Analysis of the Ion Distribution at a Charged Solid−Liquid Interface Using X-ray Standing Waves

Functionalized solid−liquid interfaces were analyzed by X-ray standing waves (XSW) combined with streaming current measurements to study surface charges, interfacial potential, and ion distributions. Thin films of aqueous solution containing Br− anions and Fe3+ cations at a concentration of 10 mg/L were prepared on functionalized silicon wafers. Functionalization of Si surfaces was accomplished by aminosilane groups shifting the interfacial potential toward positive values. The ion distribution was measured with nanometer resolution, which allows distinguishing between absorbed and mobile ions at the surface and in the diffusive layer, respectively. For Br−, different degrees of ion attraction were measured for the pH values 5.7 and 2.8. The ion Debye length values of the diffuse layer were 4 and 2 nm, respectively.