Sol to gel silica transition in fast evaporating systems observed by in situ time resolved infrared spectroscopy

The in situ observation of a sol-to-gel transition in fast evaporating
systems is a challenging task and the lack of a suitable
experimental design, which includes the chemistry and the analytical
method, has limited the observations. We synthesise an
acidic sol, employing only tetraethylorthosilicate, SiCl4 as catalyst
and deuterated water; the absence of water added to the
sol allows us to follow the absorption from the external environment
and the evaporation of deuterated water. The timeresolved
data, obtained by total reflection infrared spectroscopy
on an evaporating droplet, enables us to identify four different
stages during evaporation. They are linked to specific hydrolysis
and condensation rates that affect the uptake of water
from external environment. The second stage is characterized
by a decrease in hydroxyl content, a fast rise of condensation
rate and an almost stationary absorption of water. This stage has been associated with the sol-to-gel transition