So there are many products on the market sold as fuel additives; some (like 'Heet' brand) are nothing more than 99.9% isopropyl alcohol in a nice package.

When trying to understand how they work and what they actually do you hear vague things like "Removes water in fuel. The alcohol will bond with water and solubilize water into gasoline so it can be burned and pass out exhaust"

From what I understand alcohol and water don't 'react' -> there is no chemical formula change. They simply mix really well. But how does this make this new mixture 'mix' with gasoline better?

I know that if you take 50% isopro (other 50% is water) and light it on fire, it will burn. But I understand that it's only the 50% of alcohol molecules burning, not the water. So at the end you'll still end up with the water remaining (excluding some evaporation from the heat).

What's going on? Can someone explain the actual chemistry please of how IPA removes water from the gasoline?

UPDATE: found this technical paper from a chemical engineer on the subject: https://web.archive.org/web/20130427123941/http://www.epa.gov/OMS/regs/fuels/rfg/waterphs.pdf

However, it still doesn't spell out the exact chemistry but should add more context to my question.

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    $\begingroup$ You want the water evenly mixed throughout the gasoline (well, you really want it filtered out). You don't want a blob of water heading into the injectors to be sprayed into a cylinder and not ignite. $\endgroup$
    – Jon Custer
    Oct 27, 2022 at 23:20
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    $\begingroup$ I don't see what "chemistry" you want. Water in fuel is more of engineering issue as Jon says. If you add enough alcohol you get one phase - no separate water phase - no problem. Maybe it's that you reasoned backwards? Alcohol dissolves also in fuel and if you add enough, water has enough solubility in resultant mixture. $\endgroup$
    – Mithoron
    Oct 28, 2022 at 0:02
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    $\begingroup$ Alcohols are composed by a hydrocarbon chain (R-) and a hydroxyl group (-OH). The bigger the hydrocarbon chain is, the more soluble other organic substances (like fuel) become, but water becomes less soluble. When there is a right balance between R size and OH in terms of total alcohol composition, both substances (fuel and water) become reasonably soluble to form a homogenized mixture. Isopropanol is an example of an alcohol that qualifies. $\endgroup$
    – Sam202
    Oct 28, 2022 at 0:56
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    $\begingroup$ Here is a wrong sentence from the second paragraph : The alcohol will bond with water and solubilize water into gasoline so it can be burned. This is wrong. Twice wrong. Alcohol will not solubilize water into gasoline, and water cannot be burned. A mixture $50$% ethanol or isopropanol + $50$% gasoline dissolves less than 1 % water. $\endgroup$
    – Maurice
    Oct 28, 2022 at 9:10
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    $\begingroup$ Alcohol in the gasoline will dissolve water and the typical dose is about 250mL. If the amount of water is large it may require repeated dosed to get the engine running right again. We once bought some watered gas in Apalachicola Fl couldn't find any dry gas until Dade City hobbling across the panhandle about 25mph. It took a case of dry gas keeping the fuel level below 1/4 tank to get the car running OK, by that time we were halfway thru Georgia. this was an endemic problem in northern climates almost alleviated by todays gasoline with 10% ethanol. I still have some cans of dry gas, waiting $\endgroup$
    – jimchmst
    Nov 8, 2022 at 22:10

1 Answer 1


Water separates from gasoline and should not be injected or carbureted in large quantities because it will destabilize engine performance.

Addition of isopropanol in quantities comparable to the water may not make a homogeneous solution in gasoline, but the water-iPA phase will have a surface tension so low (50/50 at 25$^0$ C: 24.26 mN/m vs 21.22 for pure iPA Ref 1)

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that emulsification will be very easy (from shaking due to road conditions like potholes, sharp turns or braking, stopping, etc.).

This sort of behavior is evidenced in practice by formation of a foamy layer between two phases, or just a foamy mass, of tiny separate phase droplets that do not easily recoalesce into two distinct phases. This upsets lab workers who are trying to separate the phases, but the engine is much more tolerant (or could be if the amount of water is not too great).

The engine operates well with injected gasoline; less well with some emulsified water; poorly with too much water in the emulsion. But in general, small amounts of water-containing emulsion can be injected or carbureted and burned without difficulty.

In other words, the "chemistry" behind the effectiveness of isopropanol's ability to counteract the effect of water in the gas tank does not require a total clarification and transformation into a single phase, but only an assistance so as to be able to minimize the amount of water in each cylinder-load of fuel.

I suppose a surfactant that really emulsified and stabilized a cloudy gasoline-water mixture would burn quite well, even at high concentrations of water.

This is a slightly expanded form of a comment I had posted yesterday.

Ref 1. pubs.acs.org/doi/pdf/10.1021/je00019a016


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