Geometrical and chemical effects of water diffusion in silicate gels: Molecular dynamics and random walk simulations

• Published in: Journal of the American Ceramic Society 2024-12, Vol.107 (12), p.7770-7783
• Main Authors: Hatori, Takuma, Matsubara, Ryuta, Inagaki, Yaohiro, Ishida, Keisuke, Ohkubo, Takahiro
• Format: Article
• Language: English
• Subjects: Chemical bonds; Chemical effects; Diffusion layers; Diffusivity; Gels; hydration; Hydrogen bonds; Mass transport; Moisture content; Molecular dynamics; Nuclear safety; Pore size; Porosity; radioactive waste; Radioactive wastes; Random walk; silicates; Simulation; Water
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.19935

Understanding mass transport in the alteration layers of glass surfaces is a crucial component of the safety assessment of nuclear waste glass. In this work, we model such an alteration layer as a silicate gel with water through a molecular dynamics (MD) simulation with a reactive force field. Gels with various water contents (WCs) ranging from 5.1 to 30.7wt%$30.7 \,{\rm wt}\%$ are produced via high‐temperature annealing with water and silica. It is found that an increase in the water content destroys the polymerized structure of the silicate network and promotes the formation of silanol groups. The pore size and water connectivity formed by the silicate networks are investigated for the modeled gels. Gel with a WC of 5.7wt%$5.7 \,{\rm wt}\%$ is composed of isolated water in the pores; in contrast, pores filled with interconnected water are formed in gel with a WC of 30.7wt%$30.7 \,{\rm wt}\%$. The water diffusivity in the modeled gel is calculated using the mean‐squared displacement at various temperatures. An attempt is made to formulate a linear relationship between the water diffusivity and porosity derived from the MD simulation. The porosity is calculated using a probe atom with a radius, which was optimized from a linear relationship between the water diffusivity and porosity. This approach successfully explains the water diffusivity in terms of the porosity. Random walk (RW) simulations for the structures derived from the MD simulations are performed to determine the geometrical effects of the pores. The diffusivity obtained from RW simulation is compared with the results of the MD simulations, which include chemical interactions such as the formation and breakage of hydrogen bonds. This comparison highlights how geometrical effects and chemical interactions contribute to water diffusivity depending on the WC.