Surface chemistry of glasses: Sorption, ion exchange and durability

The chemical durability of silicate glasses is a key property in many of their applications and a major problem in surface chemistry. The durability or corrosion resistance of silicate glasses is a function not only of glass composition and leaching conditions (e.g., temperature) but also of the composition of the aqueous phase.;This experimental work shows that the key process in attenuating silicate glass corrosion involves the sorption of reactive metal ions, specifically Al, under controlled conditions, leading to formation of sparingly soluble (or easily adsorbable) aluminosilicate species at the glass surface. In the case of other ions, such as Mg, the interaction with the glass appears to consist of exchange with alkali ions present in the glass. In order to separate out the effect of the presence of reactive ions such as Mg and Al on the glass dissolution from the effect of pH, most of the experiments were carried out in buffered media. A simple two-species model, based on the Langumuir isotherm, was developed and used to interpret the experimental data. One of the two species is the metal ion (Mg or Al) and the other is the buffering species, wherever used.;This work also shows conclusively that glass durability is strongly correlated to the glass composition, so that when Al and Mg species are introduced subsequent leaching of silicate glasses in water is similarly retarded whether the additives (Mg or Al) are initially in the glass or in the leachant. In both cases the additives are eventually involved in the formation of corrosion resistant surface species.;Although sorption and ion-exchange are known to take place on glass surfaces, their relevance to glass corrosion has been consistently overlooked. These mechanisms are believed to be the rate controlling processes that cause the corrosion to be slowed or shut down completely. In general, the extent of sorption from solution depends on the nature of the solid sorbent, the nature of the solute and the nature of the solvent. The detailed manner in which each of these influences the extent of sorption and, thus glass durability is the objective of this work. More generally, this work focuses on the basic correlation between sorption and glass durability.