Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 41

The γ of proteins (which are large molecules with molecular weights from 6000 to
one million) also decrease when dissolved in water.
Surfactants are characterized as amphiphiles. Amphiphile in Greek means “likes
both kinds.” A part of the amphiphile likes oil or the hydrophobic (lipophilic) part,
while the other part likes water or the hydrophilic (also called lipophobic ) part.
The balance between hydrophilic and lipophilic is called HLB. It can be estimated
by experimental means, and theoretical analyses allow one to estimate its value
(Adamson and Gast, 1997; Holmberg, 2004; Birdi, 2009). HLB values are applied in
the emulsion (soaps and detergents) industry. Soap molecules are made by reacting
fats with strong alkaline solutions (a process called saponification). ). In water solu-
tion, the soap molecule, C n H 2n+1 COONa (with n greater than 12 to 22), dissociates at
high pH into RCOO − and Na + ions.
Originally, humans used soaps produced from fats (especially lard). However,
many decades ago, synthetic surfactants were made (as by-products of oil refining)
for special industrial applications such as cleaning and washing. A great variety of
surfactants were synthesized from oil by-products, especially, C 12 H 25 C 6 H 4 SO 3 Na,
and sodium dodecyl benzene sulfonates were used in detergents. Later, they were
replaced by sodium dodecyl sulfates or sulfonates because sodium dodecyl benzene
is not biodegradable. This means that the bacteria in sewage plants were not able
to degrade the alkyl group. Alkyl sulfonates were degraded to alkyl hydroxyls and
alkyl aldehydes, and later to CO 2 , etc. This problem was solved by using biodegrad-
able alkyl sulfates. In many applications, it has been found necessary to employ
surfactants that were nonionic, a variety of which have been synthesized, and one
can obtain tailor-made surfactants that suit a particular application. Further, since
nonionic detergents do not exhibit any charges, these find applications in processes
in which this property is essential.
For example, nonionic detergents as used in washing clothes are much different in
structure and properties compared to those used in dishwashing machines. In wash-
ing machines, foam is crucial as it helps keep the dirt, once it has been removed,
away from the clothes. On the other hand, dishwashing machines do not need any
foam, as it will hinder the dishwashing process. However, in these cases, surface
tension (which means wetting and other properties at the interface) needs to be low
to decrease the contact angle as well as to remove fats (through detergent action).
There are tailor-made nonionic detergents that satisfy these criteria. Another impor-
tant property of a surface-active agent is that most of the ionic detergents cannot be
used in conjunction with seawater (due high content of Ca and Mg salts). Therefore,
special detergents have been used to combat this problem.
3.2 SurFace tenSIon oF aqueouS SolutIonS
The surface tension (γ) of any pure liquid (water or organic liquid) will change when
another substance (solute) is dissolved in it. The change in γ will depend on the char-
acteristics of the solute added. The surface tension of water increases (in general)
when inorganic salts (such as NaCl, KCl, Na 2 SO 4 , etc.) are added (Figure 3.1), while
its value decreases when organic substances are dissolved (ethanol, methanol, fatty
acids, soaps, detergents, etc.) (Figure 3.1).