Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 67

separated from the dietary lipids and returned to the liver for reuse. Recycling thus
enables 20-30 g of bile salts to be secreted into the small intestine each day.
The most abundant of the bile salts in humans are cholate and deoxycholate, and
they are normally conjugated with either glycine or taurine to give glycocholate or
taurocholate, respectively. The conjugation is important in identifying the bile salt
for recycling back to the liver. The cholesterol contained in bile will occasionally
accrete into lumps in the gall bladder, forming gallstones.
In the absence of bile, fats become indigestible and are, instead, excreted in the
feces. This causes significant problems in the parts of the intestine, as normally, vir-
tually all fats are absorbed in the duodenum and the intestines, and bacterial flora are
not adapted to processing fats past this point. The role of bile salt micelles in these
biological systems is very important.
3.4 aPPlIcatIonS
The area of industry where surface-active agents are applied is extensive and beyond
the coverage of this topic. However, some main industrial applications will be
described in very general terms.
3.4.1 c l e a in I in g a n d d e T e r g e n c y
One of the most important applications of surface and colloid chemistry principles in
everyday life is in the systems where cleaning and detergency is involved. These are
some of the most important phenomena for humans (as regards health and welfare
and technology), and it has been regarded as such for many centuries. For example,
the effect of clean wings of airplanes is of utmost concern in flight security. Humans
have been aware of the role of cleanliness on health and disease for many thousands
of years. Many critical diseases, such as AIDS or similar infections, are found to be
lesser in incidence in those areas of the world where cleanliness is highest. The term
detergency is used for such processes as washing clothes , or drycleaning , or clean-
ing. The substances used are designated as detergents (Zoller, 2008). In all these
processes, the object is to remove dirt from fabrics or solid surfaces (floors or walls
or other surfaces of all kinds).
The shampoo is used to clean hair. Hair consists of portentous material and thus
requires different kind of detergents than when washing clothes or a car. A sham-
poo should not interact strongly with the hair but should remove dust particles or
other material. Another important requirement is that the ingredients in the sham-
poo should not damage or irritate the eye or skin with which it may come in con-
tact. In fact, all shampoos are tested for eye and skin irritation before marketing. In
some cases, by increasing the viscosity, one achieves a great deal of protection. For
example, a surfactant solution (ca. 20%), alkyl sulfate with two EO, gives very high
viscosity if a small amount of salt is added.
In the case of drycleaning, the aqueous medium is replaced by a nonaqueous
medium (such as tetrachloroethylene). Tetrachloroethylene can dissolve grease from
materials that should not be treated by water. Since there is also polar dirt (such as
sugars, minerals, etc.), a small amount of water needs to be added. This leads to the