MadSci Network: Chemistry

Re: What is the difference between a detergent and a solvent?

Date: Mon Mar 27 07:40:20 2000
Posted By: David Barker, Grad student, Organic Chemistry, University of Sydney
Area of science: Chemistry
ID: 954095880.Ch

Ok Ill try and keep this simple. A solvent by definition is a substance in which something can be dissolved. The thing that is dissolved in the solvent is called the solute.
For example when you dissolve salt in a glass of water, the salt is the solute and the water is the solvent.
Usually people think of solvents as things like nail polish remover, which is the chemical acetone, or cleaners such as methylated spirits, which is a mixture of methanol and ethanol. We commonly consider these chemicals as "solvents" because they dissolve things that water usually doesn't, just like acetone removes the paints off your nails but water won't.
The reason why these solvents remove the paints but water doesn't is simply explained by a simple chemistry phrase, "like dissolves like". Which means the paint on the nails , or the gluely mess on the back of stickers that you can remove with methylated spirits, is chemically just like the solvents themselves, and not like water, most of these solvents are very non-polar, or uncharged. The best example of this is petrol, or gasoline, it is very non-polar and totally unlike water, thats why it doesnt mix with water at all, they are not "alike".
Chemically salt and water are similar too, they both are charged (think can conduct electricity) thats why salt dissolves in water but not in nail polish remover.

Ok so thats solvents, detergents are different because they are chemicals have both a polar(or charged) section and a non-polar(or uncharged) section. They are usually long thin molecules that have a charged end and non charged tail, think of it like a matchstick, the head is charged and likes to be in water, the end of the match is non-polar and wants to be in a non-polar solvent like petrol or oil.

How detergents work to dissolve oil and grease for example when washing dirty dishes is that the oily dirt is surrounded by the detergent, with the non-polar end of the molecule surrounding the oily dirt. The other end of the detergent points away from the oil and sits in the water where it likes to be. So this clumps all the oilly dirt into "packets" called micelles and they can be washed away. A good picture of a micelle can be found at this web site

well I hope that helps

David Barker

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