The Foundations of Chemistry

charged Na
ions. The result is a suspension of micelles in water. These micelles are large
enough to scatter light, so a soap–water mixture appears cloudy. Oil and grease “dissolve”
in soapy water because the nonpolar oil and grease are taken into the nonpolar interior
of micelles (Figure 14-21b). Micelles form a true emulsion in water, so the oil and grease
can be washed away. Sodium stearate is called a surfactant(meaning “surface-active
agent”) or wetting agent because it has the ability to suspend and wash away oil and grease.
Other soaps and detergents behave similarly.
“Hard” watercontains Fe^3
, Ca^2
, and/or Mg^2
ions, all of which displace Na
from
soaps to form precipitates. This removes the soap from the water and puts an undesirable
coating on the bathtub or on the fabric being laundered. Synthetic detergentsare soap-
like emulsifiers that contain sulfonate, XSO 3 , or sulfate, XOSO 3 , instead of carboxylate
groups, XCOO. They do not precipitate the ions of hard water, so they can be used in
hard water as soap substitutes without forming undesirable scum.
Phosphates were added to commercial detergents for various purposes. They complexed
the metal ions that contribute to water hardness and kept them dissolved, controlled acidity,
and influenced micelle formation. The use of detergents containing phosphates is now
discouraged because they cause eutrophicationin rivers and streams that receive sewage.

Figure 14-21 (a) A representation

of a micelle. The nonpolar tails

“dissolve” in one another in the

center of the cluster and the polar

heads on the outside interact

favorably with the polar water

molecules. (b) Attachment of soap or

detergent molecules to a droplet of

oily dirt to suspend it in water.

14-18 Hydrophilic and Hydrophobic Colloids 581

Oil droplet (dirt)

suspended in water

Long-chain-hydrocarbon

portion of detergent

(oil-soluble)

Water-soluble portion

of detergent, –COO–

or –SO 4 – (polar part)

(a) (b)

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 14.11, Surfactants.

Phosphates and nonbiodegradable

detergents are responsible for the

devastation of plant life in and

along this river.