The Charge of Solid-Liquid Interfaces Measured by X-Ray Standing Waves and Streaming Current

Measurements of ion distributions at a charged solid–liquid interface using X‐ray standing waves (XSW) are presented. High energy synchrotron radiation (17.48 keV) is used to produce an XSW pattern inside a thin water film on a silicon wafer. The liquid phase is an aqueous solution containing Br and Rb ions. The surface charge is adjusted by titration. Measurements are performed over a pH range from 2.2–9, using the native Si oxide layer and functional (amine) groups as surface charge. The Debye length, indicating the extension of the diffuse layer, could be measured with values varying between 1–4 nm. For functionalized wafers, the pH dependent change from attraction to repulsion of an ion species could be detected, indicating the isoelectric point. In combination with the measurement of the streaming current, the surface charge of the sample could be quantified. Scanning the electric double layer: The distribution of ions in the electric potential of a solid–liquid interface depends on the charge of the interface. By means of X‐ray standing waves, the ion distribution in solutions containing bromine and rubidium ions is analyzed (see figure). In combination with the measurement of the streaming current, the interfacial charge is determined quantitatively.