Is the iso more aggressive b/c of the polarity?
It has more to do with sterics--and polarity by proxy, rather than as a primary source of the difference.
When we discuss dissolving power in the simplest of terms what we're talking about is polar vs non-polar character and varying degrees of both. Isopropanol inhabits a very weird middle ground between these two--sort of what you might call a "best of both worlds" or a "sweet spot".
When we talk non-polar character what we're generally discussing is either:
1. Carbon and hydrogen ONLY.
2. Carbon affixed to molecules which are polar--who's polar vectors cancel each other out (they pull/push in equal and opposite directions throughout the entire molecule--these ALWAYS have at least 1 plane of symmetry).
Let me just rewind for a second here and say that what we're REALLY trying to get at here is how these molecules are sharing electron density. If one atom in the molecule is more electronegative (a physical property), it will unequally pull electron density towards itself--and this puts the distribution of charge in the molecule out of whack. This creates a magnetic dipole--and these dipoles (from molecule to molecule) are attracted through charge interactions in the way we are all familiar with. Similarly molecules which are not symmetric were never in balance to begin with, so they are inherently polar.
Non-polar interactions depend on two things.
1. Hydrophobic interactions (or in a manner of speaking "polar-phobic" interactions). These are driven by entropy (which says in a simple explanation that the disorder of the universe is always increasing and that things progress down this sort of thermodynamic well that I described earlier). The simplest way to explain this is that, structurally and magnetically, in order for the polar molecules to mix very well with the non-polar ones--they need to form very advanced structures so that the chemical mixture will "make sense". In a sentence, the structures required would lead to negative entropy. It's hard to think of this sometimes because with thermoydynamics often we find out that what does happen is because of what "would" happen. In a sense we find out that a less favorable possibility is the driving force behind the process that actually takes place, the un-favored future affecting the present.
This has always drummed up the Einstein quote "spooky action at a distance" for me, but it is nonetheless what is happening.
In lieu of these advanced structures, the polar and non-polar elements just exclude themselves from one another--by snuggling up with one another.
2. Van der Waals interactions. Also mentions with these are London Dispersion Forces--but it all essentially means the same thing. These guys works off of what are known as "instantaneous dipoles". These occur from the actual
movement of electrons around the molecules. This is a much more nuanced magnetic interaction, but it results from the same idea--as they move through space, sometimes the molecule has more electrons on one side than the other, and this slight imbalance produces a dipole. These interactions are orders of magnitude weaker than polar interactions, and that is why non-polar solvents have much lower boiling points (and also why their boiling points go up as chain length increases).
To the meat:
With isopropanol we have a special situation--not only do we have these polar and non polar elements play off of one another as in the other short chain alcohols, but the OH group is attached to the central carbon atom of a chain of length 3 (3 carbon atoms long).
The oxygen atom in the alcohol group pulls away some of the electron density from that central carbon, which in turn pulls (equally) from each of the two remaining carbon atoms through induction.
What we end up with is a sort of super-polar propane. Because each of the carbon atoms is sharing the pull from the oxygen--they end up all donating about the same amount of their electron density and thus the non-polar "character" of this chain is mostly retained--at the same time, there is a SIGNIFICANT dipole moment which is created through this interaction even though the distribution across the chain has been equal--the molecule has still been polarized on either side.
In ethanol, for instance, what we have is a molecule that has been almost entirely polarized. It can only induct straight through the chain--this offers much less stabilization than a bi or tri-directional spread. It can still do some nonpolar-ish stuff because (for lack of a better way to say it) carbon and hydrogen chains really love other carbon and hydrogen chains, even if they've been polarized.
We
can however see this difference better with the salting out behavior of isopropanol. It's possible to separate iso from water by adding salt to the solution--whereas doing this with ethanol is mostly fruitless.
The ethanol is undisturbed by the increase polarity of the solution (due to the ionic salts now dissolved)--whereas the isopropanol can only interact with it so much before the hydrophobic interactions from the non-polar chain will dominate over those effects.
From still a third stanpoint--structurally isopropanol does some weird stuff that makes it a really really aggressive solvent. This is probably well beyond most of your capacity to understand sans considerable primer material, and is hilariously far beyond my ability to properly explain.
As with most things in chemistry, it is never so simple as "is it this because of that?"
Probably a dozen people have spent a good portion of their life trying to fully understand this one molecule, if not more.
