The synthesis of long-term stable amorphous calcium carbonate in water-free ethylene glycol system without any phase stabilizer

Porous amorphous calcium carbonate (ACC) with long-term stability is easy to obtain by mixing the glycol solution of CaCl 2 and K 2 CO 3 without any phase stabilizer. For ACC synthesized at different temperature (from 0 °C to 60 °C) aging for 30 min, the number and the size of pores decrease with the elevating of reaction temperature, and large irregular aggregations become small irregular particles even small irregular spherical particles (near ellipsoid). A dense semi-spherical particle with the diameter about 600 nm at a polar of a near ellipsoidal particle is found in ACC obtained at 60 °C. The selected area electronic diffraction supports that dense semi-spherical particle is vaterite with poor crystallinity and other part of the ellipsoidal particle is ACC, although the diffraction peaks of vaterite are not found in XRD patterns. This result indicates that ACC has the dynamic condition for crystallization into vaterite at 60 °C. • Amorphous calcium carbonate (ACC) is prepared without any phase stabilizer. • The water-free glycol solutions of CaCl 2 and K 2 CO 3 are used as reaction solution. • Effect of aging times on the polymorphs and morphologies of CaCO 3 is studied. • Effect of reaction temperatures on the polymorphs and morphologies of CaCO 3 is studied. • ACC can maintain amorphous in vacuo and in organic solvent for a long time. Amorphous calcium carbonate (ACC) plays an important role in the biomineralization process and has a wide application in biological medicine area. In the present work, stable ACC was successfully prepared in water-free ethylene glycol system, by mixing the ethylene glycol solution of CaCl 2 and K 2 CO 3 without phase stabilizer. The CaCO 3 obtained at different aging times (from 1 min to 3 months) and reaction temperatures (from 0 °C to 60 °C) were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electronic diffraction (SAED). XRD results indicate that ACC is the only phase in all samples and SAED results verify the existence of ACC. SEM and TEM images display that the irregular nano-particles were aggregated into irregular porous micro-particles, the pores become smaller with the aging time increase and the reaction temperature elevating, and some relative regular particles (near ellipsoidal or spherical) were formed when system is aged for a long time (3 months) at a low temperature (20 °C) or for a short time (30 min) at a high temperature (60 °C). As-synthesized ACC can be persistent in vacuum or organic solvent for a long time (1 month) at room temperature.