Characteristics of ambient-pressure-dried aerogels synthesized via different surface modification methods

In order to compare the various surface modification methods of ambient-pressure-dried aerogels, three different surface modification methods, namely co-precursor method, derivatization method and two-step method, were studied in this paper. The property differences of the aerogels caused by the three surface modification methods were discussed in terms of the reaction processes, micromorphology, pore size distributions, hydrophobicity and thermal stability. During the surface modification process, organic groups were attached to the wet gel skeletons transforming the hydrophilia to the hydrophobicity which were characterized by Fourier transform infrared spectrometry (FTIR). Though mesoporous structure could be obtained by all the methods, the co-precursor method had the smallest special surface area, pore volume and average pore size, while the derivatization method presented the best uniform pore size distribution with the most probable pore diameter of 7 nm and average pore size of 12 nm. Among the three methods, the samples of the two-step method had the lowest intensity of ≡Si–OH groups which indicated the excellent hydrophobicity, and the contact angle was confirmed as 162° using a contact angle meter. And two exothermic peaks on the DSC curve of the two-step method samples were observed unexpectedly, attributing to the oxidation of organic groups on the aerogel skeletons. Meanwhile, the initial temperatures of the two peaks were both higher than those of the derivatization method which suggested that the thermal stability of the silica aerogels could be improved by the two-step method. From all mentioned above, it can be drawn that surface modification methods had obvious effects on the properties of aerogels.