Electrification at Water-Hydrophobe Interfaces
Nauruzbayeva, J., Sun, Z., Gallo Jr., A., Ibrahim, M., Santamarina, J. C., Mishra, H.* (Under review: Nature Communications)
The mechanisms leading to the electrification of water when it comes in contact with hydrophobic surfaces remains a research frontier in chemical science. A clear understanding of these mechanisms could, for instance, aid the rational design of triboelectric generators and micro- and nano-fluidic devices. Here, we investigate the origins of the excess positive charges incurred on water droplets that are dispensed from capillaries made of polypropylene, perfluorodecyltrichlorosilane-coated glass, and polytetrafluoroethylene. Results demonstrate that the magnitude and sign of electrical charges vary depending on: the hydrophobicity/hydrophilicity of the capillary; the presence/absence of a water reservoir inside the capillary; the chemical and physical properties of aqueous solutions such as pH, ionic strength, dielectric constant and dissolved CO2 content; and environmental conditions such as relative humidity. Based on these results, we deduced that common hydrophobic materials possess surface-bound negative charge, for example, polypropylene has a surface charge density of -0.7 nC-cm-2, i.e. the number of electronic charges (#) per μm2 of 43 #/m2. Thus, when these surfaces are submerged in water, hydrated cations (e.g., H3O+) form an electrical double layer. Furthermore, we demonstrate that the primary role of hydrophobicity is to facilitate water-substrate separation without leaving a significant amount of liquid (as films or droplets) behind. These results advance the fundamental understanding of water-hydrophobe interfaces and support the enhanced design of materials/technologies for applications in electrowetting, omniphobicity, separation processes, and energy transduction, among others.