Polarity and Retention
Let's return to our original example. Water (the mobile phase) is more polar than paper (the stationary phase). Component B had a strong affinity for the water. This suggests that B is a polar molecule. Component A had a strong affinity for the paper. This suggests that it is relatively nonpolar. Component C has an intermediate affinity for both and therefore would be more polar than component A, but less polar than component B. To summarize:
| Component
| Polarity
| More attracted to the...
| Moved...
| Rf
|
| A
| nonpolar
| stationary phase
| a little
| small
|
| B
| polar
| mobile phase
| a lot
| big
|
| C
| in between
| both
| some
| medium
|
We can use similar arguments to predict what a different component might do. If another component, even less polar than A, were used it would move even less than A and have a smaller Rf value.
Remember that we started this discussion by saying that the mobile phase was polar and the stationary phase was nonpolar. This does not always have to be the case. If the stationary phase were polar relative to the mobile phase, the results would have been reversed.
Continue and try an example.
