Influence of Surface-Active Molecules in Solution on Charge Transfer Due to a Water-Air Contact Line Moving over a Hydrophobic Surface.

Charge transfer due to a water-air contact line moving over a fluoropolymer hydrophobic surface is investigated for an aqueous solution containing surface-active molecules. It is found that anionic (SDS) and neutral (Triton X-100) surfactants exhibit a two-stage charge transfer reduction with concentration. At low concentrations, a layer of surfactant molecules accumulates near the hydrophobic surface and partially quenches the charge transfer. Surprisingly, after this first stage, the charge transfer remains nearly constant or weakly increasing, while the concentration of surfactants increases several orders of magnitude. Eventually, for large enough concentrations, the charge transfer continues to decrease, eventually resulting in almost zero charge transfer before reaching the critical micelle concentration. For the cationic surfactant (CTAB), the behavior is entirely different and a single quenching mechanism can explain the observed reduction in charge transfer due to positively charged surface-active molecules forming a layer that electrostatically screens the water-induced negative charge residing on the hydrophobic interface. A similar behavior is observed for poly(vinyl alcohol), which is attributed to its known and strong interaction with the hydrophobic surface used in this study.