MadSci Network: Chemistry

Re: With soft water, why can't we rinse off all the soap

Date: Mon Jul 23 10:34:33 2001
Posted By: Lon Brouse, Faculty, Chemistry, Challenge Charter School
Area of science: Chemistry
ID: 993933059.Ch


This is a chemistry question that requires a chemical answer.  There are 
many 'common sense' answers that don't do the problem justice.  First, 
soap is made by heating animal fat (triglyceride fats) with lye (sodium 
hydroxide).  The lye (NaOH) gives up its OH group and separates the 
glycerine from its three fatty acid molecules.  This produces one molecule 
of glycerol and three ionically-bonded molecules of sodium stearate 
(soap).  This sodium salt will give up its sodium ion to a water 
solution.  The stearate will subsequently precipitate if it comes in 
contact with an ion that wants to hold on to it more strongly.  Calcium 
and magnesium are the usual culprits when 'hard water' is used in the 
shower.  The resulting calcium and/or magnesium stearate make the ever-
popular 'bathtub ring'.  Sodium stearate (soap) is comprised of a single 
sodium ion attached to a single stearate ion.  This combination is 
soluble.  Calcium and magnesium each have two places to form bonds and 
each of these metallic ions can combine with two stearate ions.  This 
resulting molecule is insoluble in water and is rinsed away in the shower 
spary.  This same 'bathtub ring' is an ingredient in many candy-like 
products.  Look on the ingredients list of say, Tic Tacs and you will see 
magnesium stearate!  They synthesize the product and do not scrape it from 
the inside of their bathtubs after the Saturday night cleanup.

Now enter the soft water.  You have removed the calcium and magnesium ions 
from the water and have replaced them with sodium.  There is no tendency 
to remove the sodium from the sodium stearate (soap) and therefore, no 
tendency to form an inosluble compound.  The surface of your skin has 
enough electrical charges in the form of amino acids, to cause the 
stearate ion to lightly cling to it.  The soft water has a much reduced 
ability to combine with the soap film on your body and therefore, it is 
much more difficult to rinse off.

The answer?  Use much less soap and accept the less than clean rinse 
characteristics, or change to a synthetic detergent (a 'syndet') in the 
form of a liquid body wash.  Adjust the amount of this synthetic cleaner 
and you should get much better results from your shower experience.

I hope this helps.

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