Triboelectric Charging of Insulators—Evidence for Electrons Versus Ions

Whether electrons or ions are involved in triboelectric charging of insulators remains elusive. We designed polymers whose surface compositions (determined by X-ray photoelectron spectroscopy) differed from the bulk. Charging against insulators related to their topmost surface compositions, but against metals related to their bulk compositions. We propose the hypothesis that the former involves ion exchange between the topmost surfaces and the latter involves electron tunneling into the bulk, thus postulating a relationship between charging mechanism and charge penetration depth, which is supported by the fact that ions are known to adsorb to polymer surfaces and electrons are considered to penetrate into the bulk. Integration of this with the frequently conflicting evidence and hypotheses of others has, for the first time, led to a coherent though qualitative overall understanding of brief contact charging mechanisms: ion exchange for insulator-insulator contacts, both electron and ion exchange for metal-insulator contacts, and electron exchange for metal-metal contacts, consistent with successively increasing “electron availability” at these interfaces. We suggest that this concept of relative surface “availability” of the charge exchange agent also accounts for the predominance of mobile ion exchange over hydroxide ion and electron exchange when mobile-ion-containing organic salts or polymers are involved.