Electrification at water–hydrophobe interfaces

Abstract

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 deduce that common hydrophobic materials possess surface-bound negative charge. Thus, when these surfaces are submerged in water, hydrated cations 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 behind. These results advance the fundamental understanding of water-hydrophobe interfaces and should translate into superior materials and technologies for energy transduction, electrowetting, and separation processes, among others.

Experimental Data Contributing to the Elusive Surface Charge of Inert Materials in Contact with Aqueous Media

“…In view of the contradictory conclusions, it is not unlikely that the four above-mentioned causes (and maybe others) are all simultaneously at play. The literature on the various topics is exhaustive and extending year by year [16][17][18], and it would be futile to even try and discuss all the information that is being accumulated.…”

Counterintuitive Wetting Transitions in Doubly Reentrant Cavities as a Function of Surface Make‐Up, Hydrostatic Pressure, and Cavity Aspect Ratio

“…For example, at low hydrostatic pressures, DRCs demonstrate dramatically longer entrapment of air underwater in comparison to the SCs, [61] and methods for restoring the diffused air inside the cavity should be explored [62] along with the subtle effects of surface make-up. [63] In contrast, SCs with hydrophobic make-up outperform commensurate DRCs under gradually-ramped elevated hydrostatic pressures. Thus, the pressure range must be carefully considered while choosing DRCs or SCs for practical applications.…”

A first‐principles approach for treating wastewaters

“…[ 58,72–75 ] We also note that ξ ‐potentials of hydrocarbon and perfluorocarbon surfaces, which have been investigated extensively in the past and intensely debated on regarding their influence on interfacial charges, wetting, surface forces, and adsorption. [ 44,76–78 ] However, it was not possible to measure ξ ‐potentials of our specific samples because of their buoyancy and hydrophobicity; they would not stay inside a water‐filled cuvette.…”

“…[58,[72][73][74][75] We also note that ξ-potentials of hydrocarbon and perfluorocarbon surfaces, which have been investigated extensively in the past and intensely debated on regarding their influence on interfacial charges, wetting, surface forces, and adsorption. [44,[76][77][78] However, it…”