Effects of Na/K-Cl Salts on Hydrolysis of Aluminosilicate Glass Using Ab Initio Molecular Dynamics

The structural and chemical modifications on the surface of pure and alkali-doped aluminosilicate (AS) glasses due to hydrolysis are investigated using ab initio molecular dynamics. The effects of water on the glass network are fully elucidated by analyzing the short- and intermediate-range structural orders embedded in the pair distribution function, bond length and angle distribution, coordination number, and interatomic bonding. A novel concept of total bond order is used to quantify and compare the strength of bonds in hydrated and unhydrated glasses. We show that AS glass is hydrolyzed by water diffusion near the surface and by proton (H⁺) transfers into the bulk, which increases with time. Hence, a dissolved glass-water interface becomes rich in Si-OH and Al-OH. The alkali ions associated with the nonbridging oxygen accelerate the hydrolysis by facilitating water and H⁺ diffusion. Al is more impacted by hydrolysis than Si, resulting in greater variation in the Al-O bond order than Si-O. Doping of NaCl and KCl enhances the ionization of water and the hydrolysis of ASs with increased salt concentration. The KCl doping ionizes more water molecules and causes more degradation of the glass network than NaCl. Co-doping of Na and K results in a mixed alkali effect due to complex interatomic bonding from different-sized ions. These exceptionally detailed findings in highly complex glasses with varying salt compositions provide new and unprecedented atomistic insights that can help to understand the hydrolysis and dissolution mechanisms of ASs and other silicate glasses.