37.1.3—Differences in Rf values

A solute travels farther when it is relatively soluble in the mobile phase and interacts weakly with the stationary phase. Strong attraction to the stationary phase holds it back and lowers Rf.

Polarity, hydrogen bonding and the solvent composition control the balance. “More soluble” means relative to the other phase, not simply soluble in an absolute sense.

A polar compound may remain closer to polar alumina while a less strongly adsorbed compound moves with the solvent and gives a larger Rf.

A larger Rf does not mean a larger molecule or a higher boiling point; it describes phase interactions in that chromatographic system.