True or False: A sample of aspirin has an $R_f$ value of 0.20 when measured on a silica gel TLC plate with a certain solvent mixture. If the polarity of the solvent is increased, the $R_f$ value of the aspirin will become larger.

Would this be false? Because aspirin is polar (I believe would be considered polar because it dissolves in water despite some other sources saying that it is actually non-polar) and the silica in the TLC plates are also polar, aspirin would not be able to travel very far because it would interact with the silica. Thus, shorter distance over longer distance = lower $R_f$ value, with $R_f$ = distance spot moves/ distance solvent moves.

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I wish we would stop teaching chromatography in terms of "polar" and "nonpolar."

The aspirin will interact fairly strongly with the silica due to hydrogen bonding/electrostatic interactions of the carboxylic acid and the ester with the silica. If you increased the "polar" component of the mobile phase, it would travel further due to the mobile phase competing more for binding spots of the silica gel.

So increasing the polarity of the mobile phase does raise $\mathrm{R_f}$ of aspirin (or anything) on regular silica gel.


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Since aspirin has a carboxylic acid group on it, it would be considered polar. Silica gel, consisting of $\ce{SiO2}$, is also polar. Since polar molecules attract other polar molecules, the aspirin molecules will tend to bind to the silica and not move up the TLC plate in a nonpolar eluent, resulting in a low $R_f$ value.

When the polarity of the eluent is increased, its ability to attract polar molecules is increased. This means that it can better compete with the silica in attracting the aspirin, and the $R_f$ value will be higher.


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