MadSci Network: Chemistry
Query:

Re: Why do oil and vinegar separate

Date: Wed Oct 8 02:16:37 2003
Posted By: Werner Sieber, Research Scientist, Coating Effects,
Area of science: Chemistry
ID: 1065227420.Ch
Message:

Dear Daniel,

You may know that liquids, like oil, water, or vinegar, consist of molecules. Even though these tiny bits of matter can't be seen with the best microscope, they can be characterized by various methods. In a "pure substance" like water, all molecules are of the same type. In liquids, they are relatively close to each other, yet able to move about. They ATTRACT each other to a certain extent - otherwise the liquid would evaporate in a jiffy.

Now if we take two DIFFERENT liquids A and B, and try to mix them, there are 3 different attraction forces (strictly speaking: energies) to consider: those between A and B, as well as among A's and B's themselves. The type of force easiest to understand is the DIPOLE interaction, which dominates in water (and vinegar, which is mostly water). Dipolar molecules attract each other more than they attract "non-polar" molecules (like oil), because each end of every dipole seeks oppositely charged ends of other dipoles. If you beat oil and vinegar together, and leave it to stand, the movement of molecules proceeds towards the miminum energy - that's when the dipolar molecules have found each other and puddled together - leaving the oil molecules on their own. This even happens in the absence of gravity, albeit more slowly.

Best Regards
Werner Sieber


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