XPS at solid-aqueous solution interface

Application of X-ray Photoelectron Spectroscopy (XPS) to study the solid-aqueous solution interface is reviewed. XPS provides complementary physicochemical information about electrical double layer from the perspective of the solid surface. Experimental techniques, such as differential pumping, controlled adsorption/co-adsorption, freeze-drying, and fast-freezing, are discussed for both electrochemical and dielectric solid-solution interfaces. The use of fast-freezing, as applied to wet pastes centrifuged from aqueous suspensions, makes it possible to approach a real solid-solution interface in UHV conditions. XPS data allow estimation of the surface density of counter-ions, surface point of zero charge, and in some cases the measurement of surface potential. Interfacial chemical reactions such as ion pair formation, specific adsorption and ligand exchange can be directly observed. The technique is easy to apply to any suspension including colloids and gels of inorganic or organic nature, and can be adapted for electrochemistry as complementary to traditional "emersed electrode" studies.