable Substrate Wettability by Thin Water Layer

(Graph Presented) In oil-water-mineral substrate systems, we show that the contact angle can be tuned by ionic structures in the water layer confined between an oil droplet and the substrate. We perform molecular dynamics simulations of a complex oil droplet in a NaCl aqueous solution on a mica surface; the oil is a mixture of n-decane and surfactant molecules. The surfactant head contains an OH group and an aromatic ring. A thin water layer between the oil droplet and the substrate and ionic stratification regulate the wetting behavior. The concentration of salt ions in the thin film is nonmonotonic; it first increases, then decreases, and starts increasing again as the salt concentration in the bulk increases. On the other hand, the surfactant head adsorption in the thin film first increases as the bulk salt concentration increases. Then, it decreases with further increase in the bulk salt concentration. The change of contact angle with salt concentration also shows a nonmonotonic behavior; the contact angle is first nearly constant to a low salt concentration of 0.1 wt % NaCl. Then, it decreases sharply as the salt concentration increases from 0.1 to 1.1 wt % NaCl. A reverse trend in contact angle follows with further salt concentration increase. The nonmonotonic trend unlike the monotonic trend of interfacial tension with salt concentration is in line with recent measurements of contact angle of oil-brine-substrate systems. A sharp increase of surfactant head adsorption in the thin film, the decrease of ion adsorption, and the minimum of contact angle are all related. This is the first report of such correlations with change of wetting in the brine-complex oil-mineral substrate predicted from molecular simulations.