Structure and solubility behaviour of zinc containing phosphate glasses

The structure of phosphate glasses of general composition 10Na₂O : (20 + x/2)ZnO : (20 + x/2)CaO : (50 -x)P₂O₅ (0 ≤x≤ 20) has been investigated using IR spectroscopy, 1D ³¹P and ⁴³Ca MAS Bloch decay, ³¹P-³¹P double quantum MAS-NMR and ⁴³Ca and ⁶⁷Zn static NMR techniques, as well as neutron diffraction analysis. Zinc is shown to aid glass formation in this system. Glass transition temperature and density increase with increasing cation : phosphate ratio. However, free volume calculations show structures becoming significantly more compact from x = 5 to x = 10. The structural data confirm depolymerisation of the glasses with increasing cation : phosphate ratio. Zinc oxide is found to act in a network forming role in the system, with ⁶⁷Zn NMR and neutron diffraction analysis confirming zinc exhibits predominantly four-coordinate geometry. Solubility in deionised water and tris/HCl buffer solution is seen to decrease significantly with increasing x-value. This is discussed in terms of water ingress and the degree of structural openness, associated with increased cross-linking and a decrease in concentration of P-O-P linkages. pH measurements confirm invert phosphate compositions maintain physiological pH levels on immersion in water and buffer solutions for up to four weeks.