Silica-based microcapsules: preparation and release properties

In this work, silica microcapsules of different structures containing hydrophobic substances were prepared using hyperbranched polyethoxysiloxane (PEOS) and its PEGylated derivatives (PEG-PEOS) as both silica precursor and stabilizer of oil-in-water emulsions. PEOS is a hydrophobic liquid, and it exhibits a pronounced interfacial activity in an oil/water system induced by hydrolysis at the interface or partial PEGylation. The influence of reaction conditions on the morphology and release property of the capsules was systematically investigated. In the first instance, paraffin was encapsulated with quantitative efficiency in mechanically strong submicron silica capsules simply by emulsifying the mixture of molten paraffin and PEOS in water under ultrasonication or high-shear homogenization. It is shown that the size of the capsules can be controlled by emulsification energy as for a miniemulsion process. The silica shell, whose thickness can be easily tuned by varying the paraffin-to-PEOS ratio, acts as an effective barrier layer retarding significantly the evaporation of enclosed substances; meanwhile the microencapsulated paraffin maintains the excellent phase change performance. Further, a new type of heterophase polymerization that allowed synthesizing in one step monodisperse polymer@SiO2 core-shell particles in a wide size range from tens to hundreds of nanometers has been worked out. The strategy utilizes PEG-PEOS, which can reduce the interfacial tension between oil and water close to zero. An oil phase containing such kind of surfactants can be emulsified in water spontaneously or just under low-energy stirring. Polymerization of styrene in the resulting emulsions using an oil-soluble initiator leads to the formation of monodisperse polystyrene@SiO2 particles, whose size can be precisely adjusted by the PEGylation degree of the precursor molecules and can reach as small as 30 nm. It is demonstrated that the PEGylation degree, i.e. the HLB, dictates the reaction mechanism that varies from suspension polymerization with breakup of monomer droplets and miniemulsion polymerization to microemulsion polymerization leading to exact “copying” of initial emulsion droplets. PEG-PEOS derivatives act as very efficient emulsifier to stabilized oil-in-water emulsions even for polar organic liquids. After a sol-gel reaction, oil droplets stabilized by PEG-PEOS of lower PEGylation degrees are converted to oil-containing aerogel particles. In the case of PEG-PEOS with higher degrees of modification, core-shell particles are obtained, where a liquid oil core is surrounded by a thin silica layer. Remarkably, the presence of an oil phase increases the porosity of the aerogel particles, acting as porogen, but has no influence on the porosity of the hollow particles. After freeze-drying the aerogel particles, acting as matrix-type microcapsules, can retain encapsulated volatile hydrophobic liquid, however, the liquid is evaporated from the hollow particles (core-shell-type microcapsules). The encapsulation efficiency of hydrophobic liquids in the aerogel particles can reach as higher as 99 % after drying. The encapsulation efficiency as well as barrier property of the aerogel particles are higher due to bigger particle size and higher meso- and micro-porosity. Finally, it was attempted to strengthen the shell of silica nanocapsules using monosilicic acid coating technique. For this purpose, silica hollow nanoparticles, octyl acetate@SiO2 as well as polystyrene@SiO2 nanocapsules were prepared using PEG-PEOS, where 10 mol.% ethoxy-groups are replaced by PEG, as the silica precursor, and were coated with an aqueous dispersion of silicic acid under hydrothermal conditions at pH close to 9. It is demonstrated that the silica shell obtained under these conditions exhibits a high degree of condensation and a low porosity, and the silica nanocapsules after such a treatment show an enhanced barrier property. Nevertheless, the silica shell is still not dense enough to hinder the dissolution of encapsulated polystyrene by THF