Spin probe study of acidity of inorganic materials

Acidic characteristics of various inorganic materials, such as silica gel, aluminum, and titanium(IV) oxides, original and modified with F- and SO 4 2- ions, in the form of hydrogels and nanostructured powders doped by copper(II) ions, are studied. It is shown that pH values inside pores of the studied sorbents (pHintr) differ from the pH of external solutions by 0.5-1.1 units. In the pores of original γ-Al 2O3, the pHintr values are higher, while, in pores of γ-Al2O3 samples modified with F- Ee Cyrillic sign SO 4 2- , as well as nonmodified α-Al2O3, SiO2 gel, and TiO2 hydrogels, pHintr are lower compared to the pH of the external solution. It is found that pHintr decreases, as the content of Cu2+ ions in the TiO2 phase grows, and, beginning with a certain moment, it coincides with pH of an equilibrium solution. The pK a values of functional group dissociation in some materials are determined. It is shown that, using spin probes, that is, pH-sensitive nitroxyl radicals, the electrostatic potential of the surface of nanostructured TiO 2 can be estimated. © 2007 Pleiades Publishing, Ltd