Electrochemically Enhanced Dissolution of Silica and Alumina in Alkaline Environments.

Abstract

Dissolution of mineral surfaces at asymmetric solid-liquid-solid interfaces in aqueous solutions occurs in technologically relevant processes, such as chemical/mechanical polishing (CMP) for semiconductor fabrication, formation and corrosion of structural materials, and crystallization of materials relevant to heterogeneous catalysis or drug delivery. In some such processes, materials at confined interfaces exhibit dissolution rates that are orders of magnitude larger than dissolution rates of isolated surfaces. Here, the dissolution of silica and alumina in close proximity to a charged gold surface or mica in alkaline solutions of pH 10-11 is shown to depend on the difference in electrostatic potentials of the surfaces, as determined from measurements conducted using a custom-built electrochemical pressure cell and a surface forces apparatus (SFA). The enhanced dissolution is proposed to result from overlap of the electrostatic double layers between the dissimilar charged surfaces at small intersurface separation distances (<1 Debye length). A semiquantitative model shows that overlap of the electric double layers can change the magnitude and direction of the electric field at the surface with the less negative potential, which results in an increase in the rate of dissolution of that surface. When the surface electrochemical properties were changed, the dissolution rates of silica and alumina were increased by up to 2 orders of magnitude over the dissolution rates of isolated compositionally similar surfaces under otherwise identical conditions. The results provide new insights on dissolution processes that occur at solid-liquid-solid interfaces and yield design criteria for controlling dissolution through electrochemical modification, with relevance to diverse technologies.