Liquid Chromatography - The Structure of Silica Gel 2
Employing NMR techniques Sindorf and Maciel, (16,17) has shown that the single hydroxyl group is likely to be the most prolific. The next most common is the geminal hydroxyl groups followed by the tertiary hydroxyl group. The silica surface, however, has additional complexities. Water can be hydrogen bonded to the hydroxyl groups and multi-layers of water physically adsorbed on top of these. Water can be hydrogen bonded to the silica gel surface in a number of different ways which are depicted in figure 31.
None of the above structures has been confirmed in an unambiguous manner but all are reasonably possible. The center and right hand side structures contain a type of double hydrogen bond and would have high energies of formation and, thus, more stable than the simple hydrogen bond depicted on the left. The right hand structure would be particularly stable as it constitutes a four membered hydrogen bonded ring similar to that which might be expected to form in the strong association of water with itself.
Figure 31. Different Ways in Which Water May be Hydrogen Bonded to Silica Gel Hydroxyl Groups