Polarity of solvent for TLC

[akimhaneul]

Why would increasing the polarity of the solvent increase the rf values for TLC? Wouldn't it have not much effect for nonpolar molecules or even cause it to hardly move?

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[acemason]

So remember, in TLC, we have a stationary phase and a mobile phase. The stationary phase we usually see is made up of silica gel or alumina which are very polar. The mobile phase can range from non-polar to very polar solvents. We pick the solvents based on whatever we know about the compound we are trying to separate.

The polar molecules in the compound will be attracted to the silica gel while the more non-polar molecules will stay in the solvent (mobile phase). Let's say if you developed a TLC plate and you saw all the dots very close to the bottom of the plate. This means that the solvent wasn't polar enough to kick off the polar compound off the silica or alumina and move it further up the plate. So in this case, you would have a lower rf value since the molecules did not travel far.

We could then increase the polarity of the solvent which would help "elute" the molecules and move them further up the silica plate. This would result in larger rf values.


And if you need further clarification:

Given two compounds that differ in polarity, the more polar compound has a stronger interaction with the silica and is, therefore, more capable to dispel the mobile phase from the binding places. As a consequence, the less polar compound moves higher up the plate (resulting in a higher Rf value). If the mobile phase is changed to a more polar solvent or mixture of solvents, it is more capable of dispelling solutes from the silica binding places, and all compounds on the TLC plate will move higher up the plate.

 

[Bersabeh]

Why would increasing the polarity of the solvent increase the rf values for TLC? Wouldn't it have not much effect for nonpolar molecules or even cause it to hardly move?

this is for increasing the elution factor. the more polar the TLC solvent which is polar by itself, the better it expels the nonpolar molecules to give higher rf value.

 

[akimhaneul]

So remember, in TLC, we have a stationary phase and a mobile phase. The stationary phase we usually see is made up of silica gel or alumina which are very polar. The mobile phase can range from non-polar to very polar solvents. We pick the solvents based on whatever we know about the compound we are trying to separate.

The polar molecules in the compound will be attracted to the silica gel while the more non-polar molecules will stay in the solvent (mobile phase). Let's say if you developed a TLC plate and you saw all the dots very close to the bottom of the plate. This means that the solvent wasn't polar enough to kick off the polar compound off the silica or alumina and move it further up the plate. So in this case, you would have a lower rf value since the molecules did not travel far.

We could then increase the polarity of the solvent which would help "elute" the molecules and move them further up the silica plate. This would result in larger rf values.


And if you need further clarification:

Given two compounds that differ in polarity, the more polar compound has a stronger interaction with the silica and is, therefore, more capable to dispel the mobile phase from the binding places. As a consequence, the less polar compound moves higher up the plate (resulting in a higher Rf value). If the mobile phase is changed to a more polar solvent or mixture of solvents, it is more capable of dispelling solutes from the silica binding places, and all compounds on the TLC plate will move higher up the plate.

Thanks! But would nonpolar substance such as butane really move up more under polar solvent situation as opposed to nonpolar situation? Wouldn't the nonpolar solvent be better at pulling off the butane from the silica plate?

 

[Azide047]

Here's a simplified way of looking at TLC:

The silica gel is polar. This means that it'll hold other polar molecules tightly and keep the Rf low.
If the molecules are nonpolar (ether, butane, etc) then they won't be as "sticky" and will be allowed to continue up the strip resulting in a higher Rf.

Now for the solvents: The more polar solvent, then smaller Rfs and less separation you will see. Vice versa for nonpolar solvents.

Hope this helps


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