Silica oligomers/small molecules

Starting in the 1950s a process was developed that leads from small-molecule silicon alkoxides such as tetraethoxysilane (tetraethyl orthosilicate), to organosiloxane oligomers and low polymers, and eventually to silica via a low temperature synthesis route.14 24 A simplified outline of the basic chemistry is shown in reactions (l)-(4), where R is an ethyl or higher alkyl unit. Any or all of the Si-OR bonds can be... 

Figure 24.1. Snapshots from molecular dynamics simulations of oligomerization in aqueous silicic acid with r = 11. Only silica polymorphs containing at least two silicon atoms are shown all silicic acid monomers and water molecules (which completely fill the remainder of the simulation cell) are removed for clarity. Only silicon and oxygen atoms are shown terminal hydrogens have been removed. Snapshots are taken from the simulation at times A. 1.0, B. 2.0, C. 3.0, and D. 4.8 ns. The evolution of small oligomers into compact sol particles is clearly visible. Reprinted with permission from [49]. Copyright 2007 American Chemical Society.

In the case of polysilicic acid of very low molecular weight, such as cyclic tetramer and similar oligomers no liquid phase can be salted out. It appears necessary to let the silica polymerize to three-dimensional units or small particles before it can be covered by oriented organic molecules to form a hydrophobic complex that will form a second phase. On the other hand, if the particles further polymerize by aggregation to form microgel, then only a gelatinous emulsion is formed. 

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