Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets
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, thin films
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.
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.
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 on the opposite surface. As a result, even though the the net energy difference between the two surfaces is very large, there is no way for the charges to relax.
The authors point out that charging effects are merely half of the effect; as surfaces are pulled apart, if the charge imbalance is sufficiently large, electrons can be transferred in a variety of ways. The primary means are electron tunneling, electron field emission, or dielectric breakdown of air. When the surfaces are first pulled apart, the electric fields can be extremely high due to the small distances involved, and the authors examine which effects dominate under which conditions. They find that the most common limiting factor is ultimately dielectric breakdown, hence why humidity can help prevent static electricity buildup: thin layers of water on the surfaces of materials can provide a conducting path to ground for the breakdown.
The applications of ionic electrets are briefly discussed - one clear one is in "electrophotography" (such as laser printing), where charged particles are a key component of adhesion. Another one they have explored is self-assembly, though they don't go into much detail in the review.
The Role of Water
One of the main sections of the review article focuses on the role of water: it is noted that basically all surfaces adsorb a thin layer of water under atmospheric conditions; even fluorocarbons are expected to have 2 monolayers at 80% humidity). After a brief review of the double-layer and the general behavior and distribution of ions near the surface, the authors discuss the effect of water on contact electrification in general. On the most basic level, it is clear that if much water is present, it will provide a discharge path to ground. However, in 0% humidity, another study observed no contact electrification whatsoever, which suggests that water is important for the transfer of ions.
A proposed theory is that water forms a bridge between the two surfaces; the ions can transfer back and forth until the bridge is broken and the ions are left behind. However, this model would seem to favor high-mobility ions as having a large affinity for contact electrification (though I'm not sure I agree with the author's point here); in reality, the magnitude of the contact electrification seems more strongly correlated with how likely the mobile ions are to be adsorbed directly on the surface (called the Stern layer) - the more they diffuse in the aqueous phase, the more likely they are to get left behind.
In fact, the article proposes that this aqueous behavior can lead to contact electrification even when there aren't any mobile ions: some surfaces have preferential hydroxide adsorption, which would lead to net charging when the surfaces are separated. This is merely a proposed theory, with many possible similar effects proposed (eg. dissolved carbon dioxide participating), but an important circumstantial piece of evidence in the authors' favor is that polymers with a greater index of refraction tend to accumulate more negative charge - both hydroxide adsorption and index of refraction are tied to the dispersion forces of the polymer.
The review ends on this note, acting guide to many possible experiments that should help truly elucidate the mechanism behind contact electrification. The full review article is lengthy and contains far more information than included here, and is a highly worthwhile read.