The effects of additives on the porosity and stability of amorphous calcium carbonate

Abstract Amorphous calcium carbonate (ACC) stabilized by various carboxylic additives was synthesized by incorporating the additives into an ACC suspension in methanol. The additives studied included polyacrylic acid, citric acid, adipic acid, 6-aminocaproic acid, 4-aminobutyric acid and hexanoic acid. The stabilized ACC samples (ACC-additives) exhibited similar characteristics to ACC alone. They appeared X-ray amorphous, contained characteristic infrared bands and had the same nanoparticle aggregated microstructure as ACC. The porosity of the ACC-additives was, however, markedly improved, with Brunauer-Emmett-Teller (BET) surface areas of up to ∼640 m2/g. The BET surface area of ACC-citric acid was close to double that of a highly porous ACC sample. The structure and amount of the additive had a noticeable effect on the porosity of the ACC-additives. When the additive was adsorbed onto the surface of the ACC nanoparticles, their growth was restricted. The restricted growth reduced the size of the ACC nanoparticles, which increased the BET surface area of ACC. Finally, the long-term stability study revealed that the stability of all the ACC-additives was markedly enhanced when stored in ambient or semi-airtight conditions (in a closed falcon tube). In particular, ACC stabilized with adipic acid (ACC-AA-267) had excellent stability, remaining in an amorphous phase for more than one year under ambient conditions and retaining ∼87% porosity for 48 weeks under semi-airtight conditions. The extremely high porosity and excellent long-term stability make these ACC-additives promising candidates for applications where porosity and stability are critical, such as those involving adsorption, bone regeneration or drug delivery.