Micro-Emulsion Mediated Silica Formation

The diversified porous patterns of diatomaceous silicas are on the nano- to submicrometer scale ( 10-300 nm) and these meso- and macropores cannot be mediated by single macromolecules, not even proteins. To mimic these meso- and macroporous structures, a different approach can be applied based on a phase separation process as in the vesicle-mediated macromorphogenesis processes extensivily reviewed in Pickett-Heaps et al.I 1 In this case oil-in-water (0/W) emulsions are applied as a model system. 0/W emulsions are isotropic and thermodynamically stable liquid media with a continuous water domain and an oil domain, which are thermodynamically stabilized by a surfactant as micrometer-sized liquid entities. 

At pH 5.2, hollow spheres are formed with a uniform size distribution and an average diameter of approximately 1 fj,m. These silica spheres are thermally very stable and hollow (Fig. 9.17), possessing a relatively thin shell with a very regular thickness of 80-130 nm composed of 7-8 nm thick layers of silica with an interlamellar spacing of 3-4 nml l.  

The biomimetic studies of biomineralization have also provided inspiration to explain biopolymer-mediated silica formation in the diatoms. As mentioned in the previous section, biogenic silica contain small proteins and polyamines such as the silaffins. 

The silaffin-lA proteins are small (2.4-3.1 kDa) polycationic proteins with highly modified amino acid residues. A particular well-characterized p eptide contain seven phospho-rylated serine residues and one phosphorylated trimethylhydroxylysine moiety. 

The zwitterionic structure of the native silaffins leads to the self assembly of these molecules, which explains their extremely efficient induction of silica precipitation. Following up on this model, Sumperl l proposed, on the basis of simulations, a model that consists of multiple steps in which phase separation processes occur. At the initial stage phase separation of protein phase and silica permits the formation of the large, honeycomb structures, followed by several intermittent steps of silica formation - each mediated by phase separation processes to create smaller structures (see Fig. 9.18). 

---