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 ν₂CO₃²⁻ and ν₃CO₃²⁻ B-type of the Na-apatites, suggesting a longer C–O bond length, while K-apatites had a slight blue-shift for only ν₂CO₃²⁻. 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.