Signatures of a drying SiO2 · (H2O)x gel from Raman spectroscopy and quantum chemistry

The dehydration process, induced by thermal treatment, of a silica gel, obtained from colloidal nano-particles, has been investigated. At sufficiently low water contents in the structure; water molecules wet the hydrophilic interior surface of the pores rather than forming droplets of pore water. This is manifested by elongation of the OH-bonds of the acidic silica groups and a dramatic downshift in the OH-vibrational frequency is observed. Further dehydration results in a dry, partially hydroxylated, porous silica consisting of a three dimensional network SiO2 nano-particles. Upon further thermal treatment the remaining hydroxyl groups condense to Si–O–Si bridges between the colloidal particles.