Résumé
We compare traditional zeta potential measurements with a surface-induced quadrupolar relaxation-based approach to characterize the surface electric properties of oxides in aqueous media, particularly at high ionic strengths and solid contents where classical methods fail. Three oxide materials (two TiO2 and one α-alumina) were tested in 1 M NaBr aqueous solutions. Zeta potential measurements are expected to yield reliable isoelectric points (IEP) at moderate solid contents and up to 0.1 M ionic strengths. In contrast, NMR relaxation rate measurements, exploiting surface-induced quadrupolar relaxation (SIQR) of 81Br and 23Na, successfully provided IEP even at 1 M ionic strength and high solid content, although they are usually much more challenging under conditions suitable for conventional zeta potential measurements. Our NMR results correlated well with zeta potential trends at lower concentrations. Notably, TiO2_325 showed only acidic surface behavior, likely due to surface carbonate and/or phosphate groups. Surface acidity constants derived from NMR data allowed IEP estimation consistent with the literature values. This demonstrates that NMR relaxation rate measurements provide a robust alternative to zeta potential experiments under conditions where the latter are unreliable.