Why are Water-Hydrophobe Interfaces Electrically Charged?

Mechanisms underlying the electrification of water-hydrophobe interfaces are of much interest in chemical science, but are not entirely clear. In response, a systematic investigation of excess electrical charges carried by water droplets dispensed from capillaries was performed by: (i) studying the deflections of pendant droplets under uniform electric fields, and (ii) the direct measurement of electrical charges of the dispensed droplets using an ultrasensitive electrometer coupled with a Faraday cup. Thus, the effects of the following crucial factors were unraveled: hydrophobicity/hydrophilicity of the capillary, the presence/absence of a water reservoir inside the capillary, water pH, ionic strength, dielectric constant, the dissolved CO₂ content, and the relative humidity. The emerging picture is that the electrification at interfaces of common hydrophobic materials, e.g., polytetrafluoroethylene and polypropylene, is not limited to interfaces with water alone; nor is this phenomenon entirely dependent on the specific adsorption of OH^- or H₃O⁺ ions, as commonly believed. Our exhaustive study also draws together an extensive body of literature on this subject