Probing Surface Charge Densities of Evaporated Polymers

Electrified interfaces of water manifest across natural and applied sciences, e.g., thundercloud charging,

aerosols, pipetting, emulsion stability, Kelvin dropper, and triboelectric nanogenerators. Curiously, the

origins of the electrification of water as it interfaces with common hydrophobic dielectric media, air,

hydrocarbons, and perfluorocarbons, have remained controversial. For example, the proposed

mechanisms range from the specific interfacial hydroxide or hydronium or bicarbonate ions to partial

charge transfer between water and dielectrics to environmental contamination. In 2020, Nauruzbayeva

et al., utilized a parallel plate capacitor and an electrometer to quantify the electrification of water

droplets in contact with polypropylene and perfluorinated capillaries. They demonstrated that the

electrification of water was because those solid surfaces were negatively charged, even in the air, with

an approximate surface charge density of -1 nC-cm -2 . We have utilized a Surface Force Apparatus (SFA)

to quantify surface charge densities of polymer surfaces in air and water. Protocols were developed to

deposit films of polypropylene and polytetrafluoroethylene onto ultrasmooth mica films. Force–distance

curves were obtained using the SFA and the surface charge densities were estimated in the air and

aqueous solutions. Methods to deduce the surface charge densities for polymers

in our experimental configurations will also be presented.