Small-angle x-ray scattering study of the structural evolution of the drying of sonogels with the liquid phase exchanged by acetone

Abstract

The structural evolution on the drying of wet sonogels of silica with the liquid phase exchanged by acetone, obtained from tetraethoxisilane sonohydrolysis, was studied in situ by small-angle x-ray scattering ͑SAXS͒. The periods associated to the structural evolution as determined by SAXS are in agreement with those classical ones established on basis of the features of the evaporation rate of the liquid phase in the obtaining of xerogels. The wet gel can be described as formed by primary particles ͑microclusters͒, with characteristic length a ϳ0.67 nm and surface which is fractal, linking together to form mass fractal structures with mass fractal dimension Dϭ2.24 in a length scale ϳ6.7 nm. As the network collapses while the liquid/vapor meniscus is kept out of the gel volume, the mass fractal structure becomes more compacted by increasing D and decreasing , with smoothing of the fractal surface of the microclusters. The time evolution of the density of the wet gels was evaluated exclusively from the SAXS parameters , D, and a. The final dried acetone-exchanged gel presents Porod's inhomogeneity length of about 2.8 nm and apparently exhibits an interesting singularity D →3, as determined by the mass fractal modeling used to fit the SAXS intensity data for the obtaining of the parameters and D.

Structural Changes to the Calcium–Silicate–Hydrate Gel Phase of Hydrated Cement with Age, Drying, and Resaturation

“…This phenomenon is known in the gel literature as “constant rate period” drying, 31 because the decrease in the volume of the gel is equal to the volume of liquid lost by evaporation. A similar result was obtained from a SAXS study of silicate sonogels, 23 which had a lower initial packing density and thus exhibited relatively larger increases in density on drying.…”

“…This phenomenon is known in the gel literature as ''constant rate period'' drying, 31 because the decrease in the volume of the gel is equal to the volume of liquid lost by evaporation. A similar result was obtained from a SAXS study of silicate sonogels, 23 which had a lower initial packing density and thus exhibited relatively larger increases in density on drying. At the macroscopic level, the bulk shrinkage of cement paste on drying is quite small compared with that of the gel component, generally o1% by volume, 29 due to the restraining effect of nonshrinking solid phases.…”

“…The first term in , which gives the volume‐fractal 19 and single‐particle scattering, has also been used to model the scattering from fine silica gels prepared using sol–gel methods 20–23 . The second term in is the surface fractal scattering 24 .…”