Tunable Substrate Wettability by Thin Water Layer.

In oil-water-mineral substrate systems we show that the contact angle can be tuned by an ionic structure in the water layer confined between an oil droplet and the sub strate. We perform molecular dynamics simulations of a complex oil droplet in an NaCl aqueous solution on mica surface; the oil is a mixture of n-decane and surfac tant 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 first decreases as the salt concentration in the bulk increases. With further increase in bulk salt concentration, the adsorbed ion concentration in the thin film 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 non-monotonic behavior; the contact angle is first nearly constant to a low salt con centration of 0.1 wt% NaCl. Then decreases sharply as salt concentration increases from 0.1 to 1.1 wt% NaCl. A reverse trend in contact angle follows with further salt concentration increase. The non-monotonic trend unlike 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 droplet 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.