How can you tell which mobile phase is more polar in TLC? (hexane/ethyl acetate/ethanol vs. isopropanol/ammonia)? the hexane/ethyl acetate/ethanol travelled up the TLC faster, does this make it more polar?

What's the substrate on the TLC? SiO2? If so that is polar and it should hold the more polar components back (travel slower). My best intuition thinks that ammonia would be more polar than ethanol.

the TLC is silica gel

Since the substrate is silica gel (SiO2), that is a polar substrate and the more polar solvent should move slower than the less polar solvent.

thank you!

To determine which mobile phase is more polar in thin-layer chromatography (TLC), you need to consider the relative polarities of the individual solvents that make up each mobile phase. In TLC, the mobile phase is a mixture of solvents that carries the sample along the stationary phase.

In this case, you are comparing two mobile phases:

1. hexane/ethyl acetate/ethanol
2. isopropanol/ammonia

The speed at which the mobile phase travels up the TLC plate is an indication of its polarity. Generally, a more polar mobile phase will cause the sample to move more slowly up the plate, while a nonpolar mobile phase will allow the sample to move quickly.

In your specific scenario, you mentioned that the hexane/ethyl acetate/ethanol mobile phase traveled up the TLC faster, while the isopropanol/ammonia mobile phase must have traveled slower for the same sample. This indicates that the hexane/ethyl acetate/ethanol mobile phase is less polar compared to the isopropanol/ammonia mobile phase.

The reason behind this observation lies in the relative polarities of the individual solvents within each mobile phase. Hexane is a nonpolar solvent, while ethanol is a polar solvent. Ethyl acetate falls somewhere in between in terms of polarity. Since the hexane/ethyl acetate/ethanol mobile phase traveled faster, it suggests that hexane (being the least polar component) is the dominant solvent and contributes to the overall nonpolarity of the mobile phase.

On the other hand, isopropanol is a moderately polar solvent, while ammonia is a highly polar solvent. Therefore, the isopropanol/ammonia mobile phase is expected to be more polar overall.

In summary, based on the observation that the hexane/ethyl acetate/ethanol mobile phase traveled faster in TLC, it suggests that this mobile phase is less polar compared to the isopropanol/ammonia mobile phase.