Small-angle neutron scattering studies of water and ice in porous Vycor glass

Small-angle neutron scattering has been used to study both water condensation and freezing/ defreezing processes in porous Vycor glass. When the system is partially saturated with a matched D₂O/H₂O water mixture, an interesting new feature appears in the scattering which gives a straight line on a log-log plot of scattering intensity versus scattering vector, Q. This feature extends over about one and half orders of magnitude in Q, but with a substantially different gradient for desorption and adsorption, the gradients being 1.75 ± 0.1 and 0.5 ± 0.1 respectively. The overall form of the scattering is modelled in terms of a percolating system of empty pores connected to the surface during desorption and of a random distribution of clusters of filled pores on adsorption. Further, when water in porous Vycor undergoes a freezing/defreezing cycle, the density of the water changes gradually over a large temperature range and there is also ~ 10 K hysteresis between the freezing and defreezing processes.