Surface-Induced Quadrupolar Relaxation (SIQR): Probing Adsorption of Monovalent Ions (NH₄ ⁺, NO₃ ^-, Li⁺, Na⁺, Rb⁺, Cs⁺, Br^-) on Anatase Titanium Dioxide in Aqueous Media by NMR

The adsorption of small ions on the surface of inorganic material has been studied extensively through the lens of nuclear magnetic resonance relaxation enhancements. In mixtures of a commercial titanium dioxide (TiO2) powder with various aqueous electrolytes based on ammonium nitrate (NH4NO3) and alkali (Li, Na, Rb, Cs) bromide, the adsorption of Li+, Na+, Rb+, Cs+, Br-, NH4+, and NO3- onto TiO2 was investigated with N-14, N-15, Li-7, Na-23, Rb-87, and Cs-133 and Br-81 NMR pi/2 single-pulse spectra and longitudinal relaxation curves analysis (T-1). In the TiO2/NH4NO3 slurry, the preferential adsorption of NH4+ ions on titania was reflected in the broadening of their N-14 peak and the increase of the longitudinal relaxation rates of both (NH4+)-N-14 and (NH4+)-N-15, with a more significant effect for N-14 (undergoing surface-induced quadrupolar relaxation, SIQR) than N-15 (experiencing dipolar relaxation). To probe alkali ion adsorption on TiO2, single pi/2 pulse NMR spectra were first recorded on slurries containing a single alkali ion. Fractions of adsorbed ions and the corresponding density per unit area were derived from the signal losses for Na-23(+) and Rb-87(+). The latter approach was limited to Li-7(+) and Cs-133(+), for which no signal loss was recorded, although SIQR was observed for all ions. To compare the alkali adsorption properties, simultaneous exposures of TiO2 surfaces to pairs of cations allowed SIQR to be measured and surface affinities to be ranked in the following order Cs+ > Rb+ > Na+ > Li+, which is strongly correlated to the water structure-breaking properties of both TiO2 and ions.