Inclusion of Hydrophobic Liquids in Silica Aerogel Microparticles in an Aqueous Process: Microencapsulation and Extra Pore Creation.

Due to its extraordinary properties, silica aerogel has a high potential for a number of applications; however, the state-of-the-art technique of its production involves cost-intensive supercritical drying or solvent exchange with a nonpolar solvent. Here, we report on a pure aqueous process for the preparation of silica aerogel particles as well as silica hollow nanoparticles, which is based on the self-assembly of amphiphilic silica precursor polymers, PEGylated poly(ethoxysiloxanes) (PEG-PEOS), in water and subsequent conversion under basic conditions. Addition of a hydrophobic organic liquid to the aqueous dispersions of PEG-PEOS results in the spontaneous formation of oil-in-water emulsions, which resemble the self-assembled structures of PEG-PEOS in water swollen by the organic liquid. The products of basic conversion of the emulsions are silica aerogel particles as well as hollow nanocapsules loaded with organic liquid. Remarkably, the oil phase significantly increases the porosity of the aerogel particles by acting as a porogen; meanwhile, it only decreases the silica shell thickness of the hollow nanoparticles. During freeze-drying, the aerogel particles, acting as matrix-type microcapsules, can efficiently retain the encapsulated volatile hydrophobic liquid; at the same time, the liquid is completely evaporated from the hollow particles (core-shell-type microcapsules). The encapsulation efficiency of hydrophobic liquids in the aerogel particles can reach as high as 99% after drying. The barrier property of the aerogel particles is higher with PEG-PEOS of lower PEGylation degrees due to bigger particle size and higher meso- and microporosity. This work opens a new avenue to prepare particulate silica aerogel for different promising applications including microencapsulation.