Counteracting Interfacial Energetics for Wetting of Hydrophobic Surfaces in the Presence of Surfactants

Surface active agents (surfactants) are commonly used to improve the wetting of aqueous solutions on hydrophobic surfaces. The improved wettability is usually quantified as a decrease of the contact angle theta of a droplet on the surface, where the contact angle theta is given by the three surface tensions involved. Surfactants are known to lower the liquid-vapor surface tension, but what they do to the two other surface tensions is less clear. We propose an improved Zisman method for quantifying the wetting behavior of surfactants at the solid surface. This allows us to show that a number of very common surfactants do not change the wettability of the solid: they give the same contact angle as a simple liquid with the same liquid-vapor surface tension. Surface-specific sum-frequency generation spectroscopy shows that nonetheless surfactants are present at the solid surface. The surfactants therefore change the solid-liquid and solid-vapor surface tensions by the same amount, leading to an unchanged contact angle.