Hydrogen bond formation between citrate and phosphate ions in spherulites of fluorapatite

Samples of carboxylate-fluorapatite are prepared with citric, tricarballylic, and glutaric acids under hydrothermal conditions. The size of the hexagonal rods differs significantly for the three samples, of which the citric-acid sample exhibits the smallest dimension along the [h00] direction. The solid-state NMR data reveal that all the citrate molecules of citrate-fluorapatite are in direct contact with the fluorapatite surface and that there are at least two binding modes accounting for the interaction between citrate and fluorapatite surface. In addition to the electrostatic interaction between the carboxylate carbons and the calcium ions, some citrate molecules also form hydrogen bond between the hydroxyl group of citrate and the orthophosphate ion of fluorapatite. This hydrogen-bond interaction is highly ordered and may play an important role in the formation of the spherulites.