# How does the Rf value change when changing the eluent composition?

A non-polar molecule X is analyzed by TLC chromatography on silica to determine its $R_f$ value.

The first eluent was hexane:EtOAc = $9:1$ and the $R_f$ value was $0.5$.
In the second run, the eluent was changed to hexane:EtOAc = $7:3$. Will the $R_f$ value be smaller or greater than $0.5$?

I am confused because there are two important interactions: because the 2nd system is more polar, it will form weaker interactions with substance X, and the $R_f$ value will decrease; on the other hand, the molar polar eluent will form stronger interactions with stationary phase, so the $R_f$ value will increase because the substance X is more easily eluted.

The most important interaction is that of the mobile phase with the stationary phase. If you increase the mobile phase’s polarity, that interaction will increase, decreasing the interaction of the solute with the stationary phase. Therefore, a higher ethyl acetate content in the mobile phase will increase $R_f$ values.
• @Uros If the interactions with the mobile phase are dominant, the $R_f$ value in pure hexane will be $> 0.5$. Source: Selling my soul to the chemistry lab for the last five years. – Jan May 30 '16 at 8:25
• Part two: Yes, in these lab years I had my fair share of compounds whose $R_f$ values in pure isohexane were larger than $0.8$ (which technically makes them impossible to isolate in standard silica chromatography). No matter which other mobile phase mixture I used, their $R_f$ value never sank. – Jan May 30 '16 at 8:31