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Name: Mashudu
Status: student
Grade: 9-12
Country: South Africa
Date: Fall 2009


Question:
What is the advantages of using a synthetic detergent in place of soap in harder water?



Replies:
The main advantage is that it will actually work.

In hard water, soaps exchange their cations for the hard water cations, and the resulting complex is insoluble, precipitating from the water. That means that it will not work to lower the water's surface tension or incorporate dirt into micelles, because the soap no longer has a hydrophilic head.

Most synthetic detergents do not have this drawback: their calcium and magnesium complexes are still hydrophilic.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Hi Mashudu,

Traditional soap is made up of a long chain-like, non-polar, molecule with a one polar end. This polar end is usually made up of an ionic bond such as Na(+) to an O(-). This arrangement allows the long-chain to attract non-polar molecules like oil, while the polar end attracts water. Thus, something like oil and water, which normally do not mix, will dissolve through the "bridge" created by the soap.

Since hard water contains many different cations and many of these ions have a 2+ oxidation state - Mg(2+), Ca(2+). These ions can now ionically bond twice. When this happens the soap molecule now looks like a long-chain, followed by the polar ionic bond, followed by another long-chain. The effect of this is to place the polar group in between two non-polar chains. This arrangement inhibits the "bridge" effect mentioned in the previous paragraph.

Synthetic detergents are designed differently - and there are many ways to do this, so that the effect of 2+ and 3+ cations does not reduce too much the wanted bridging effect. For example, some synthetic detergents will have a polar end that is normally bonded to a Ca(2+). This means that the detergent is designed to carry the bridging effect even with 2+ ions present. Some synthetic detergents carry multi-headed polar ends so that the polar end is not dependent only on one cation for its polar effect.

Greg (Roberto Gregorius)
Canisius College


Traditional "soap" is a mixture of carboxylic acids and its esters. These carboxylic acids form water insoluble salts with magnesium and calcium (hard water). This renders them inactive as detergents. Synthetic detergents have different chemical groups that remain soluble in the presence of salts like magnesium and calcium. Consequently, they remain active as detergents. This is a very simplified response to an entirely complicated technology of "detergence".

Vince Calder



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