In primitive societies, even today, clothes are cleaned by beating them on rocks
near a stream. Certain plants, such as soapworts, have leaves that produce sapions,
chemical compounds that give a soapy lather. These were probably the first detergents
people used.
If you look up detergent in a dictionary it is simply defined as cleaning agent.
During the last two to three decades, however, the word detergent has tended to imply
synthetic detergent, or syndet for short, rather than the older soap. In fact, commercial
formulations consist of a number of components, and we shall use the term surface-active
agent, or it's abbreviation surfactant, to describe the special active ingredients that
give detergents their unusual properties.
Soap, by this definition, is a surfactant. In fact, it is the oldest one and has
been in use for over 4500 years. Some soap manufacture took place in Venice and Savona in
the fifteenth century, and in Marseilles in the seventeenth century. By the eighteenth
century, manufacture was widespread throughout Europe and North America, and by the
nineteenth century the making of soap had become a major industry. As a matter of fact,
soap became a detergent in 1907 when a German company put the product "Persil"
on the market. In addition to the carboxylic acid soap, "Persil" contained
sodium perborate, sodium silicate and sodium carbonate. Hence perborate + silicate =
"PERSIL".
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 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.
The relationship between foaming power and detergency has always been of
interest, and foaming power has become associated in many consumers' minds with high
detergent power. The first liquid detergent on the Australian market was "Trix".
It was non-foaming, so was soon replaced because of consumer resistance. However, it is
generally conceded by detergent technologists that foam height has no direct relationship
to cleaning power in ordinary fabric washing systems.
In systems where the amount of washing fluid is low, foam may play an important
role. The individual foam films tend to take up and hold particles of soil that have been
removed from the item, preventing them from being re-deposited and allowing them to be
washed or scraped away. Front loading washing machines work by bashing clothes against the
side of the tub - the high tech version of beating clothes on rocks. Front loaders clean
clothes better than top loaders, but only if a low-suds detergent is used, because the
suds cushion the impact and reduce the cleaning action.
Synthetic detergents dissolve or tend to dissolve in water or other solvents. To
enable them to do this, they require distinct chemical characteristics. Hydrophilic (water
loving) groupings in their molecular structure, and hydrophobic (water hating) groupings,
help the detergent in it’s “detergency” action.
This detergency depends on the balance of the molecular weight of the
hydrophobic to the hydrophilic portion. This is called the HLB value, and can range from 1
upwards. HLB is Hydrophilic-Lypophilic Balance. As the 0HLB value increases, the product
can tend towards being a paste or solid. The lower number HLB values tend to be less water
soluble, and more oil soluble. The higher the HLB the more water soluble the product.
Mixtures of low and high HLB detergents produce good detergents to handle oil,
fat and grease, the higher HLB detergent helps solubilise the less water soluble, low HLB
detergent into an aqueous system.
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