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Synthetic
Surfactant or Soap ?
You may well ask why soap, which served well for so many years, was eventually
displaced. Soaps are cheap and they are manufactured from a renewable source, whereas many
of the synthetic detergents are made from petrochemicals. Soaps are also biodegradable;
that is, they are readily broken down by bacteria, and thus they do not pollute rivers.
However, due to their gelling properties, soaps do have a greater tendency to clog
sewerage reticulation systems than synthetic detergents. The grease trap of a non-sewered
house was often laden with soap. But the most important reason for the displacement of
soap is the fact that, when a carboxylic acid soap is used in hard water, precipitation
occurs. The calcium and magnesium ions, which give hardness to the water, form insoluble
salts with the fatty acid in soap and a curd-like precipitate occurs and settles, of
course, on what ever is being washed. By using a large excess of soap, it is possible to
redisperse the precipitate, but it is extremely sticky and difficult to move. This problem
with soap can be demonstrated by a simple experiment in which a concentrated solution of
hard-water salts is added to a 0.1% solution of soap and also to a 0.1% solution of
synthetic surfactant. The soap precipitates, but the synthetic surfactant remains clear
because it's salts are water soluble.
You may live in an area where the water is extremely soft. But calcium and magnesium
ions are present in the dirt that you wash out of your clothes, so that some precipitation
still occurs if soap is used, and gradually deposits are built up in the fabric.
There are other disadvantages with soap; it deteriorates on storage, and it lacks
cleaning power when compared with the modern synthetic surfactants, which can be designed
to perform specialised cleaning tasks. Finally and very importantly from a domestic
laundry point of view, soap does not rinse out; it tends to leave a residue behind in the
fabric that is being washed. A residue gradually builds up and causes bad odour,
deterioration of the fabric and other associated problems.
What's the
Difference ?
What's the difference between a surfactant and soap ? In general terms, the difference
can be likened to the difference between cotton and nylon. On the one hand, soap and
cotton are produced from natural products by a relatively small modification. On the other
hand, synthetic surfactants and nylon are produced entirely in a chemical factory.
Synthetic surfactants are not very new, either. Back in 1834 the first forerunner of
today's synthetic surfactants was produced in the form of a sulfated castor oil, which was
used in the textile industry.
The development of the first detergents in an effort to overcome the reaction of soaps
with hard water provides a good illustration of one of the standard chemical approaches.
If a useful substance has some undesirable property, an attempt is made to prepare an
analogue, a near chemical relation, which will prove more satisfactory.
The petroleum industry had, as a waste product, the compound propylene, CH3-CH=CH2,
which used to be burnt off. By joining four of these propylene molecules together and if
benzene is attached at the double bond, the resulting compound reacts with sulphuric acid.
Then sodium hydroxide is added to neutralise the sulfonic acid and a sodium salt is
obtained. The new substance is closely related to an ordinary soap, and is an excellent
detergent.
High Tech Detergents
- http://www.chemistry.co.nz/stain.htm - More information
where this came from!
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