Detergent, Soap, Hard Water
Country: South Africa
Date: Fall 2009
What is the advantages of using a synthetic detergent in
place of soap in harder water?
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
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)
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
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Update: June 2012