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I remembering reading that the interaction between a polar molecule and non-polar molecule is stronger than the interaction between two non-polar molecules.

However, in analytical chemistry classes, we are told that polar molecules elute first in a Gas Chromatograph that has a non-polar column. According to http://www.chem.ucla.edu/~bacher/General/30BL/gc/theory.html

If the polarity of the stationary phase and compound are similar, the retention time increases because the compound interacts stronger with the stationary phase. As a result, polar compounds have long retention times on polar stationary phases and shorter retention times on non-polar columns using the same temperature.

These two concepts seem to conflict. Is there something else that is going on? Shouldn't polar compounds elute last even if the stationary phase is non-polar.

(Also sidenote: most mobile phases in Gas Chromatography are non-polar: helium, argon, or nitrogen)

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I think you're incorrectly remembering something you've been told. People speak of the adage "like-dissolves-like" which really means that polar things interact best with polar things and nonpolar interact best with nonpolar. There are always exceptions and fringe cases, but that adage works pretty well here. Nonpolar molecules interact with other nonpolar molecules pretty much exclusively through dispersion forces. These are the so-called "instantaneous dipoles" you have probably heard of. Polar molecules usually have some way of interacting through dipole-dipole interactions, hydrogen bonding, or simple electrostatics.

Note that the mobile phase, i.e. the carrier gas, does not have a large effect on the retention time in GC, so it really is mostly due to the simplified explanation of intermolecular forces given above.

One might ask why the polar molecules don't also have large dispersion interactions with the stationary phase (they certainly have some), and this is due to two effects. First, polar molecules tend to interact fairly strongly with each other in the gas-phase, and so there is less of an energetic gain for interacting with the stationary phase. This is a problem because interacting with the stationary phase is entropically unfavorable. Second, many polar molecules are smaller than a lot of the nonpolar systems that are studied using GC, and dispersion is correlated with surface area, so the polar molecules tend to have weaker dispersive interactions even ignoring the first point.

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  • $\begingroup$ I am surprised by the statement that "polar molecules tend to interact fairly strongly with each other in the gas-phase". Most of the time, molecules in the gas phase don't interact with other molecules at all. $\endgroup$ – Karsten Theis Jan 21 at 19:19
  • $\begingroup$ Looking at a gas chromatogram region, it might seem that polar molecules are smaller than nonpolar molecules with similar retention times because if you compare molecules of similar molar mass, the polar ones bind more tightly to the stationary phase and will have much longer retention times. See discussion here $\endgroup$ – Karsten Theis Jan 21 at 19:23
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Your statement about intermolecular forces are correct.

Your statement about polar molecules eluting first from a non-polar gas chromatography column is debatable. It really hinges on comparing apples with apples instead of with oranges.

If we group molecules according to shape and molar mass, we ensure that molecules within a group will have similar strength of dispersion forces. Then, we can compare molecules that are non-polar with ones in the same group that are polar, for example nonane vs 1-octanol or dibutyl ether. It turns out that 1-octanol has a higher retention time (elutes later, i.e. interacts stronger) than nonane. Dibutyl ether is similar in retention time (and boiling point) to nonane.

There is no need to classify the mobile phase as polar or non-polar. The interactions in the gas phase are minimal (otherwise it would condense and form a liquid).

So there is a conflict between your two statements (strength of IMF, behavior on non-polar GC column) which resolves once you realize that one of the statements is incorrect.

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