Hydroxyl group ordering and its effect on post-crystallization structural memory retention in hydroxyapatite

Hydroxyapatite (HAp), the sole inorganic material endogenous to the human body, is similar to water in a sense that they both mesmerize with their anomalies. One peculiarity of HAp lies in its capacity to display the structural memory retention effect, where the mechanistic pathways governing the formation or transformations of HAp remain impressed in the material, predisposing it for particular properties and performance. Because hydroxyl groups are most challenging of all atomic constituents of HAp to discern during structural characterizations, it is hypothesized that their arrangement may explain why HAp materials obtained via different routes, yet exhibiting nearly identical microstructural characteristics, behave differently. To that end, metastable amorphous calcium phosphate was made to mature into HAp via two different routes, one relying on maturation in liquid water and one in ambient air. Then, the hydroxyl group characteristics in these two materials were compared against each other and against the metastable amorphous precursor and HAp synthesized directly using three distinct spectroscopic methods: nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) and Raman vibrational spectroscopies. A lower degree of deshielding of OH protons caused by the increased compactness of the lattice and by the establishment of the polarized arrangement of OH groups inhabiting the c-axis channel was observed in HAp formed directly compared to HAp materials formed from the metastable amorphous precursor. This, in turn, was shown to create a more isotropic distribution of electronegativity around the OH protons, thus, rather counterintuitively, liberating channel OH ions from the influence of the adjacent atomic groups. Based on the least deshielded OH protons, the freest OH stretching mode and the narrowest distribution of states associated with OH stretches in HAp obtained directly as compared to HAp materials obtained from a metastable amorphous precursor through different routes, it is concluded that the structural memory effect displayable by forms of HAp obtained via different mechanistic pathways can be traced to the characteristic structuring of the OH channel and ions occupying it.