Chemistry Reference
In-Depth Information
While an increase in the hydrocarbon chain length in a series of normal alkyl
surfactants between C
8
and C
20
will have a minor effect on the effectiveness of
a surfactant, other structural changes can produce much more dramatic effects. We
have seen that structural features such as branching and multiple-chain hydrophobes
will generally result in increases in the cmc of surfactants with the same total car-
bon content. Those changes seem to have a much smaller effect on the efficiency
of the surfactant (C
20
) than on its effectiveness. For sodium n-dodecylbenzene
sulfonate the cmc and
s
min
are 1.2
10
3
M and 36 mN/m, while the same values
for sodium(2-methylundecylbenzene)sulfonate are 2.3
10
3
M and 27 mN/m,
respectively.
The introduction of slightly polar groups such as unsaturation, ether, ester, or
amide linkages, or hydroxyl groups located well away from the head group will
usually result in a significant lowering of both the efficiency and effectiveness of
the surfactant as compared to a similar material with no polar units. Such a result
has generally been attributed to changes in orientation of the adsorbed molecule
with respect to the surface due to interactions between the polar group and the
water (Figure 3.12). If the polar group is situated very near the primary hydrophilic
group, its orientational effect will be less dramatic, although it may still have a sig-
nificant effect on the cmc of the material.
Changes in the hydrophobic group in which fluorine atoms are substituted for
hydrogen will usually result in significant increases in the efficiency and effective-
ness of the surfactant. The substitution of fluorine for hydrogen in a straight-chain
surfactant results in a relatively small increase in chain cross-sectional area, as
compared to a methyl branch, for example, so that the changes must be related
to the chemical nature of the substitution. As has already been pointed out, fluori-
nated organic materials have a relatively low cohesive energy density and therefore
little interaction with adjacent phases, or themselves, for that matter. They therefore
have very favorable thermodynamic driving forces for adsorption (leading to high
efficiency), as well as low surface energies. Their effectiveness is reflected in the
very low surface tension values produced (as low as 20 mN/m in some instances).
(a)
(b)
Figure 3.12.
The effect of additional polar groups on the adsorption of surfactants:
(a) additional polar groups along the hydrophobic chain; (b) multiple, separated head groups.
