[Seraph2284]

Anyone have any idea what order compounds will travel further on a TLC plate (between Aspirin, Methyl Salicylate, and a mixture of the two) using a 75% Hexanes 25% Ethyl Acetate mobile phase?

[discodermolide]

Aspirin is a carboxylic acid, methyl salicylate is an ester.
On silica-gel with this solvent that the former will not move on TLC and the latter, being less polar, will run up the plate.

[trinitrotoluene]

Aspirin is a carboxylic acid, methyl salicylate is an ester.
On silica-gel with this solvent that the former will not move on TLC and the latter, being less polar, will run up the plate.

I have done my fair share of TLC, yet I have never had it explained like this before (I am not doubting you, this makes a lot of sense). I might be jumping the gun here, but is this equivalent to saying that the solvent gradient throughout the plate is different? If true, can the the same be said for a column?

[discodermolide]

If the TLC is dome properly the solvent gradient should be the same all over the plate.
What I said will hold true for a column.
In that solvent system a carboxylic acid will not move off the baseline.
With these two compounds you don't need TLC to separate them, but I suppose this is an example?
How else would you explain it?

[trinitrotoluene]

My comment was based purely speculation and perhaps overthinking. I find it difficult to explain the way I think of TLC in regards to trying to develop solvent systems. I think about it as being the more polar the solvent system the further polar products which move up the plate. Which I feel doesn't have a lot of depth.

Something about your comment prompted me to think about the possibility that the more polar solvents themselves (as opposed to simply the solutes), would stick to the gel  and less polar solvents would keep cruising on by. In turn, changing the concentration gradient of the solvent system along the plate and or gel. Futhermore, the separation that you would achieve.

I suppose it just got me thinking that there was more to TLC/column chromatography than my elementary knowledge. The way I interpreted your logic is with the focus being on the solvent as opposed to my logic where the focus is the solute. (Probably a terrible description of what I am trying to say, but its the best I can think of)

Again, I may be overthinking this... but I have had problems in the past where the separation I get on a TLC plate does not match my results when I run a column and this post got my neurons firing as to an explaination.

Cheers

[discodermolide]

My comment was based purely speculation and perhaps overthinking. I find it difficult to explain the way I think of TLC in regards to trying to develop solvent systems. I think about it as being the more polar the solvent system the further polar products which move up the plate. Which I feel doesn't have a lot of depth.

Something about your comment prompted me to think about the possibility that the more polar solvents themselves (as opposed to simply the solutes), would stick to the gel  and less polar solvents would keep cruising on by. In turn, changing the concentration gradient of the solvent system along the plate and or gel. Futhermore, the separation that you would achieve.

I suppose it just got me thinking that there was more to TLC/column chromatography than my elementary knowledge. The way I interpreted your logic is with the focus being on the solvent as opposed to my logic where the focus is the solute. (Probably a terrible description of what I am trying to say, but its the best I can think of)

Again, I may be overthinking this... but I have had problems in the past where the separation I get on a TLC plate does not match my results when I run a column and this post got my neurons firing as to an explaination.

Cheers

Effectively you are correct, the more polar a solvent the further polar compounds will travel up the plate.
It the tank is in equilibrium then there should be no gradient on the TLC plate.
Getting the solvent system correct can be time consuming and mean some experimentation, especially with mixed systems. On a column I usually begin somewhat non-polar and gradually increase the polarity. But, this depends upon the separation of the components.
To get a carboxylic acid to move on silica-gel you need a very polar solvent system, we had several mixtures in my lab for example chloroform/methanol/acetic acid/water and the like. The only problem here is you can'd run a column in such a mixture, you will end up with more silica dissolved possibly more than your product if you work on smallish scale.