Non-random bonding of mono/divalent cations in mixed phosphate glasses

Incorporation of Sr to biocompatible Ca-phosphate glasses opened many possibilities for clinical applications. Transport properties relevant to the cation release in physiological conditions depend, among other factors, on the interaction with the PO4 forming groups. To understand these issues, three series of mixed glasses with mono (Na+) and divalent (Ca2 +/Sr2 +) cations were considered here: polyphosphates (0.57 − x) Na2O ∙ x MO ∙ 0.43 P2O5 (MCa, Sr; 0 ≤ x ≤ 0.57), and metaphosphate (0.50 − x) Na2O ∙ x SrO ∙ 0.50 P2O5 (0 ≤ x ≤ 0.50). Molar volumes and glass transition temperatures were analyzed as a function of the concentration of the alkaline earth. 31P and 23Na nuclear magnetic resonance techniques were applied to analyze local structure around phosphate groups and Na sites. As a function of the alkaline earth content, transitions in the evolution of properties (molar volume, glass transition temperature and 31P resonance of Q1 chain-end tetrahedra) were detected in the polyphosphate series. This behavior can be interpreted in terms of preferential bonding of the divalent cation to the oxygens in (PO3.5)2 − anions (Q1), instead of (PO3)− (Q2). This non-statistical bonding to the available O is only possible up to a maximum concentration of the alkaline earth oxide (x = 0.28), in good agreement with the observed break in properties of NaSr polyphosphate (x = 0.30). A similar behavior seems plausible for Ca2 + in NaCa polyphosphate, but changes in the medium range order and progressive structural distortions induced by the stronger ion cause smooth variations of the molar volume. These findings may be relevant to understand the ion dissolution rates in biocompatible glasses, as the more hydrolysable Q2 groups concentrate the weaker cations, while the stronger Sr2 + or Ca2 + are located near chain-end Q1 groups.