Question

What trend emerged from changing the polarity of the mobile phase in terms of the Rf of the pigments? What happened to the separation as you increased polarity of the mobile phase? What happened to the separation as you decreased the polarity of the mobile phase? Explain the process also on the molecular level describing what happens during the TLC process.

Answer 1

First, recall that Rf is the retention value for a pigment, it is "how high" does a pigment go in a TLC column.

The more polar the pigment is, the lower Rf value it will have, since it will attach mostly to the stationary phase which is polar. For the mobile phase, which is a mix of nonpolar-polar solvents, expect the least solvent material to go to the highest Rf values.

The separation will perform better with nonpolar solvents, in order to split polar + nonpolar solvents in the mobile phase

If you decrease polarity of mobile phase, then the nonpolar speices will favour a higher Rf value and polar pigments will favour a much lower Rf value.