Question

What characteristics of silicon and what characteristics of oxygen that are contained in silica contribute to creating a very polar stationary phase?

Answer 1

The electropositive character of the aluminum or silicon and the electronegative oxygen create a very polar stationary phase.

Therefore, the more polar the molecule to be separated, the stronger the attractive force to the stationary phase. In some sense, the old adage “like-dissolves-like” can be applied here.

The polar stationary phase will more strongly attract like or polar molecules. The equilibrium will be shifted as the molecules remain on the stationary phase.

Nonpolar molecules will have a lower affinity for the stationary phase and will remain in the solvent longer.

This is essentially how the partitioning separates the molecules. The equilibrium governs the separation, but the component’s attraction to the stationary phase versus the mobile phase determines the equilibrium.

In general, the more polar the functional group, the stronger the bond to the stationary phase and the more slowly the molecules will move.

In an extreme situation, the molecules will not move at all. This problem can be ovecome by increasing the polarity of the mobile phase so that the equilibrium between the free and absorbed state is shifted towards the free.

Although alumina and silica are the most common stationary phases used for TLC, there are many different types.

They range from paper to charcoal, nonpolar to polar and reverse phase to normal phase. Several different types of stationary phases are listed according to polarity in Figure 1.

Chromatography Stationary Phase Polarities

Polydimethyl siloxane*

       Methyl/Phenylsiloxane*

      Cyanopropylsiloxane*

     Carbowax (polyethyleneglycol)*

   Reverse Phase (hydrocarbon-coated silica e.g. C-18)

       Paper

    Cellulose

      Starch

    Calcium sulfate

   Silica (silica gel)

    Florisil (magnesium silicate)

    Magnesium oxide

    Alumina (aluminum oxide; acidic, basic or neutral)

    Activated carbon (charcoal; Norit pellets) *

Figure 1 : Common Stationary Phases listed by Increasing Polarity

How does thin layer chromatography work?

The stationary phase - silica gel

Silica gel is a form of silicon dioxide (silica). The silicon atoms are joined via oxygen atoms in a giant covalent structure. However, at the surface of the silica gel, the silicon atoms are attached to -OH groups.

So, at the surface of the silica gel you have Si-O-H bonds instead of Si-O-Si bonds. The diagram shows a small part of the silica surface.

The surface of the silica gel is very polar and, because of the -OH groups, can form hydrogen bonds with suitable compounds around it as well as van der Waals dispersion forces and dipole-dipole attractions.