31P NMR characterisation of phosphate fragments during dissolution of calcium sodium phosphate glasses

Abstract

Phosphate glasses in the system P₂O₅–CaO–Na₂O dissolve in aqueous solutions, and their solubility can be varied by changing the glass composition. This makes them of interest for use as controlled release materials, e.g. as degradable implants, devices for the release of trace elements or as fertilizers, but in order to tailor glass solubility to meet specific requirements, we need to further our understanding of their dissolution behaviour and mechanism. The structure of P₂O₅–CaO–Na₂O glasses (P₂O₅ between 55 and 35 mol%; glass structure analysed by ³¹P MAS NMR) changed from a network (55 mol% P₂O₅) to short chains (35 mol%) with decreasing phosphate content. Solubility in Tris buffer showed significant differences with phosphate content and glass structure; dissolution varied between 90% (50 mol% P₂O₅) and 15% (35 mol%) at 24 h. Glasses with high phosphate contents significantly lowered the pH of the solution, while glasses with low phosphate contents did not. Glasses consisting of a phosphate network dissolved by a mechanism involving P–O–P bond hydrolysis, as no Q³ groups but increasing concentrations of Q⁰ (orthophosphate) were found in solution by solution ³¹P NMR. Glasses consisting of chains, by contrast, can dissolve by hydration of entire chains, but hydrolysis also occurred, resulting in formation of Q⁰ and small ring structures. This occurrence of hydrolysis (and thus formation of P–OH groups, which can be deprotonated) caused the pH decrease and explains the variation in solution pH with structure.