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

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Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets Logan S Reviews G. M. Whitesides and L. S. McCarty DOI: 10.1002/anie.200701812 Ionic Electrets Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets Logan S. McCarty and George M. Whitesides* Keywords: charge transfer · electron transfer · electrostatic interactions · interfaces · polymers Angewandte Chemie 2188 www.angewandte.org 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 2188 – 2207 Angewandte Ionic Electrets Chemie This Review discusses ionic electrets: their preparation, their mecha- From the Contents nisms of formation, tools for their characterization, and their appli- cations. An electret is a material that has a permanent, macroscopic 1. Introduction 2189 electric field at its surface; this field can arise from a net orientation of 2. Background 2189 polar groups in the material, or from a net, macroscopic electrostatic charge on the material. An ionic electret is a material that has a net 3. The Contact Electrification of electrostatic charge due to a difference in the number of cationic and Materials Containing Mobile anionic charges in the material. Any material that has ions at its Ions 2194 surface, or accessible in its interior, has the potential to become an 4. The Role of Water in Contact ionic electret. When such a material is brought into contact with some Electrification 2199 other material, ions can transfer between them. If the anions and cat- ions have different propensities to transfer, the unequal transfer of 5. Summaryand Outlook 2205 these ions can result in a net transfer of charge between the two materials. This Review focuses on the experimental evidence and theoretical models for the formation of ionic electrets through this ion- 2. Background transfer mechanism, and proposes—as a still-unproved hypothesis— 2.1. Properties and Uses of Electrets that this ion-transfer mechanism may also explain the ubiquitous contact electrification (“static electricity”) of materials, such as organic The two-volume series Electrets, polymers, that do not explicitly have ions at their surface. edited by Sessler and Gerhard-Multh- aupt, provides a comprehensive treat- ment of the fabrication, properties, and applications of electrets from the vant- 1. Introduction age of engineering and condensed-matter physics;[1] we offer only a brief introduction. The two basic types of electrets— An electret is a material that has a permanent, macro- dipolar electrets and space-charge electrets—are fabricated scopic electric field at its surface. There are two distinct by quite different techniques. Slow cooling of a dielectric classes of materials, both of which are called electrets: material from above its glass-transition temperature in the 1) dipolar electrets, which are overall electrically neutral, presence of a strong electric field (typically several hundred but have a macroscopic electric dipole moment, and 2) space- kV per cm) yields dipolar electrets: as the material solidifies, charge electrets, which have a net, macroscopic electrostatic individual molecular dipoles become “frozen” with a net charge. We define an ionic electret as a space-charge electret orientation in the direction dictated by the applied field. that has a net electrostatic charge due to a difference in the Classical wax electrets, known since the 19th century, are number of cationic and anionic charges in the material. For examples of dipolar electrets. (Oliver Heaviside coined the example, we discuss later the characteristics of polystyrene term “electret” in 1885 to describe a material that is an beads that contain a number of covalently bound sulfonate electrostatic analogue of a permanent magnet.[2]) Today, ions and a smaller number of sodium counterions. Although many dipolar electrets are made from poly(vinylidine difluor- the difference in the number of charges is small (ca. 0.5% of ide) (PVDF). This material contains the CF2 moiety, a group the ions at the surface of the beads), the electric fields that with a high electric dipole moment, and has a structure in arise from this imbalance of charge can be substantial: they which these polar groups orient cooperatively within crystal- can exceed 30 kVcmÀ1, the typical threshold for the dielectric line domains.