Influence of water on the tracer diffusion of sodium in glasses

Experimental results for the diffusion of the radioactive tracer Na-22 in Corning Code 1737 glass (an alkaline-earth aluminoborosilicate glass), in type I silica glass (Infrasil 302) and in model glasses of the type (CaO·Al2O3)x(2SiO2)1−x are reviewed. In addition, data for the incorporation of water into these glasses and for the influence of this water on the diffusion of sodium are considered. It was observed by FTIR spectroscopy that the overall concentration of water incorporated into glass in the form of OH groups as a function of the water uptake time follows a parabolic rate law. The water uptake leads to a very significant decrease in the sodium diffusivity in the near-surface region of the glass. The width of the near-surface region with a reduced sodium mobility as a function of the water uptake time follows a parabolic rate law. The decrease in the sodium tracer diffusion coefficient in the near-surface region is on the order of 100 to 1000, depending on the glass considered, the temperature, and, possibly to a small extent, the amount of OH groups generated. The observed decrease in the sodium diffusivity in the near-surface region is attributed to a structural relaxation that is enabled by the OH groups generated upon the uptake of water.