Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 45

Surface
FIGure 3.5 Orientation of soap (surface-active substance) at the surface of water (alkyl
group:----, polar group: 0).
surface-active substance at the surface looks like alkane, it would thus be expected
that the surface tension of the surface-active substance's solution would decrease
from 72 mN/m (surface tension of pure water) to alkane-like surface tension (close
to 25 mN/m).
The orientation of the surface molecule at the interface will be dependent on the
system. This is shown as follows:
Air-water: polar part toward water and hydrocarbon part toward air
Oil-water: polar part toward water and hydrocarbon part toward oil
Solid-water: polar part toward water and hydrocarbon part toward solid
3.2.1.1 aqueous Solution of Surfactants
The solution properties of various surfactants in water are very unique and complex
in many aspects compared to such solutes as NaCl or ethanol. However, in the fol-
lowing text, a very simplified but useful and practical description will be given (for
details, the reader is advised to consult the relevant references [Birdi, 2002; Tanford,
1980]). The solubility of charged and noncharged surfactants are very different,
especially regarding the effect of temperature and salts (such as NaCl). These char-
acteristics are important in the application of these substances in diverse systems.
For instance, one cannot use the same soap with seawater, the main reason being that
salts (such as Ca and Mg) found in seawater affect the foaming and solubility char-
acteristics of major surface-active substances. For similar reasons, one cannot use
a nonionic detergent for shampoos (only amnionic detergents are used). Therefore,
tailor-made surface-active agents have been devised by the soap and detergent indus-
try to meet these specific demands.
3.2.1.1.1 Solubility Characteristics of Surfactants in
Water (Dependence on Temperature)
The solubility characteristics of surfactants (in water) is one of the most studied
phenomena. Even though the molecular structures of surfactants are rather simple,
their solubility in water is rather complex as compared to other amphiphiles such
as long-chain alcohols, etc., in that it is dependent on the alkyl group. This is easily
seen since the alkyl groups will behave mostly as alkanes. The hydrophobic alkyl
part exhibits solubility in water, which has been related to a surface tension model
of the cavity (see Appendix B). However, it is found additionally that the solubility