Why doesn't soap lather in sea water?

Answer 1

Soap molecules are composed of long nonpolar "tails" with polar, negatively charged "heads" that arrange in a micelle formation: nonpolar interior, with a polar exterior shell surrounded by polar water. The polar exterior of soap micelles is negatively charged, and these negative charges repel one micelle from another, preventing coagulation. Soap is more effective in freshwater because higher salt ion concentration in seawater disrupts the ability of water to interact with the polar shells of micelles (fewer micelles can form in seawater because of the high concentration of salt ions in solution).

Answer for Ott's Chem 102

Answer 2

Sometimes salt helps soap lather and sometimes not.

First, here is a short answer. Soap creates a lather because soap molecules tend to aggregate as a consequence of the forces from electric charge on the soap molecules and the effectiveness of the electric charge is strongly affected by the polar character of water and these both (water and soap) are further affected when charged ions are in the water, with salt being a prime example. The ultimate answer as to why soap "lathers" requires an explanation of the arrangements of the soap molecules in water, but the basic answer to the question is that the stability of the arrangement of soap molecules is affected by the presence of ionic charges, such as from salt, in the water.

Now, if one considers the molecular processes, one can expand on the short answer.

As you add salt to water you affect the properties of soap quite a lot. Pure water is not as effective with soap as water with a small amount of salt. As you increase the salt concentration even further, complex things happen with the soap, and it can become less effective.

Soap or surfactants in general are complex materials, but the primary constituents are long molecules with polar head groups. Surfactants all have this property of containing molecules with one portion having different electric characteristics than another portion. One need only consider the example of a molecule of perhaps fifty or so atoms, mostly a carbon hydrogen chain, but with one end of the chain having a polar head group, like a carboxylate. (One uses the term head group for the polar end and hydrocarbon tail for the rest of the molecule.) The polar head group interacts strongly with the polar water molecules due to the simple electrostatic forces present and those are attractive, over all, and soap molecules will dissolve in water. The long hydrocarbon chain is not so attracted to water molecules and the energetics are such that (like oil and water) the chains are overall repulsed by the water molecules. To minimize this energetically unfavorable repulsive effect, soap molecule come together so the hydrocarbon tails are close to each other and water molecules are largely excluded from regions were the tails are close and the polar head groups tend to stick out of these regions and retain most of the energetic benefits of interacting with the water.

(Obviously, the details depend on the particular soap molecule, its length and type of head group and details of bonding of atoms in the chain.)

The upshot is, however, that the presence of ions, such as sodium, Na+, and Chlorine, Cl-, have a very large effect on the energetics of the process and structure of the water and the structures formed by the soap. There are thousands of combinations of ions, and soap molecules and soaps are not even formed by just one or two types of molecules, so what types of soap structure form depends on many factors.

In the end, the final answer is that salt at some concentrations will help form stable arrangements of soap molecules and at other concentrations or with other types of soap molecules, the effect will not be stable or not favorable to "lather."