Can someone explain the chemistry around adding Isopropyl Alcohol to water contaminated gasoline - why does this help?

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One molecule larger than the other (regardless of which), would net the same result.
Actually that's not the correct explanation, and in fact volumes are(more or less) directly additive when you add together molecules with dramatically different sizes, such as mixing pentane and octane.

The likely explanation is a lot more nuanced and has to do with the nature of intermolecular interactions in liquid water and how hydrogen bonding causes the molecules to arrange themselves and create "voids" that other molecules can fit in. That's especially true when you involve something like alcohols that will themselves hydrogen bond with water and disrupt the bonding.
 
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Actually that's not the correct explanation, and in fact volumes are(more or less) directly additive when you add together molecules with dramatically different sizes, such as mixing pentane and octane.

The likely explanation is a lot more nuanced and has to do with the nature of intermolecular interactions in liquid water and how hydrogen bonding causes the molecules to arrange themselves and create "voids" that other molecules can fit in. That's especially true when you involve something like alcohols that will themselves hydrogen bond with water and disrupt the bonding.
Or in other words it forms a solution rather than a mixture.
 
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Little bit of a quick experiment this afternoon and I was curious about just how effective products like HEET were...

I had a can of Toro branded 50:1 "engineered fuel" here(I'd been looking at it in the GC-MS) so took that, put 45mL in a graduated cylinder, and added in 5mL of ethanol to roughly simulate E10. I should add also that I used anhydrous(200 proof) ethanol from a sealed bottle.

I then used a burette to add water. It took ~.2mL to start getting phase separation, or water/ethanol on the bottom of the graduated cylinder.

I was then took 100% IPA and started adding it back hoping I could get the phases to recombine. It took roughly 20mL to get the water fully back into solution.

Considering the scale of materials I was dealing with, it makes me question if a little ~300mL bottle of HEET(don't know the exact usual packaging, just guessing from the normal size) could have any meaningful effect added to ~16 gallons of gasoline. Note too that I started with gasoline that presumably was relatively dry. This particular "engineered fuel"does not seem to contain any oxygenates at least that I've seen on GC-MS, and instead seems to mostly use aromatics and branched alkanes to get its relatively high octane rating. Because of that, I'd not expect it to be able to hold a lot of water.
 
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