Graz Advanced School of Science PHYSICS COLLOQUIUM OF THE UNIVERSITY OF GRAZ AND THE GRAZ UNIVERSITY OF TECHNOLOGY

Vortragender:

Dr. Matthew A. Brown

Laboratory for Surface Science and Technology

Department of Materials, ETH Zürich

Abstract:

Almost all surfaces develop an electric charge when in contact with aqueous solution, causing ions in the adjacent electrolyte to rearrange and form an electric double layer that screens the charge. The structure of this electric double layer has been debated for well over a century as it mediates colloidal interactions, regulates surface structure, controls reactivity, sets capacitance and represents the central element of electrochemical supercapacitors. The electrostatic potential of such surfaces generally exceeds the electrokinetic potential, often substantially. Traditionally, a Stern layer of non-specifically adsorbed ions has been invoked to rationalize the difference between these two potentials; however, the inability to directly measure the absolute surface potential of dispersed systems has rendered quantitative measurements of the Stern Layer potential, and other quantities associated with the Outer Helmholtz

Plane, impossible. Here we use X-ray photoelectron spectroscopy (XPS) from a liquid microjet to measure the absolute surface potentials of silica nanoparticles dispersed in aqueous electrolytes. We quantitatively determine the impact of specific cations (Li+, Na+, K+, and Cs+) in chloride electrolytes on the surface

potential, the location of the shear plane and the capacitance of the Outer Helmholtz Plane. We find that the magnitude of the surface potential increases linearly with hydrated cation radius. Interpreting our data using the simplest assumptions and most straightforward understanding of Guoy-Chapman-Stern theory reveals an Outer Helmholtz Plane whose thickness corresponds to a single layer of water molecules hydrating the silica surface, plus the radius of the hydrated cation. Our results subject electrical double layer theories to direct and falsifiable tests that have proven elusive since the pioneering work of Helmholtz, and reveal a physically intuitive, but quantitatively verified picture of the Stern Layer that is consistent across multiple electrolytes and solution conditions.