Emulsion-based synthesis of porous silica

We review the use of various types of emulsions as media for synthesis of porous silica particles. The use of high internal phase emulsions, i.e. emulsions with a high ratio of dispersed to continuous phase, is an approach that has attracted considerable attention. Polymerization of the continuous phase followed by removal of the dispersed phase leads to a material with an even distribution of pores if the emulsion droplets are uniform in size. Another route is to use particle stabilized emulsions as template. This will lead to either hydrophilic or hydrophobic porous silica particles depending on whether the templating emulsion is oil-in-water or water-in-oil, respectively. Use of double emulsions as templates is a way to obtain porous particles with hierarchical porosity, usually both macropores and mesopores. Templating amphiphiles, which are often polyoxyethylene-polyoxypropylene-polyoxyethylene block copolymers, are needed in order to obtain materials with highly ordered pore structure. Non-ordered mesoporous silica with small particle size and relatively large pores can be obtained by emulsifying a silica precursor together with an inert solvent in water and allowing the silica to gel within the drops. Hollow silica spheres, i.e. spherical particles with a void in the middle, can be prepared by using emulsion drops as templates around which silica polymerizes. The particles have nanometer-sized pores penetrating the shell.