Liquid Chromatography - The Structure of Silica Gel 1

The Structure of Silica Gel

The matrix of the primary silica gel particle consists of a core of silicon atoms joined together with oxygen atoms by siloxane bonds (silicon-oxygen-silicon bonds). On the surface of each primary particle some residual, uncondensed hydroxyl groups from the original polymeric silicic acid remain. These residual hydroxyl groups confer upon silica gel its polar properties. These hydroxyl groups react with the silane reagents to form bonded phases. The silica surface is quite complex and contains more than one type of hydroxyl group, strongly bound or 'chemically' adsorbed water and loosely bound or 'physically adsorbed' water. There are three types of hydroxyl group. The first is a single hydroxyl group attached to a silicon atom which has three siloxane bonds joining it to the gel matrix. The second is one of two hydroxyl groups attached to the same silicon atom which, in turn, is joined to the matrix by only two siloxane bonds. These twin hydroxyl groups are called Geminal hydroxyl groups. The third is one of three hydroxyl groups attached to a silicon atom which is now only joined to the silica matrix by only a single siloxane bond. An example of each type of hydroxyl bond is shown in Figure 30.

Figure 30. Different Forms of Hydroxyl Group that can Occur on the Surface of Silica Gel.