The Mechanism of Chromatographic Retention - Chromatographic Interactions > Polar Interactions > Page 7

 

If the stationary phase is largely dispersive in character then the retention mechanism will be predominantly dispersive and in LC the eluting mobile phase would be predominantly polar (cf. reverse phase chromatography). If the stationary phase is largely polar in character then the retention mechanism will be predominantly polar and in LC the eluting mobile phase would-be made dispersive (cf. normal phase chromatography).

An example of dispersive and polar interactions is afforded by the separation of the gasoline sample on both a highly dispersive stationary phase, and a strongly polar stationary phase. The separations are shown in figure 2. GC gives a clear indication of the retentive character of the stationary phase as there are no significant interactions in the mobile phase. Gasoline has a relatively high proportion of aliphatic hydrocarbons which can only interact dispersively with any stationary phase. However, it has also a significant number of different aromatic hydrocarbons present which, as already has been discussed (book 1), can be polarized and consequently interact with a polar stationary phase. Thus if a sample of gasoline is chromatographed on a strongly dispersive stationary phase the components would be separated roughly on a basis of molar volume. This is shown in the top chromatogram in figure 2.

Polar Interactions

Polar interactions can take two forms; those that result from direct dipole-dipole interaction (involving only molecules that have permanent dipoles) and those that result from dipole-induced dipoleinteraction Dipole-induced dipole interaction results from the interaction of a molecule with a permanent dipole and one that is polarizable. In general, dipole-dipole interactions are very strong polar interactions (e.g. the interaction of methanol with water) and dipole-induced dipole interactions relatively weak (e.g., the interaction of an ester with an aromatic hydrocarbon). Depending on the structure of the molecule, both types of interaction can take place in addition to the underlying dispersive interactions.