Question

Discuss the reason behind different Rf values of dye by considering the chemical structures of dyes,mobile, and stationary phase

Rhodamine B- Ethanol Rf=0.3955

Rhodamine B- water Rf=0.039

Rhodamine B- silica gel (discuss only interactions between them)

Answer 1

Dye Rhodamine

Rhodamine B is a hydrophobic molecule.

It does not partition between the stationary phase and the polar solvent ethanol but stays on the tlc plate. It does not move with the mobile phase ethanol which is polar solvent. Non-polar moelcule dissolves in non-polar solvent and not in polar solvents. Thus the dye does not move well on the tlc plate and Rf value is lower.

In water which is more polar, the dye has even lesser interaction with the mobile phase and thus, Rf value drops further in this case when compared to ethanol which has low polarity as compared to water.

Rhodamine B-silica gel has weak london-disperson and dipole-induced dipole force of attraction whcih helps keep dye adsorbed to the surface of silica gel solid support.