The processes of gel drying produce xerogels or aerogels, depending on the method used.The method of xerogel production is lyofilization or frozen-drying, i.e. evaporation of the solvent from frozen gelator skeleton, at low pressure.The gelator network structure, with enormous surface area and small pore size, can be maintained, but such drying is accompanied with unhindered shrinkage that can lead to a change of wet gel structure.The aerogel is produced by drying the wet gel in various environments, often in super-critical condition and/or with the solvent change. The gel does not shrink, so the fragile gelator network can be maintained.
The GEL1 was prepared of methyl-4,6-O-benzylidene alpha-D-glucopyranoside gelator with diphenyl ether as a solvent.This molecule is one of the smallest among monosaccharides and apolar, as well as the solvent is. The GEL2 was built of methyl-4,6-O-(p-nitrobenzylidene)-alpha-D-glucopyranoside gelator with water as a solvent.Both of them are polar, and the gelator molecule size is slightly bigger. The aerogels were produced by leaving obtained wet gels in open vessel till they become dry. Xerogels were produced by frozen-drying of the gels in Labconco lyph.lock 1l apparatus at about -50C and 10mm Hg.
The SAXS measurements of these materials were performed at HASYLAB synchrotron and analyzed using fractal method. The fractal results of GEL1 xerogel shows that drying induced stress caused a collapse and formation large-scale morphological features, so SAXS scattering is related to their smooth surface. The collapse takes place also at aerogel formation, where evaporated solvent is replaced by its vapor molecules and finally by air, so stresses should be much smaller. The fractal results for GEL2 xerogel suggest similar collapse, as above. But GEL2 aerogel structure is no a product of collapse as well as no modification of the previous wet gel structure. It must be formed by it dissolutions and precipitations (Ostwald ripening).
The all xerogels and aerogels, obtained by us from the monosaccharide gels, are of essentially changed structures in comparison with related wet gels. |