Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets

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Original entry: Tamas Szalay (APPHY225 2012)

"Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets"

Logan S. McCarty and George M. Whitesides

Angew. Chem. Int. Ed. 2008, 47, 2188 – 2207


An electret is the electrostatic cousin of a permanent magnet: a material with a permanent charge at the surface, either due to a static charge or a dipole moment. An ionic electret is defined to be an electret where the net charge is due to a difference in anionic and cationic concentrations at the surface; when two are brought in contact, ionic exchange can occur, and this can produce a net transfer of charge. This article examines the theoretical and experimental evidence for the mechanism by which these charge transfers occur, and uses this to propose a model for the ubiquitous static electricity behavior of organic polymers.

Soft matter keywords

Charge separation, dispersion force, static electricity, electret

Background Information

The authors first spend some time explaining types of electrets and their uses. Dipolar electrets are those with aligned electric dipoles, and are often formed by cooling glassy polarizable materials in the presence of a strong electric field; one common modern example is the material PVDF (polyvinylidine difluoride). Space charge electrets, with a permanent static charge, are formed in a couple of ways, but some of the most common include strong electron or ion irradiation.

The process of contact electrification (tribocharging) is then discussed, first in the context of static charging (in the classic sense, when materials are rubbed together). If one tries to rank different materials by those which acquire a negative charge and those which acquire a positive one, it can be seen that cycles exist; this suggests that static charging is the combination of different effects.


The authors then provide some details on experiments performed in order to measure the contact electrification of various materials, as well as details about neat tricks one can accomplish with the contact electrification of mm-scale beads, such as self-assembled lattice structures.

Ultimately, though, they make the point that the question here is the precise mechanism of tribocharging. Many people have tried correlating material triboelectric properties with other ostensibly electronic properties, such as the dielectric constant or atomic ionization potentials and electronegativity, but they seem to often be unrelated - this would suggest that the effect is not electron-based in nature (and may be ionic instead). Furthermore, transferring an electron from one material to another would require an extremely high energy cost, and in the case of organic molecules, require well-matched donor and acceptor orbitals, which are uncommon.

Instead, the one material property contact electrification seems to correlate with is the acidity or basicity of insulators, which would indicate a proton-transfer (or generally ion transfer), and in fact that the charge transfer mechanism is hardly ever due to electrons.

Contact Electrification

In this section, the authors experimentally demonstrate that when the surface of polystyrene is functionalized with various organic groups that have a bound ion, the sign of the net static charge on the sphere ends up being the same as the charge of the covalently bound ion, indicating it is the mobile ions that are stripped.

(experimental data)

A mechanism is then suggested for contact electrification, whereby when two surfaces are brought in close proximity, the mobile surface ions can thermally fluctuate between potential wells between them (provided that they are close enough that the barrier between them drops below kT). Then, as the surfaces are mechanically separated, the ions can occasionally inadvertently get stuck in

Experimental Results