Distribution of equilibrium constant k and hydroxyl S in silica surface layers

The concentration of water penetrated into silica depends strongly on the stress state in the surface region. In the water diffusion zone hydroxyl water is generated by the water/silica reaction that is at temperatures <450°C a first-order re action. When [2S ]=[2 SiOH] is the concentration of the im movable hydroxyl, the reaction equation reads

≡Si-O-Si≡ +H$_2$O ↔ [2 SiOH]

For this reaction the equilibrium constant is strongly affected by stresses, internal swelling stresses and externally applied stresses. Since no activation volumes for this first- order reaction are available in literature, we will show here the principal influence by a parameter study. As the main results we concluded:

1) With increasing reaction volume ∆V, the concentration of the hydroxyl S(z) at the surface is significantly reduced and slightly increased inside.
2) The shape of the S-distribution deviates strongly from the profile of the total water concentration, C$_w$(z).
3) The product of the "effective layer thickness" and the level of the S- concentration, S(0)×z$_{eff}$, is largely independent of the assumed reaction volume.