Title: Charging Colloidal Particles in Apolar Media
Time and Date: October 14, 2022 at 1 p.m. Refreshments served at 12:45 p.m
Location: CHBE Room 102
The charging of colloidal particles in apolar media (dielectric constant ≈ 2) is a subject of much current interest, underlying such applications as electrophoretic displays (e.g., the Amazon Kindle® reader) and new printing devices (e.g., the HP Indigo® Digital Press). It is often of interest to charge a particular particle type in a given medium with a given polarity and magnitude, and for this purpose it is desirable to have sufficient mechanistic understanding of the charging process to provide guidelines for the creation of the desired result. Understanding the creation and stabilization of electric charge in apolar media, and in particular the charging of colloidal particles with them has been challenging owing to the complexity of the systems involved and the large number of factors that appear to be important. It is generally accepted that charges in measurable amount can exist in apolar media only when housed in reverse micelles, or similar structures, composed of appropriate surfactant monomers, and it is likely that the charge-bearing species in the cores of the micelles are hydroxyl and hydronium ions arising from traces of water housed there. With respect to the charging of particle surfaces in apolar media, however, a number of different, and sometimes conflicting, theories have been advanced. It appears now that most observations, obtained both in the author’s laboratory and elsewhere, are explainable in terms of an acid-base mechanism, and this seminar reviews this evidence. Adducts formed between chemical functional groups on the particle surface and monomers of reverse micelle-forming surfactants dissociate, leaving charged groups on the surface, while the counter-charges formed are sequestered in the reverse micelles. For a series of mineral oxides in a given medium with a given surfactant, surface charging was found to scale linearly with the aqueous point of zero charge (PZC) values of the oxides, i.e., the pH at which the particles in water carry zero charge. Different surfactants, with the same oxide series, yielded similar behavior, but with different PZC crossover points between negative and positive particle charging, and different slopes of charge vs. PZC. Thus the oxide series could be used as a yardstick to characterize the acid-base properties of the surfactants. This has led directly to the study of other materials, including surface-modified oxides, carbon blacks, pigments (charge transfer complexes), and polymer latices. This review focuses on unraveling the acid-base mechanism of particle charging in apolar media with the goal of constructing a road map for particle charging behavior in a wide variety of systems, assisting in the choice or development of materials for specific applications.
JOHN C. BERG is Rehnberg Professor of Chemical Engineering at the University of Washington. The author or co-author of some 230 refereed articles, as well as two books, he is recognized internationally as a researcher and educator in interfacial and colloid science. His textbook: “An Introduction to Interfaces and Colloids: The Bridge to Nanoscience” is a bestseller, having been adopted at more than 100 colleges and universities worldwide. He has supervised the research of 101 graduate students, including 56 for Ph.D. Degrees, seven of whom have gone on to become Professors at different institutions. He served twelve years as Editor of Advances in Colloid and Interface Science, and on the Editorial Advisory Boards for Langmuir, Colloids and Surfaces, Journal of Colloid and Interface Science, and the Journal of Adhesion Science and Technology. He is a Fellow of the American Institute of Chemical Engineers. Some of his recognitions include: the J. S. Guggenheim Fellowship for studies at the E.T.H., Zürich, Switzerland, where he served as a Guest Professor; the Alpha Chi Sigma Award of the AIChE for his research in Interfacial Hydrodynamics, the UW College of Engineering Innovator Award, and the Visiting Eminent Scholar Award at the École Polytechnique in Lausanne, Switzerland.