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Posted by George in Michigan on Wednesday, October 22, 2008 at 11:10:10 :
In Reply to: Re: OT Alcohol and diesel posted by D Sherman on Tuesday, October 21, 2008 at 20:36:23 :
Chemists have a saying...like dissolves like. So things that are polar molecules, dissolve other similar ones. Water and methanol are similar enough that they are miscible...that is they form a solution in any proportions. Ethanol and isopropanol do the same with each other or water. But once you get to the alcohols with 4 or more carbon atoms, they have some solubility in water but are not miscible. The alcohols with lots of carbon atoms have low solubity water.
It works the same way with hydrocarbons which are nonpolar molecules. Gasoline, diesel fuel, motor oil etc, are mostly made up of carbon and hydrogen atoms...so they are "like" molecules and are soluble (most would be miscible) with each other. Now water and methanol have oxygen in them and are thus more polar. The more carbons/hydrogens you have in the alcohol, the more soluble it will be in a hydrocarbon like diesel fuel. I think that ethanol is miscible with gasoline, but I don't think methanol (methy alcohol) is.
If you have two layers, like water and diesel fuel, and you also have methanol (or anything else) present, the methanol will partition between the two layers...in this case mostly concentrating in the solvent it likes, water.
And this why you can add ethanol or methanol to get water condensation out of your fuel tank. A little alcohol can dissolve in the hydrocarbon making the solution just polar enough to dissolve a bit of water.