Characterization of active phosphorus surface sites at synthetic carbonate-free fluorapatite using single-pulse 1H, 31P, and 31T CP MAS NMR

The chemically active phosphorus surface sites defined as equivalent to POx, equivalent to POxH, and POxH2, where x = 1, 2, or 3, and the bulk phosphorus groups of PO43- at synthetic carbonate-free fluorapatite (Ca-5(PO4)(3)F) have been studied by means of single-pulse H-1,P-31, and P-31 CP MAS NMR. The changes in composition and relative amounts of each surface species are evaluated as a function of pH. By combining spectra from single-pulse H-1 and P-31 MAS NMR and data from P-31 CP MAS NMR experiments at varying contact times in the range 0.2-3.0 ms, it has been possible to distinguish between resonance lines in the NMR spectra originating from active surface sites and bulk phosphorus groups and also to assign the peaks in the NMR spectra to the specific phosphorus species. In the P-31 CP MAS NMR experiments, the spinning frequency was set to 4.2 kHz; in the single-pulse H-1 MAS NMR experiments, the spinning frequency was 10 kHz. The P-31 CP MAS NMR spectrum of fluorapatite at pH 5.9 showed one dominating resonance line at 2.9 ppm assigned to originate from PO43- groups and two weaker shoulder peaks at 5.4 and 0.8 ppm which were assigned to the unprotonated equivalent to POx (equivalent to PO, equivalent to PO2-, and equivalent to PO32-) and protonated equivalent to POxH (equivalent to PO2H and equivalent to PO3H-) and surface sites. At pH 12.7, the intensity of the peak representing unprotonated equivalent to POx surface sites has increased 1.7% relative to the bulk peak, while the intensity of the peaks of the protonated species equivalent to POxH have decreased 1.4% relative to the bulk peak. At pH 3.5, a resonance peak at -4.5 ppm has appeared in the P-31 CP MAS NMR spectrum assigned to the surface species equivalent to POxH2 (equivalent to PO3H2). The results from the H-1 MAS and P-31 CP MAS NMR measurements indicated that H+, OH-, and physisorbed H2O at the surface were released during the drying process at 200 degrees C.