Drivers of Water Transport in Glass: Chemical or Topological Effect of the Glass Network?

International audienceThe relation existing between water transport in glass, topology, chemical elements (network former and charge compensator), and their structural role in glass were investigated through glass topology modeling, glass and water structure analyses, and water diffusion characterization. Two series of aluminosilicate glasses with and without boron and having various ratios of charge compensators CaO/Na 2 O were used. The glass structure was characterized using Raman spectroscopy and nuclear magnetic resonance. Their topologies, i.e., the density of bottlenecks and the interstitial sites, were obtained by molecular dynamics. For glasses without boron, the substitution of Na by Ca leads to the strengthening of the glass network. The results are similar for glasses with boron except if the amount of Na is not sufficient. In this case, a part of Ca is required as a charge compensator for AlO 4 and BO 4 units. This last result is important since it highlights that the presence of boron and the CaO/Na 2 O ratio drives the roles of Ca inside the glass structure. Moreover, glass topology is driven by boron presence and to a lesser extent by CaO/Na 2 O ratio. Water transport characterized by the duration of the predominance of the hydration/interdiffusion processes, the apparent water diffusion coefficient, and the water structure in the hydrated glass were studied and determined using X-ray reflectivity and attenuated total reflectance infrared spectroscopy. The results show that the duration of the predominance of the hydration/interdiffusion processes is driven by the ability of Si-OX bonds to be hydrolyzed as much as the fraction of free water clusters in hydrated glass. Moreover, for aluminosilicate glasses, we show that water transport is mainly driven by glass topology through the role of Ca and its amount in glass. Indeed, Ca strengthens the glass network by decreasing the density of the bottleneck allowing the diffusion of water molecules. When boron is added to the glass, water transport may be mainly driven by the chemical interactions between the water molecules, Ca, and the network former of glass matrix