Non-linear dissolution mechanisms of sodium calcium phosphate glasses as a function of pH in various aqueous media

TL;DR

This study investigates the complex dissolution mechanisms of phosphate glasses used in bioresorbable implants, revealing a two-stage process influenced by pH and composition, and proposing a new diffusion-controlled model.

Contribution

It introduces a novel two-stage dissolution model for phosphate glasses, highlighting the impact of pH and composition on dissolution behavior and transition dynamics.

Findings

Dissolution follows a two-stage process with parabolic then linear time dependence.

Lower Ca concentration and pH accelerate dissolution.

A new diffusion-controlled formation of a conversion layer mechanism is proposed.

Abstract

Phosphate glasses for bioresorbable implants display dissolution rates that vary significantly with composition, however currently their mechanisms of dissolution are not well understood. Based on this systematic study we present new insights into these mechanisms. Two-stage dissolution was observed, with time dependence initially parabolic and later linear, and a two-stage model was developed to describe this behaviour. Dissolution was accelerated by lower Ca concentration in the glass, and lower pH in the dissolution medium. A new dissolution mechanism is proposed, involving an initial stage where diffusion-controlled formation of a conversion layer occurs. Once the conversion layer is stabilised, layer dissolution reactions become rate-limiting. Under this mechanism the transition time is sensitive to the nature of the conversion layer and solution conditions. These results reveal the dependence of P$_{2}$O$_{5}$-CaO-Na$_{2}$O glass dissolution on solution pH, and provide new insight into the dissolution mechanisms, particularly regarding the transition between the two dissolution stages.