Ammonium-to-sodium ion-exchange process at the interlayer of octacalcium phosphate

Octacalcium phosphate (OCP) has been considered as the layer component of calcium phosphate, but whether it achieves the ionic-exchange ability of conventional layer components is unclear. As OCP is highly biocompatible, understanding its ionic-exchange properties would potentially expand its pharmaceutical and medical applications. Herein, we demonstrate that the substituted cations in ammonium (NH₄)-substituted octacalcium phosphate (OCP-NH₄) and sodium (Na)-containing ammonium phosphate solutions undergo ion exchanges with OCP interlayers. Replacing NH₄ ⁺ with Na⁺ did not alter the crystal structure of OCP, confirming that a substituted cation exchange process similar to that in other layered compounds occurs in OCP.