Activity3 1-13-11

Show a chemical structure for soap.

What is unique about the polarity(charge characteristics) of soap?

 

The oil is a pure hydrocarbon so it is non-polar. The non-polar hydrocarbon tail of the soap dissolves into the oil. That leaves the polar carboxylate ion of the soap molecules are sticking out of the oil droplets, the surface of each oil droplet is negatively charged. As a result, the oil droplets repel each other and remain suspended in solution (this is called an emulsion) to be washed away by a stream of water. The outside of the droplet is also coated with a layer of water molecules.
The graphic on the left although not strictly a representation of the above description is a micelle that works in much the same fashion. The oil would be a the center of the micelle.

 

How does soap work?

Most soaps are soluble sodium or potassium salts of carboxylic acids.  The most common commercial soap is sodium stearate, Na[C₁₇H₃₅CO₂].    It dissolves in water, forming the sodium and stearate ions.  Even though most of the stearate ion is a hydrocarbon chain, it dissolves in water because of the carboxylate group.  The carboxylate end is called hydrophilic (water-loving), and the hydrocarbon tail is called hydrophobic (water-fearing).