Effect of the degree of supersaturation on the synthesis of the layered double hydroxides of Ca and Al

The layered double hydroxide (LDH) of Ca and Al is more soluble than the hydrotalcite family of compounds. The pH at precipitation is in the range 11.5–12. At this high pH, the Al³⁺ ion tends to re-dissolve given the amphotericity of the hydroxide of Al³⁺. These factors introduce a degree of irreproducibility in the outcome of a coprecipitation reaction. This work showed that the outcome is critically dependent on the degree of supersaturation. By carrying out a number of coprecipitation reactions in a fixed volume of reaction mixture, the exact base concentration required to crystallize the LDH was obtained, paving the way for the synthesis of LDHs with different anions intercalated. While a quantitative treatment of the ionic strength facilitating the nucleation of the desired phase is beyond the scope of this work, it is empirically seen that moderate base concentrations result in the desired outcome. Higher and lower base concentrations lead to the crystallization of competing phases.