Section 17.13 Soaps, Detergents, and Micelles 701Na+CO 2 −H 2 O−O
2 C−O
2 C−O
2 C−O
2 CCO 2 −
CO 2 −CO 2 −CO 2 −CO 2 −Na+Na+Na+Na+Na+Na+Na+Na+nonpolar tailpolar head
groupcounterionFigure 17.4
In aqueous solution, soap forms micelles, with the polar heads (carboxylate groups) on the
surface and the nonpolar tails (fatty acid R groups) in the interior.
MAKING SOAP
For thousands of years, soap was prepared by
heating animal fat with wood ashes. Wood ashes
contain potassium carbonate, which makes the solution basic.
The modern commercial method of making soap involves
boiling fats or oils in aqueous sodium hydroxide and adding
sodium chloride to precipitate the soap, which is then dried
and pressed into bars. Perfume can be added for scented
soaps, dyes can be added for colored soaps, sand can be added
for scouring soaps, and air can be blown into the soap to make
it float in water.Making soapBecause the surface of the micelle is negatively charged, the individual micelles
repel each other instead of clustering to form larger aggregates. As river water flows
over and around rocks, it leaches out calcium and magnesium ions. The concentration
of calcium and magnesium ions in water is described by its “hardness.”Hard water
contains high concentrations of these ions; soft water contains few, if any, calcium and
magnesium ions. While micelles with sodium and potassium cations are dispersed in
water, micelles with calcium and magnesium cations form aggregates. In hard water,
therefore, soaps form a precipitate that we know as “bathtub ring”or “soap scum.”
The formation of soap scum in hard water led to a search for synthetic materials
that would have the cleansing properties of soap, but would not form scum when they
encountered calcium and magnesium ions. The synthetic “soaps”that were developed,
known as detergents, are salts of benzene sulfonic acids. Calcium and magnesium
sulfonate salts do not form aggregates. “Detergent”comes from the Latin detergere,