Here we report that citrate plays a key dual role in the apatite crystallization: driving a growth pathway via an amorphous precursor and controlling the size of nanocrystals by the non-classical oriented aggregation mechanism. These results provide new insights into bone mineralization, where the role of citrate might be wider than has been thought to date.

The affinity towards water of a selection of well-defined, nanostructured hydroxyapatite (HA) samples was investigated by H ₂ O vapour adsorption microcalorimetry and infrared (IR) spectroscopy. A large hydrophilicity of all investigated materials was confirmed. The surface features of hydrated HA were investigated on the as-synthesized samples pre-treated in mild conditions at T =303 K, whereas dehydrated HA features were characterized on samples activated at T =573 K. The relatively large hydrophilicity of the hydrated surface (−Δ _ads H∼100–50 kJ mol ⁻¹ ) was due to the interaction of water with the highly polarized H ₂ O molecules strongly coordinated to the surface Ca ²⁺ cations. At the dehydrated surface, exposing coordinatively unsaturated ( cus ) Ca ²⁺ cations, H ₂ O was still molecularly adsorbed but more strongly (−Δ _ads H∼120–90 kJ mol ⁻¹ ). The use of CO adsorption to quantify the Lewis acidic strength of HA surface sites revealed only a moderate strength of cus Ca ²⁺ cations, as confirmed by both microcalorimetric and IR spectroscopic measurements and ab initio calculations. This result implies that the large HA/H ₂ O interaction energy is due to the interplay between cus Ca ²⁺ sites and nearby hydrophilic PO ₄ groups, not revealed by the CO probe. The lower density of cus Ca ²⁺ cations at the 573 K activated HA surface with respect to the pristine one did not affect the whole hydrophilicity of the surface, as the polarizing effect of Ca sites is so strong to extend up to the fourth hydrated layer, as confirmed by both high-coverage microcalorimetric and IR spectroscopic data. No specific effects due to the investigated specimen preparation method and/or different morphology were observed.