Ionic Double-layer Adsorption on Immersed Electrets by Laser Interferometry

Abstract

The structure of the electrical double layer at interfaces between solids (conducting and non-conducting) and solutions of ions has usually been inferred from capacitive measurements where the solid is a conductor (electrodes) and from zeta potential where the solid (a non-conductor) is the stationary phase or the mobile phase (colloids). The measurements of capacitance of an electrode are easier and more reproducible in most cases. It would be helpful, however, to be able to follow the formation of a double layer by immersing a clean metal surface in a solution of its ions by observing the concentration change. This has been attempted several times using copper and zinc electrodes and multiple beam laser interferometry where the concentration change is manifested as a bend in the interference fringes, that is, a refractive index gradient. This can be transformed into a concentration gradient from the plot of concentration against refractive index for the solution. Unfortunately several other variables may affect the refractive index. If the metal and the solution are not at the same temperature, a thermal gradient may obscure or confuse the concentration data. Similarly a chemical reaction (as between an oxide layer and an oxidizable ion) will interfere with interpretation. Finally, the possibility of charge transfer, or some Faradaic current, cannot be overlooked. The use of an electret as the absorbing (charged) surface seems to eliminate these problems. There is no possibility of charge transfer if the electret is bulk rather than surface charged, and because the electret must be an insulator (with a specific resistivity greater than 10¹² Ω m⁻¹) the electret is also a poor conductor of heat.