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The reaction equation for the reaction between water and silica reads ≡Si-O-Si≡ +H$_2$O ↔ [2 SiOH] For this reaction the equilibrium constant, k, is affected by internal swelling stresses. We investigated the behaviour of erfc-shaped distributions of the total water amount in earlier Reports. The present report is intended to evaluate more complicated water profiles. Water concentrations at silica surfaces were measured by Zouine et al. Some of their data showed deviations from the expected erfc-shape. The associated datasets were used to demonstrate the procedure of determining the local equilibrium constant and the distributions of the hydroxyl and molecular water species on irregular water distributions. In an Appendix we discuss what could be the reason for the deviations in the distribution of the total water C$_w$ from the erfc-shape. The occurrence of back-diffusion after aging is assumed.

In the present Report, we will discuss the irregular water distributions at silica surfaces as were reported by the measured data from Zouine et al. [1], especially those at room temperature. The question was whether the deviations from the usually found erfc-shaped water pro- files can be understood by competing diffusion and surface dissolution and whether back-diffusion after water or air storage, may affect the type of water profile. We will come to the conclusion that the irregular distributions of the measured data by Zouine et al. [1], especially those at room tem- perature, can be understood by back-diffusion after water or air storage, respectively.

In previous reports on the mass transfer coefficient of quartz glass in water, we were able to determine that this is significantly larger in liquid water than in saturated water vapor as the surrounding medium. From literature results by Zouine et al. (2007) a lower limit value of the time parameter h/√D in eq.(4) of >10$^{-6}$ h$^{-1/2}$, could be deduced for 200°C and liquid water. One explanation for this result could be, among other things, an increased roughness due to the dissolution of the surface layers of the glass in water than is to be expected for water vapour.

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

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