Decomposition of silicate glasses in alkaline solutions

Abstract

WHEN a silicate glass comes into contact with a neutral or a slightly alkaline aqueous solution the only reaction which occurs is a diffusion controlled hydrogen–alkali ion exchange. The silica network is stable. In solutions with pH > about 9, the network starts to break down at its interface with the solution. The rate of breakdown increases sharply with increasing pH; simultaneously the ion exchange rate decreases sharply. In solutions with pH > about 12 it is difficult to detect the alkali released in solution by classical microanalytical methods as the solution is primarily made up of alkali hydroxide. It has been repeatedly suggested that the ion exchange reaction ceases to occur under these conditions and that the decomposition process becomes only a network breakdown. Surface and in-depth concentrations of alkalis in a glass which has been subjected to a highly alkaline solution have been determined by Auger electron spectroscopy (AES). Our results indicate that the ion exchange does not cease to occur in highly alkaline solutions as commonly believed.