Topics - Siloxane

Siloxane The siloxane bond is important in chromatography due to its stability and strength when linking organic moieties to the silica gel surface as in the preparation of the bonded phases. There are three important silicon bonds in the synethsis of bonded phases the siloxane bond vis Si-O-Si , the siloxyl bond vis Si-O-C and the silyl bond vis Si-C and Si-X. The first bonded phases to be prepared (the ‘brush’ phases) were by Halasz who reacted silica gel with a long chain alkyl alcohol (C8) to link the hydroxyl groups on the surface of the silica to the C8 alkane chain via the siloxyl bond. This material was of historical importance and certainly established the value and practical use of bonded phases. However, it was not stable as the siloxyl bond is very labile. It was found that the C8 moiety of the ‘brush’ phase was rapidly lost from the silica gel surface during chromatographic use. Nevertheless, the work of Halasz provoked the further development of more stable bonded phases. The bond that was established to be the most stable was the siloxane bond (Si-O-Si) and, thus. to form a stable C8 chain attached to the silica, a reagent such as dimethyloctyl silyl chloride was used which, itself, contained silyl bonds Si-C and Si-Cl). The chlorine in the dimethyloctyl silyl chloride reacts with the hydrogen of the silica hydroxyl groups producing hydrogen chloride and forming a stable -Si-O-Si- bond to the silica gel surface. Ragents other than the chlorosilanes such as esters can also be used in the same way. Silicon chemistry has made a very important contribution to design and synthesis of chromatography stationary phases