Modeling the interfacial properties of an amorphous aluminosilicate dispersed in aqueous NaCl solutions

The structural and interfacial properties of an amorphous aluminosilicate prepared from aluminum chloride and sodium silicate solutions are reported. IR and Al-27 NMR spectroscopy revealed that the structure of the studied sample was similar to that of(hydrous) feldspathoids and amorphous aluminosilicate catalysts, which are characterized by the presence of negatively charged Si-O-Al groups (SiAlO-) and hydroxylated surface groups. The particles were non-porous and dissolved appreciably at pH < 5.2. H+, Cl- and Na+ adsorption data (Gamma(H), Gamma(Cl) and Gamma(Na)), together with electrophoretic measurements, revealed, however, that the point of zero saline effect (PZSE) was different from the isoelectric point (IEP) and from the pH where Gamma(Cl) - Gamma(Na) = 0. This behavior differs from that of (hydr)oxides that contain only variable charge (amphoteric) surface sites. A simple model of the solid-NaCl solution interface is proposed to account for the cation exchange properties of SiAlO- sites and for the acid-base character of the hydroxylated ones. The lack of coincidence between the PZSE and the IEP and the invariability of the IEP with the electrolyte concentration is attributed to the presence, and cation exchange properties, of SiAlO- sites. However, the model suggests that a high proportion of structural charge sites are neutralized by sodium ions in the particle bulk and cannot be assessed for cation exchange. The ability of the model to fit experimental adsorption data suggests that it can be used to improve the understanding of the amorphous aluminosilicate-electrolyte solution interface.