Dynamics of water confined in gel formed during glass alteration at a picosecond scale

This paper reports on the characterization at a picosecond scale of water dynamics in gel formed during glass alteration by neutron scattering. The water dynamics in gel are compared with the dynamics of water confined in porous silica-based material (MCM41) having pore wall surfaces with Si-OH, Al-OH, or Zr-OH terminal groups and pore sizes around 2.3 nm in the presence of ions existing in a leachate derived from glass alteration. It is shown that the presence of ions does not impact the water mobility in model materials whatever the pore surface composition. In the gel, water is two to three times slower than in the bulk, and the fraction of fixed protons is similar to the model material having an Al-OH surface. Two hypotheses are proposed to explain these results: the ion concentrations in pores of the gel can be higher than in the leachate located outside the gel forming hydration shell around ions, thereby hindering the water motions, and/or the pore wall composition of the gel consisting of elements such Al that can immobilize water molecules. In addition to the Al effect, the presence of Ca can slow down water diffusion due to the binding of H₂O molecules in strong complexes with Ca²⁺. (authors)