Characterization of aqueous interactions of copper-doped phosphate-based glasses by vapour sorption

Owing to their adjustable dissolution properties, phosphate-based glasses (PGs) are promising materials for the controlled release of bioinorganics, such as copper ions. This study describes a vapour sorption method that allowed for the investigation of the kinetics and mechanisms of aqueous interactions of PGs of the formulation 50P2O5–30CaO–(20–x)Na2O–xCuO (x = 0, 1, 5 and 10 mol.%). Initial characterization was performed using 31P magic angle spinning nuclear magnetic resonance and attenuated total reflectance–Fourier transform infrared spectroscopy. Increasing CuO content resulted in chemical shifts of the predominant Q2 NMR peak and of the (POP)as and (PO−) Fourier transform infrared absorptions, owing to the higher strength of the POCu bond compared to PONa. Vapour sorption and desorption were gravimetrically measured in PG powders exposed to variable relative humidity (RH). Sorption was negligible below 70% RH and increased exponentially with RH from 70 to 90%, where it exhibited a negative correlation with CuO content. Vapour sorption in 0% and 1% CuO glasses resulted in phosphate chain hydration and hydrolysis, as evidenced by protonated Q0(1H) and Q1(1H) species. Dissolution rates in deionized water showed a linear correlation (R2 > 0.99) with vapour sorption. Furthermore, cation release rates could be predicted based on dissolution rates and PG composition. The release of orthophosphate and short polyphosphate species corroborates the action of hydrolysis and was correlated with pH changes. In conclusion, the agreement between vapour sorption and routine characterization techniques in water demonstrates the potential of this method for the study of PG aqueous reactions.