[3] breakdown of air. These ionic electrets defy the conventional Space-charge electrets result from adding charge to the assumption in chemistry that bulk matter is electrically surface or the bulk of a material by bombarding it with an neutral and that ionic materials must have an equal number electron beam or ion beam, spraying it with ions from the of cationic and anionic charges. corona discharge of a high-voltage electrode, contacting it This review provides a brief survey of the chemistry of directly with a charged electrode, or transferring ions to (or ions at interfaces, and explains how the transfer of mobile from) the material by other means (some of which we discuss ions, facilitated by adsorbed water, can yield macroscopically herein). Electronic or ionic charges in space-charge electrets charged ionic electrets. We discuss applications of ionic electrets in Xerography (electrophotography) and self-assem- bly. Finally, we propose—as a still-unproved hypothesis—that [*] Dr. L. S. McCarty, Prof. G. M. Whitesides the well-known phenomenon of contact electrification (the Department of Chemistry andChemical Biology HarvardUniversity transfer of charge from one material to another upon contact) 12 OxfordSt., Cambridge,MA 02138 (USA) may, in many circumstances, be due to the unequal partition- Fax : (+1)617-495-9857 + À ing of aqueous ions (particularly H and OH ) between E-mail: [email protected] interfaces. Homepage: http://gmwgroup.harvard.edu Angew. Chem. Int. Ed. 2008, 47, 2188 – 2207 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2189 Reviews G. M. Whitesides and L. S. McCarty may reside on the surface or at sites deeper in the bulk of the 2.2. Contact Electrification (Tribocharging) material. Most insulating materials, especially organic mate- rials, that are space-charge electrets probably contain ionic When two solid surfaces are brought into contact and charges, regardless of the process by which they are charged: separated (with or without intentional rubbing or frictional injection of an electron into an insulating organic material will contact), charge often transfers from one surface to the other; result in its attachment to a functional group with a high this process is known as contact electrification or tribocharg- electron affinity (e.g. C=O, CÀCl, or aromatic groups), or to ing.[11,12] Reports of contact electrification date back to impurities such as O2 or CO2. These electron-injection ancient Greece; our word “electricity” is derived from the processes may also break bonds in the material. Greek word elekt1on, meaning amber—a material that Cellular polymer ferroelectrets share properties of both develops a negative charge when rubbed with animal fur. space-charge and dipolar electrets.[4] These materials result The “triboelectric series” (Figure 1a) is a list of materials from the application of a strong electric field to a cellular foamed polymer. The electric field causes dielectric break- down (corona discharge) inside each polymer cell; this dielectric breakdown deposits equal and opposite electrical charges on opposite interior surfaces of the cell. Each cell in a cellular ferroelectret has space charges on its walls, but the ferroelectret overall is electrically neutral and acts like a dipolar electret with a large dipole moment. Many applications of electrets involve coating an object with an electrically charged powder. Electrophotography uses a pattern of charge, formed by the selective discharge of the photoconductive surface of an imaging drum, to arrange oppositely charged toner particles on that drum;[5] both the drum and the toner particles are electrets. Electrostatic powder coating and electrostatic spray painting, by giving particles of polymer or paint a net charge, can coat large objects with a uniform layer of these materials.[6] Sessler discusses other applications of electrets, including electro- static filters, electret microphones, and radiation dosimeters (which monitor the radiation-induced discharge of an elec- tret).[1] Electrostatic charging can also be undesirable: elec- trical discharges can destroy sensitive electronic equipment,[7] and discharges between surfaces charged by friction, or by the flow of liquids, can result in sparks and explosions if flammable liquids or vapors are present.[8] Electrets need not be solids: any liquid that has a net Figure 1. a) A triboelectric series that combines several different series electrostatic charge may be considered a liquid electret. The from the literature. This series is excerptedfrom reference [13], which charged droplets formed during electrospray ionization are contains a more extensive list andhighlights some of the inconsisten- examples of liquid electrets. Most electrospray mass spec- cies between various series. For example, the positions of poly(methyl methacrylate), polystyrene, andpoly(vinyl chloride)vary somewhat trometers produce charged droplets by direct contact among different published series. b) An example of a set of five [9] between a metal electrode and the liquid analyte; a strong materials that form a cyclic triboelectric series (adapted from refer- electric field can also cause a drop of liquid to break apart into ence [11]). small charged droplets.[10]
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