The Chemical Nature of the Silica Gel Surface

 

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). However, on the surface of each primary particle some residual, uncondensed hydroxyl groups from the original polymeric silicic acid remain.  It is these residual hydroxyl groups that confer upon silica gel its polar properties.

 

The silica gel surface, however, does not simply consist of uncondensed hydroxyl groups. The silica surface can be quite complex and contain more than one type of hydroxyl group, strongly bound or 'chemically' adsorbed water and loosely bound or 'physically adsorbed' water depending on the history of the gel. Any surface hydroxyl group can be one of three types as shown in figure 24.

 

Firstly, it may be a single hydroxyl group that is attached to a silicon atom, which has three siloxane bonds joining it to the gel matrix. Secondly, it can be 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 are called Geminal hydroxyl groups. Thirdly it can be one of three hydroxyl groups attached to a silicon atom, which is now only joined to the silica matrix by a single siloxane bond.

 

The probability of any one type of hydroxyl group being present and consequently, its fraction of the total has been estimated by Sindorf and Maciel employing NMR techniques and it appears a reasonably safe assumption that the single hydroxyl group is likely to be the most prolific. The next most common appeared to be the geminal hydroxyl groups followed by the least common, tertiary hydroxyl group.

 

However, the silica surface is even more complex than the above diagrams would imply as water can be hydrogen bonded to the hydroxyl groups and multi-layers of water physically adsorbed on top of these or onto siloxane groups.

 

a/ The single hydroxyl group (one hydroxyl group on a single silicon atom)

 

b/ The Double or Geminal Hydroxyl Group (Two hydroxyl groups on one silicon atom).

 

c/ The triple hydroxyl group (three hydroxyl groups on a single silicon atom).

 

Figure 24. Different Forms of Hydroxyl Group that can Occur on the Surface of Silica Gel.

 

Various types of hydrogen bonded water that might occur on the silica gel surface are depicted in figure 25.

 

Figure 25. Different Ways in Which Water May be Hydrogen Bonded to Silica Gel Hydroxyl Groups

 

It must be emphasized that none of the above structures has been experimentally confirmed in an unambiguous manner but all are reasonably possible. The center and right hand side structures are particularly interesting as they both constitute a type of double hydrogen bond and would have high energies of formation and consequently, would be more stable than the simple hydrogen bond depicted on the left. The right hand structure, if it existed, might be particularly stable, as it constitutes a four membered hydrogen bonded ring, which might be expected to form in a very strong association of water with itself.

 

So much for the first layer of adsorbed water. The high capacity of silica gel to adsorb relatively large quantities of water was known in the early work on silica gel and this high adsorptive capacity was explained on the basis of multi-layer adsorption. This concept was supported by Vleeskens and was experimentally validated by thermogravimetric measurements. An example of one type of multi-layer adsorption is shown in figure 26.

 

Figure 26.    Multi-Layers of Physically Adsorbed Water

 

The multi-layer adsorption depicted in figure 26 is probably much over simplified, as adsorption could also take place on either form of the double hydrogen bonded water (should they exist) shown in figure 25, and certainly on any surface siloxane bonds that might be present. Solvents, other than water, are also adsorbed on the surface of silica gel but only the simple adsorption of water will be considered here.

 

It should be pointed out that early workers investigating the surface of silica gel, did not consider that there were two types of adsorbed water-only physically adsorbed water and water that was evolved as a result of the condensation of surface silanol groups.

 

Early workers considered that all water evolved below 120^oC was physically bound water and any water evolved above 120^oC was derived from silanol group condensation. However, this criterion was questioned by Lange in 1965 who suggested that some of the water evolved at 300^oC to 500^oC was far more strongly held than by simple physical adsorption but still did not necessarily result from silanol condensation. He suggested, as one possible explanation, that the water evolved at higher temperatures could come from very small pores in the silica, which required significant thermal energy to release it. Another possibility he suggested was that some of water is held by strong hydrogen bonding forces to the surface that would also require substantial thermal energy to remove. Today, as a result of the use of modern instrumental methods of analysis, such as solid state NMR, FTIR and other physical chemical techniques, far more is known about the surface of silica gel, but there still remain a number of uncertainties.