Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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Z.R.Tan, C.Pagnoux, V.Sarou-Kanian, M.Deschamps, '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', J. Phys. Chem. C 127 19717–19727 (2023) doi:10.1021/acs.jpcc.3c03178

High-field Nuclear Magnetic Resonance (NMR) of quadrupolar nuclei is widely used to describe local structural environments and dynamics at the atomic scale in heterogeneous solid-liquid media. In the specific case of inorganic materials (oxides, clays, concrete, …), these studies are most often limited to systems with low ionic strength (< 0.1 mol/L) and/or low solids content (< 1 g of solid/g of liquid or <50 w%). Such conditions generally promote the dispersion of the solid phase into stable colloidal suspensions during measurements. In this study, NMR is used to probe ion-surface interactions in media with high ionic strength (1 and 2 mol/L) and high solid contents (0.5-4 g/g, i.e. 33-80 w% in solids), which are very hard to characterize with conventional interface characterization methods. The systems consisted in mixtures of a commercial titanium dioxide powder with various aqueous electrolytes based on ammonium nitrate (NH4NO3) and alkali (Li+, Na+, Rb+, Cs+) bromide. 14N, 15N, 7Li, 23Na, 87Rb and 81Br NMR spectra and longitudinal relaxation curves (T1) were recorded. Increased NMR relaxation rates and/or signal losses for cations showed that our TiO2 particles have negative surface charges in aqueous media, leading to NMR detected cation-surface interactions, while no effects were observed for anions, indicating that anionic adsorption is not taking place. Simultaneous exposures of surface to pairs of cations allowed us to compare their adsorption behaviors and surface affinities according to Cs+ > Rb+ > Na+ > Li+ which happens to be strongly correlated to their water structure breaking properties. Collaborative adsorption effects between Li+ and Rb+ ions are also discovered.