A reason for the occurrence of mass-transfer coefficients at silica surfaces

At silica surface exposed to a water vapour environment, there is a monotonously increasing water concentration observed that calls for a limited mass transfer from the humid environment to the surface. Such behaviour is characterized by a “mass transfer coefficient". We have demonstrated in several studies that silica glass surfaces may present a barrier to the diffusion of water into silica. We could interpret the experimental diffusion results of Oehler and Tomozawa (2004) and those of Helmich and Rauch (1993). Such a surface layer formed by diffusion and water reaction with the surface material, was studied by Mahadevan and Garofalini (2008). Compressive hydrostatic stresses in the surface layers reach a maximum value of σ$_{sw,h}$ = − 2.8GPa. Due to such high compressive stresses, the water diffusivity must be strongly reduced within this thin layer and, consequently, also the mass transfer coefficient. These effects will be considered in detail in this report.