NOVEL IN SITU METHOD FOR PAIR DISTRIBUTION FUNCTION ANALYSIS MEASURES STRUCTURE AND EVOLUTION OF HYDROXYLAPATITE PRECURSOR PHASES

The crystallization pathway of hydroxylapatite has been a focus of study for over forty years, specifically (carbonated) calcium phosphate bone minerals. This makes synthetic hydroxylapatite an ideal proxy for investigating biomineralization processes and for the development of bone replacement materials. Crystallization of hydroxylapatite is known to proceed through the formation of precursor phases, such as crystalline brushite and amorphous calcium phosphate (ACP). However, previous investigations into the structure of these precursors have primarily fallen into two categories; ex situ analysis of dried ACP or ab initio molecular dynamic models. A limited number of experimental studies of calcium phosphate precursors have attempted to examine the short-range structure of ACP in its native hydrated form.

Our experiments use a novel mixed-flow rector (MFR) synthesis method to examine the precursor phases in situ. Calcium phosphate forms in the MFR from reaction of calcium chloride and sodium phosphate solutions. Structural information for the resulting precipitates in solution is obtained via synchrotron total scattering for pair distribution function (PDF) analysis. The MFR set-up enables precise control of system chemistry and sample age, enabling investigations into the structural characteristics of the solids phases that occur early in the crystallization process. Results show the synthesis of hydroxylapatite and its precursor phases by this method is consistent between replicates and highly adaptable to various initial conditions.

Our investigation has focused on how initial solution conditions, like the ratio of calcium to phosphate during mixing, can affect the structure and evolution of the precursor phases. As the Ca/P ratio increases, the bonding geometry between calcium and phosphate exhibits more monodentate geometries. In addition, crystallization is occurring more rapidly at ratios at 1.0 and slightly higher. This is similar to results reported by classical growth studies on the impact of initial ion ratios on growth rate for both calcite and celestite. Further investigations with the MFR will allow us to examine the impacts of other initial conditions such as supersaturation, sample age, and varying initial chemistries.