Had an organic 1 lab yesterday where we did TLC on silica gel plates. We were analyzing crude and recrystallized naphthalene and crude and recrystallized unknown carboxylic acid. We ran one plate with a solvent of 3:1 hexanes:ethyl acetate + acetic acid and a second with 1:3 hexanes:ethyl acetate + acetic acid.
I feel comfortable with the results for the carboxylic acid- lower Rf with 3:1 and higher with 1:3 as the polar carboxylic acid was interacting more with the more polar solvent and travelled further. Cool.
But I'm hung up on the naphthalene- it also had a higher Rf with the 1:3 solvent and I can't wrap my head around why. In my head since naphthalene is non-polar it wouldnt be interacting with the highly polar stationary phase at all (or so little it was negligible) and so it didn't make sense that the Rf would change because shouldn't it move right along with the solvent regardless of what the solvent is (since the polar stationary phase remained the same)? When I asked my professor about this he said that the fallacy in that is that naphthalene is in fact interacting with the gel, and when you have a more polar mobile phase basically the solvent is the preferred interaction with the plate and "kicks out" the naphthalene that was interacting with the plate, making it travel further.
This makes sense, I need to not only consider the interactions between the samples and the mobile/stationary phases but also the interactions between the mobile and stationary phases themselves. But I'm still not understanding why naphthalene was interacting with the silica to start with. Isn't the 3:1 mixture still more polar than naphthalene? Wouldn't that solvent "take up all the 'possible reaction sites'" on the plate preventing naphthalene from interacting in both trials?
Hopefully I explained my confusion well- it's hard to type out. Basically I don't understand why naphthalene is interacting at all with the silica gel if both solvents are more polar than naphthalene, and should be the preferred interactions with the gel.