Separation based on affinity for stationary vs mobile phases.
Components separate based on their differing solubilities in the mobile phase and adsorption to the stationary phase. This depends on Intermolecular Forces.
The solvent (moves up/through the system).
The paper/silica (stays still).
\( R_f = \dfrac{\text{Distance moved by Component}}{\text{Distance moved by Solvent Front}} \)
The Rf value is always less than or equal to 1. It is specific to a solvent and can be used for identification.
Component A has stronger IMF with the mobile phase (solvent) so it spends more time dissolved and travelling. Component B has stronger IMF with the stationary phase and is retained more.
Problem: A spot travels 4.2 cm. The solvent front travels 7.0 cm. Calculate the Rf value.
1. \( R_f = \frac{4.2}{7.0} = \mathbf{0.60} \)
Interpretation: The component has moderate affinity for the mobile phase. It can be compared with known Rf values to identify the substance.