In living organisms, compartmentalization facilitates the separation of molecules that normally would intermix or react with one another. In addition, such compartments also offer the possibility of combining components on a small scale that would otherwise segregate macroscopically. Fundamental examples include closed-cell foams and sponges1 and controlled-release applications.2 Controlling the transport of molecules through the wall of such compartments creates the potential for materials with time-dependent and reactive properties. Prominent examples are self-healing materials,3 as well as the mobility through the turgor effect, which was described for plants. This implies that compartmentalization can be a potent tool for materials engineering, yet developments are often still in the early stages. The spatially controlled incorporation of fluids and components requires the process of encapsulation or else the formation of a cellular structure. Capsules in various sizes as well as open- and closed-cell foam structures have been developed on the basis of a wide variety of polymeric materials and have achieved great commercial success in various fields of application.4 However, the size of such cells is usually in the range of a few micrometers, and smaller structures are not accessible. The comparison with natural materials illustrates the significance of much finer cell structures, yet simultaneously the macroscopic homogeneity of a material necessitates the use of capsules with diameters in the submicron range.5 Enabling optical transparency and good resistance of the capsules to shear forces and compression during processing can only be achieved with capsules of such small size. In nature, compartmentalization is typically obtained on basis of self-assembled cell membranes, e.g., lipid bilayers, capsid-forming protein complexes, and primary cell walls of plants composed of polysaccharides.6
Synthetic microcapsules, on the other hand, are formed and filled by the dispersion of two immiscible liquids and the formation of a solid wall at the interface. As a result, the minor component is efficiently encapsulated. The reduction in the size of such capsules cannot be easily tuned to the submicron range. One way to achieve nanometer-sized capsules is the use of significant amounts of surfactants and energy-consumptive mechanical shearing.7 Excessive use of surfactants can be detrimental, especially in coating applications, therefore we have developed a silica precursor that enables formation of silica capsules without the use of any conventional surfactants.8-10 The key issue of this encapsulation approach has been that the precursor turns into a surfactant itself during hydrolysis, thereby efficiently reducing the interfacial energy between dispersed and continuous phase. Uniform capsules with submicron size filled with high-molecular polydimethylsiloxane (PDMS) were accessible using this procedure, however, only when using ultra-high-shear methods like homogenization. Within this work, we present an elegant and industrially feasible way to prepare silica nanocapsules using conventional shear methods like a rotor-stator dispersion process.