Stability of Calcium-Alumino Layered-Double-Hydroxide Nanocomposites in Aqueous Electrolytes

Calcium-alumino layered-double-hydroxide (LDH) nanocomposites of the alumino-ferrite monosubstituent subgroup hosting alkyl sulfates and poly(ethylene glycol) were synthesized by coprecipitation over the temperature range 5-75 °C. The stability of these nanocomposites was examined following exposure to aqueous solutions conditioned to a range of concentrations of intercalant anions including sulfate, carbonate, chloride, and phosphate. Careful analysis of these "organic-inorganic" nanocomposites reveals that their gallery (interlayer) spacing can vary, and the gallery height is controlled by the chain length (size) and orientation of the surfactant substituents, where, expectedly, anionic surfactants intercalate more robustly than nonionic surfactants. Upon anion exposure, in general, the nanocomposites (i) are unstable at SO₄ ^2-(aq) ≥ 10 mM, wherein they convert to the AFt (ettringite) phase, (ii) form solid solutions containing carbonate for CO₃ ^2-(aq) ≤ 1.5 mM and which coexist with calcite at higher carbonate concentrations, (iii) form solid solutions containing phosphate at PO₄ ^3-(aq) ≤ 1.45 mM and convert to hydroxyapatite at higher phosphate abundance, and (iv) form solid solutions for Cl^-(aq) ≤ 100 mM, but at higher concentrations, Cl^- displaces the surfactants, forming Friedel's salt. These insights into the stability and phase relationships for calcium-alumino LDH nanocomposites improve our understanding of these materials for applications including contaminant sorption, as carriers for the functional release of chemical additives in construction materials, and in biotherapeutic applications.