Wetting transition of n-alkanes on concentrated aqueous salt solutions. Line tension effect

The spreading of n-octane droplet at the surface of sodium chloride aqueous solutions was investigated as a function of the salt concentration in a saturated closed cell. As long as the salt concentration is lower than 1.2 M, the macroscopic drop takes the form of a lens in a stable coexistence with its adsorbed thin film. A wetting transition occurs for a salt concentration of about 3 M: spreading is considerably enhanced and the lens becomes a metastable pancake with a contact angle smaller than 1 degrees. After a few hours, it breaks up into a multitude of stable microdroplets which disappear 24 h later. Drop size dependent contact angles reveal a salt concentration dependent positive line tension. The values of line tension are very near. the-theoretical value. It is confirmed that the positive line tension stabilizes the microdroplets. The discussion of the results is based on the specific behavior of the interaction free energy Delta F of the oil due to the very high salt concentration of the liquid substrate.