acetate is a water-soluble, ionic solid that dissociates completely
into ions in aqueous solution. Since acetic acid is a weak acid,
a solution of sodium acetate in water is slightly basic. Some of
the acetate ions recombine with H
ions from water and leave an excess of OH
CH - COOH +
The salts of long-chain fatty acids are more soluble than the acids
themselves are. Soaps are salts of fatty acids in the 12-
to 18- carbon range, which usually are obtained from animal fats.
Sodium stearate is a common soap. In aqueous solution, stearate
ions form monolayers at the air-water interface like fatty acids
and long-chain alcohols do, with their hydrocarbon chains in the
air. When air is blown through a soap solution, soap bubbles are
formed from a double layer of soap molecules, with hydrocarbon tails
exposed to the air on either side, and charged heads meeting in
a layer of water at the centre of the film. The structures of both
surface films and bubbles are shown to the right.
The water in the centre of the soap bubble film is still connected
with the bulk water below it. As this water in the centre of the
"sandwich" gradually drains back down and the film thins,
the iridescent interference colors that we associate with soap bubbles
are produced. When too much water has drained back, electrostatic
repulsion between heads of molecules on the two sides of the bilayer
breaks the film, and the bubble bursts.