X-Ray Reflectivity analysis of SiO2 nanochannels filled with water and ions: a new method for the determination of the spatial distribution of ions inside confined media

Chemical reactions occurring at the material–aqueous solution interface are controlled by an interfacial layer of a few nm where conceptual models such as the Electrical Double or Triple Layer models can be applied. These models describing the spatial distribution of ions in term of perpendicular distance from the planar surface and ignoring topography or structure parallel to the surface are not validated in confined media. In order to investigate the critical dimensions of these models, our first approach was to use a model system consisting of two parallel plane surfaces of SiO2 spaced of 5 nm (nanochannels) filled with salt solutions XCl2 (X = Ca2+, Mg2+, Ba2+). These filled nanochannels were characterized using hard X-Ray reflectivity for the determination of electron density profiles perpendicular to the surface. From these results, the surface densities of adsorbed ions at SiO2 surface were calculated and the solution density inside the nanochannels was determined. This method opens new perspectives to a better understanding of water and ion distribution inside nanoconfined media.