Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 43

80

Formic
Acetic
Propionic
Butyric

40

0 0.5

Concentration (Mol/Liter)

FIGure 3.3 Surface tension data of a homologous series of short-chain acids in water.

The surface tension data in the case of a homologous series of alcohols and acids
show a simple relation to the alkyl chain length (Figure 3.3). It is observed that each
addition of the -CH 2 - group gives values of concentration and surface tension such
that the value of concentration is lower by about a factor 3 .
However, it should be noted that such dependence in the case of nonlinear alkyl
chains will be different. The effective -CH 2 - increase in the case of nonlinear chains
will be lesser (ca. 50%) than in the case of a linear alkyl chain. The tertiary -CH 2 -
group effect would be even lesser. In general, though, one will expect that the change
in γ per mole substance will increase with any increase in the hydrocarbon group of
the amphiphile.
The effect of chain length on surface tension arises from the fact that, as the
hydrophobicity increases with each -CH 2 - group, the amphiphile molecule adsorbs
more at the surface. This will thus be a general trend in more complicated molecules
also, such as proteins and other polymers. In proteins, the amphiphilic property arises
from the different kinds of amino acids (25 different amino acids). Some amino
acids have lipophilic groups (such as phenylalanine, valine, leucine, etc.), while oth-
ers have hydrophilic groups (such as glycine, aspartic acid, etc.) (Figure 3.4).
In fact, one finds from surface tension measurements that some proteins are consid-
erably more hydrophobic (such as hemoglobin) than others (such as bovine serum albu-
min, ovalbumin). These properties of proteins have been extensively investigated.

3.2.1 S u r if a c e -a c T I V e S u b S T a n c e S ( A m p h i p h i l e s )

All molecules that, when dissolved in water, reduce surface tension are called sur-
face-active substances (e.g., soaps, surfactants, detergents). This means that such
substances adsorb at the surface and reduce surface tension. The same will happen if
a surface-active substance is added to a system of oil-water. The interfacial tension
of the oil-water interface will be reduced accordingly. Inorganic salts, on the other
hand, increase the surface tension of water.