Effect of Solvent Polarity on Rf Value in TLC

[silversurf]

How does polarity of the solvent affect the Rf value? I was going over a problem in my book that said that polarity would increase the Rf value of both the polar and nonpolar analyte due to increased solubility. This doesn't make sense to me because wouldn't the solubility of the nonpolar analyte decrease thus, based on that explanation, only the Rf value of the polar molecule would increase and the Rf value of the nonpolar solvent should decrease right? Also, how does solubility of the solvent affect the Rf value if we only have a small pool of solvent at the bottom of the TLC plate? Does it somehow allow more compounds to move up the paper?

 

[epenguin]

How does polarity of the solvent affect the Rf value? I was going over a problem in my book that said that polarity would increase the Rf value of both the polar and nonpolar analyte due to increased solubility. This doesn't make sense to me because wouldn't the solubility of the nonpolar analyte decrease thus, based on that explanation, only the Rf value of the polar molecule would increase and the Rf value of the nonpolar solvent should decrease right? Also, how does solubility of the solvent affect the Rf value if we only have a small pool of solvent at the bottom of the TLC plate? Does it somehow allow more compounds to move up the paper?

Yes, anything that increases the analyte's solubility in the mobile solvent should increase the R_f. Your post is not very clearly written and you talk of increased polarity not saying of what, and that could be origin of puzzle. We don't know what your book actually says.

 

[chemisttree]

It isn't helpful to think of analytes in black and white terms... polar and nonpolar. Most analytes that have a measurable rf are polar enough to interact with the stationary phase to some degree. They are various shades of 'gray'. A truly nonpolar analyte would not interact with polar stationary phase and would move with the solvent front. Adding a more polar mobile phase would do little in that case. A change of stationary phase would do more to help separate very nonpolar analytes.

Of course if you are dealing with compounds that have some degree of polarity, it makes sense that increasing the polarity of the mobile phase increases the rf. In normal phase chromatography, the analyte is interacting somewhat with a fairly polar stationary phase. The effectiveness of the mobile phase to drag away the analyte from the stationary phase tracks well with what the mobile phase is competing with... a polar surface. More polar solvents are stronger in that system.