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°. 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 Δ F of the oil due to the very high salt concentration of the liquid substrate.