Solubility -> Rt

[Mao]

This is why I don't particularly like "rules of thumb" because you can't derive them deductively from anything... >_>

Agreed. I don't think there can be a 'rule of thumb' for this. Polarity is a sketchy enough concept as is. This is my reply to a question received via pm. Reposted here for anyone interested:

Paper is made of cellulose, and cellulose is polar.
Does this mean that the mobile phase will also adsorb to the cellulose is what i am wondering.

Just another follow up question,
the size of the substance would not be as relevant Cf the intramolecular forces at play are, would it?
For example, a larger component of a substance, that is polar will have a higher than a different small component of the substance which is non-polar, correct? (assuming our solvent/M.P is also polar)
But in the case where the same large component is tested twice, once where it is of low concentration and one where it is of high concentration, will the lower concentrated sample move slower?
I thought that since it was large, it would have more polar areas on it to desorb to the water, and thus a larger concentrated component would move faster.

You are correct in saying that cellulose is polar, thus both mobile phase and stationary phase are polar. It is logical to say interaction between solvent and stationary phase will be strong.

However, you should realise that separation is not as simple as dipolar interaction. Any type of attractive interaction will play a part. This includes dispersion forces and so on. The rate of adsorption and desorption really depend on how the interaction plays out. Solubility can play a factor. You already know that solubilities are rather 'unpredictable'. So when both mobile phase and stationary phase are polar, it is quite hard to predict what comes out first. The ability to H-bond is also important for both phases.

Generally, larger molecules prefer to stick with the stationary phase (stronger dispersion interaction). Larger molecules also tend to be less soluble than smaller molecules. I would try to give a more complete picture, but the actual interaction model behind it is quite complicated.

Concentration generally does not affect retention. Size does affect retention.

So what I'm really saying is there isn't a reliable way to tell. You have to consider the polarity, as well as whether the solvent is protic or not, and whether your eluent is protic or not. At this point, it is far easier to do the experiment, characterise the results and come up with a theory than apply the theory to predict the results.