XPS with fast-frozen samples: A renewed approach to study the real mineral/solution interface

The procedure for XPS study of quick-frozen solutions developed at the end of 1970s [K. Burger, E. Fluck, Inorg. Nucl. Chem. Lett. 10 (1974) 171] was modified and applied to investigate the mineral/solution interface formed at the surface of silica, gibbsite, goethite and manganite in aqueous sodium chloride solution at variable pH and ionic strength. /Cl atomic ratio (frozen samples) allows determination of the pH range where the mineral surface charge is close to zero. This ratio can be used also to prove an ion pair formation at the surface. From the intensity of the O 1s component corresponding to water, an interface thickness of 0.5 nm was estimated. A “built-in” potential at the interface, evincing as matrix elements photoelectron lines shifts after water loss, was found. This potential arises due to the spatial separation of the charged mineral surface and counter-ions in the electrical double layer (EDL). Its value does not exceed 250 mV and the sign depends on the charge of the mineral surface.