The position on the TLC, in addition to the polarity of the molecule, must also consider the nature of the TLC, whether it is silica gel, alumina or resin, because different TLCs have different effects on the molecule. There are many hydroxyl groups on the silica gel. Your substance moves on the plate by forming hydrogen bonds with the silica gel - elution - and then forming hydrogen bonds. At this time, the material with a large polarity has a strong effect on the silica gel, which is reflected in the position of the TLC. However, for some resins, it interacts with your molecule through hydrophobic action, and the resin with a large polarity has a small adsorption effect on it, which is reflected in the position of the TLC.
In addition, the properties of a developing agent must also be considered. If the polarities of the two substances are less different, and the different developing agents (polarity and other properties), even on the same TLC, the unfolded position will change. I have encountered such an example where the front and rear positions of two molecules are reversed after unfolding under different unfolding conditions. If the polarities of the molecules are very different, such as one is a salt and the other is a neutral molecule, as long as on the same TLC, such as silica gel, the salt is always behind (usually at the origin) after unfolding, and the neutral molecule is ahead. But if the polarities are not much different, then various influencing factors must be considered.
The polarity of the deployment agent. On the silica gel plate, the polarity of the expansion agent will push the same substance forward. The polarity of methanol is greater than that of acetonitrile, but as mentioned earlier, the deployment process is very complicated, which is not the only consideration.
This is usually the case, but there are exceptions, such as saying: 'For example, if one is a salt and the other is a neutral molecule, as long as it is on the same TLC, such as silica gel, the salt is always behind after unfolding (usually at the origin) ', I have made a heterocyclic aromatic amine, and the Rf value of its hydrochloride and free amine on the same TLC is the same, which may be a special case, basically agree.
Everyone always likes to talk about polarity during TLC. I never consider who has the largest polarity and who has the smallest polarity. I only see who runs above and who runs below! When passing the column, the one with the largest Rf value comes out first, and the one with the small Rf value comes out later! Whatever the polarity!
Generally, for the same sample, the greater the polarity of the developing agent, the faster the expanded substance will crawl.
Silicone panels generally belong to a normal-phase system, with a large fixed-phase polarity and a small expansion agent polarity. Therefore, in traditional silica gel TLC, the separated substances run faster with a smaller polarity, and the slower ones run with a larger polarity.
Whether it is TLC or HPLC, the speed at which a substance runs is related to the polarity of the substance itself and the mobile phase. The closer the polarity of the substance is to the polarity of the mobile phase, the faster it runs.
The polarity of the two is similar in structure. When selecting a development agent, the two substances are always close together. To reduce the polarity, climb to the head, blow dry, and continue to climb the board. Repeat a few times to separate.
Let's talk about the usual experience of TCL. Although climbing board separation is a very common thing, there are many points to pay attention to:
