Supercritical drying of aerogel: In situ analysis of concentration profiles inside the gel and derivation of the effective binary diffusion coefficient using Raman spectroscopy

We here experimentally visualize in situ temporally and spatially resolved composition and concentration fields developing inside silica aerogels during the supercritical drying process. For the extraction of the pore liquid ethanol we used supercritical carbon dioxide. The evolution of the composition profiles inside the gel was measured via one-dimensional Raman spectroscopy. The composition profiles were converted into concentration profiles using an equation of state. The evolution of the concentration fields shows that ethanol is continuously transported out of the gel. On the contrary CO 2 is first transported into and at later drying times (before the depressurization) out of the gel structure. The binary diffusion coefficient was derived as a function of the composition and found to vary by two orders of magnitude for different stages of the drying process.