|Aq Type Stationary Phases
One method of producing a water wettable phase involves close control of the spacing between the bonded phase ligands on the silica surface. Knowing the surface area of the silica and adjusting the carbon loading allows the ‘inter ligand distance’ to be controlled. This gives more (or less) access to the silica (silanol) surface, allowing the polar influence of low energy surface silanol groups to alter the selectivity of the separation.
Stability in highly aqueous mobile phases is achieved via the adsorption of a layer of water at the silica surface. The vicinal (low energy) silanol groups become hydrated and so the driving force towards ligand self-association is lost as the layer of adsorbed water at the silica surface effectively repels the ligands and they remain ‘activated’.
The alternative method of introducing stability in 100% aqueous mobile phases is to use polar end-capping reagents. These reagents are chosen to retain good peak shape with almost all applications whilst again introducing a polar characteristic to the separation and hence altering the column selectivity. The mechanism of high aqueous stability is similar to the previous case where an adsorbed layer of water at the surface prevents phase collapse