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Why does increasing the alkali content of oxide glass reduce chemical durability?
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Increasing the alkali content in oxide glass reduces chemical durability primarily by altering the glass's atomic structure and increasing the mobility of ions within the material.1
In silicate glasses, alkali ions act as network-modifying (NWM) ions. Their addition increases the concentration of nonbridging oxygens, which lowers the connectivity of the silica network. This structural change has two main effects:1
- Increased Ion Mobility: Alkali ions are the most mobile species within the interstices of oxide glass. A higher concentration of these ions facilitates an ion exchange reaction where alkali ions in the glass are exchanged with hydrogen or hydronium ions from water or atmospheric humidity. This process leads to surface weathering and the formation of deposits.21
- Reduced Network Integrity: The presence of more NWM ions creates a more open structure, which allows for greater permeation of gases and moisture into the glass.1
Consequently, glasses with lower alkali concentrations generally exhibit superior weathering resistance, whereas high-alkali compositions are more susceptible to chemical attack by water and acidic solutions.23