Analysis of the effect of drying conditions on the structural and surface heterogeneity of silica aerogels and xerogel by using cryogenic nitrogen adsorption characterization

Monolithic silica aerogels were prepared by means of sol–gel method followed by carbon dioxide (CO2) supercritical drying (SCD). By changing the pressure of liquid CO2 used to replace ethanol in the autoclave before CO2-SCD from 8.5 MPa to 4.5 MPa in steps of 1 MPa, a series of silica aerogels with different ratio of micropore, mesopore and macropore volume, having a BET surface area of 678–949 m2 g−1, a pore volume of 1.63–5.16 cm3 g−1 and a average pore width of 9.61–21.76 nm were obtained. Silica aerogel and xerogel were also prepared by ethanol-SCD and ambient pressure drying respectively for comparison. N2 adsorption isotherm was used to characterize the porous texture of the silica aerogels as prepared. Surface energy distributions were calculated by Density Functional Theory (DFT) method deduced from N2 adsorption data. Infrared spectroscopy (IR) was used to determine the surface groups of these samples. All samples exhibited a multimodal pore size distribution with maxima in the micropore and meso-/macropore regions. The pore size distribution of CO2-SCD silica aerogels was narrowed by increasing the pressure of liquid CO2, and the position of maximum also shifted from large pore width region to small pore width region. Maximum in pore volume was attained at 6.5 MPa for mesopores with little decrease of surface area. Surface energy distribution depended not only on the pore size but also on the pore size distributions and surface properties. Different surface groups lead to significant change of surface energy distribution between ethanol supercritical drying and CO2 supercritical drying silica aerogels. The relationship between surface energy distribution and pore size was introduced.