Dissolution of (50-x)Na₂O-xCaO-50P₂O₅ metaphosphate glasses in different saline Solutions: mechanism and kinetic control

Abstract

The dissolution of (50-x)Na₂O-xCaO-50P₂O₅ metaphosphate glasses has been investigated at pH 3 in Solutions containing in Na⁺, Ca²⁺, Cl⁻, NO₃⁻ and (PO₃⁻)n ions. Whatever the conditions, a hydrated layer develops over a surface covered with etch pits. Na⁺ and Cl⁻ have no specific effect on the dissolution kinetics and mechanism. In Ca²⁺-containing Solutions, XPS, electrophoretic mobility and ³¹P NMR show that Na⁺ is exchanged for Ca²⁺ in the hydrated layer, resulting in a decreased dissolution rate. Polyphosphate anions, (PO₃⁻)n, act as complexing agent for Ca²⁺, and counterbalance its inhibiting effect on the dissolution rate.