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

Abstract

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 θ of a droplet on the surface, where the contact angle θ 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.

Wetting of two-component drops: Marangoni contraction versus autophobing

“…Contrary to many previous works on autophobing [34,[36][37][38][39][40][41], we do not see an initial spreading phase followed by a retraction to the quasi-steady θ (see Supplementary Material [16]). This is likely due to the relatively high diffusion coefficient of 1,2-HD, which is a result of its small molecular size in comparison to other more common surfactants [42,43].…”

Wetting and coalescence: beyond single-phase flows

“…Contrary to many previous works on autophobing [141][142][143][144][145][146][147], we do not 4.5. Effect of the molecular structure see an initial spreading phase followed by a retraction to the quasi-steady θ (see Fig.…”

“…[126] and [127]. Molecules may also adsorb on the solid-liquid interface, which we are unable to measure using our experimental setup [142]. Such adsorption, if dominant, could lower γ SL , increase γ SV − γ SL , and thus lead to a decrease in θ.…”

Antisurfactant (Autophobic) Behavior of Superspreader Surfactant Solutions

“…The concentration of the surfactant solution is varied during an experiment using preset automatic dilution steps. For a detailed account of the procedure, examples of surface tension data as well as discussion about the anomalies in such measurements, the reader is referred to our previous work. ,− …”