Chemistry Reference
In-Depth Information
(a) (b) (c)
Figure 3.10.
Schematic representation of the role of the hydrophobe in determining the
effectiveness of surfactant adsorption: (a) n-alkyl—area determined by head group;
(b) branched or double-tailed—area determined by bulk of tail relative to head; (c)
polyoxyethylene nonionic—area determined by coiling of POE chain.
of material that can be adsorbed in a given surface area will reduce the ultimate
surface tension lowering attained. The efficiency will change more or less regularly
with the chain length. The sign of the charge on the ionic surfactant has only a
minor effect on the ultimate surface tension attained, indicating that the geometric
requirements (including electrostatic effects) are relatively constant from one head
group to the next. In the presence of neutral electrolyte, of course, electrostatic
repulsions between adjacent molecules will be reduced, making their effective
areas smaller. The net result will be a slight increase in surfactant effectiveness.
The complex relationship between the molecular structure of a surfactant and
its impact on surfactant efficiency and effectiveness can be illustrated by the case
of a series of nonionic POE surfactants in which the number of OE units is held
constant and the hydrocarbon chain length is increased (Table 3.3). In the series
it is found that the surface excess at surface saturation
m
decreases regularly
10
10
for C
6
, while the
s
min
remains
relatively constant. This would indicate that while the efficiency of surfactant
adsorption is increasing with the length of the hydrocarbon chain, the overall effec-
tiveness of the material is relatively unchanged.
It can be seen from Table 3.3 that in the cases of the C
16
and p,t-C
8
H
17
C
6
H
4
hydrophobic groups, as the size of the hydrophilic group (n) increases, the effec-
tiveness (as
s
min
) decreases. This effect can be related to the fact that each addi-
tional OE group added to the head of the surfactant increases the total area required
for adsorption of the molecule, reduces the packing density of hydrophobic groups
at the interface, and therefore results in a smaller reduction in the surface tension
of the system. If the area per molecule a
o
required for the adsorption of the
C
16
H
33
(POE)
x
series of surfactants is examined, it can be seen that the addition
of each OE unit increases the requirement by an average of 5
˚
2
or 0.05 nm
2
.
From insoluble monolayer experiments, it has been shown that the surface tension
10
10
mol/cm
2
from 4.4
for C
16
to 2.7
