Carbonate Environment Changes with Na or K Substitution in Biomimetic Apatites

Biological and biomimetic apatites allow incorporations of many cationic and anionic substituents, namely Na+, CO32- , and potentially K+, that influence apatite's physicochemical, thermodynamic, and structural properties. Carbonate substitution can modify these properties depending on the CO32- type, such as hydroxyl (A-type) or phosphate (B-type) sites, or labile ions within the non-apatitic surface layer. While Na+ co-substitutions can affect CO32- integration, it is unknown if and how K+, a larger cation, may affect CO32- substitution and related apatite properties. Therefore, the purpose of this study was to compare the effects of K+ and Na+ on the physicochemical incorporation of CO32- in biomimetic apatites. To do so, 2-6 wt% CO32- apatites were precipitated in either Na- or K-rich solutions under optimized synthesis and maturation conditions. Atomic absorption data showed more Na+ substitution into the apatite structure than K+. FTIR and XRD results indicated that Na-apatites primarily contained B-type CO32- , while K-apatites included more A-type CO32- at higher total wt% CO32- . A deep FTIR analysis of the CO32- vibration modes showed that a red-shift occurred for the nu 2CO32- and nu 3CO32- B-type of the Na-apatites, suggesting a longer C-O bond length, while K-apatites had a slight blue-shift for only nu 2CO32- . Apatitic and non-apatitic HPO42- retention was also higher for Na-apatites than K-apatites. Together, these observations suggest that modification of the local CO32- environments depends on the monovalent cation. Overall, our data reveals the distinct mechanisms for Na+/CO32- and K+/CO32- co-substitution, which may shed light on the carbonation of biological apatites and their structural features.