Evidence for a threshold in the biosolubility of aluminosilicate vitreous fibers

Two series of aluminosilicate glasses have been synthesized with the nominal composition (64 − x) SiO₂–x Al₂O₃–36 Na₂O/CaO with x varying from 9 to 19 mol%. They have been corroded in static conditions in a solution that mimics in a simplified manner the intracellular medium of the lung alveolar macrophages (37 °C, pH 4.6, citric acid). The original and corroded glasses have been studied by ²⁷Al and ²⁹Si MAS NMR. Both series display a sharp increase in the silicon dissolution rate with the alumina content. The glass network dissolves extremely slowly, whereas the release of excess sodium is very fast, for the glasses with low alumina content. On the opposite, the glasses with high alumina content dissolve much more rapidly in a nearly congruent manner. The crossover between the two behaviors occurs for x = 13, which corresponds to 33% of aluminum in the glass-former network. The sharp crossover from slow to fast network dissolution is explained in terms of connectivity of the silica sub-network. Above a certain amount of alumina, the silicon sub-network is no more percolating and the corroded glass breaks up into colloids. The sharpness of the transition and the relatively low alumina content required for fast dissolution are related to a structural feature of the aluminosilicate glasses, namely the aluminum self-avoidance that decreases the connectivity of the silica sub-lattice.