Bonding of alkaline and alkaline earth cations in Na-Ca-Sr polyphosphate glasses

Dissolution of phosphates bioglasses, for bone regeneration or controlled ion release, is a key property to be tuned by adequate selection of chemical species. Mixed Na-Sr and Na-Ca polyphosphate glasses exhibit definite bonding preferences of the stronger cations to O from chain-end phosphates (Q(1)) instead of random bonding to the network (Q(1) and Q(2)). This behavior is important for the control of dissolution, as hydration starts from the reaction of H with O in Q(1). To analyze this bonding preference in glasses with two alkaline earths competing for O, mixed Na-Ca-Sr polyphosphates (0.57-x) Na2O center dot x/2 SrO center dot x/2 CaO center dot 0.43 P2O5 (0 <= x <= 0.30 molar fraction) were considered. P-31 nuclear magnetic resonance was used to analyze the short-range environments of Q(n) groups, detecting preferential bonding of Ca and Sr to Q(1). To probe variations in the medium-range organization of the network induced by Ca or Sr, glasses with compositions (0.57-x) Na2O center dot x MO center dot 0.43 P2O5 (M = Ca,Sr; 0 <= x <= 0.30 molar fraction), were analyzed using P-31-double-quantum techniques. An increase in mutual Q(2) self-connectivity relative to the base Na-polyphosphate network was detected as Ca or Sr were added, indicating conversion of linear phosphate chains into rings.