Chemistry Reference
In-Depth Information
ciency of adsorption will be directly related to the length of the hydrophobic chain.
If the energy of such transfer is divided into components related to that for the
terminal methyl group in a straight-chain molecule (
G
trm
-CH
3
), subsequent
methylene groups (
G
trs
-S), and a stan-
dard reduction level of 20 mN/m is chosen, the surfactant efficiency can be defined
as the negative logarithm of the bulk-phase concentration required to produce a
reduction of 20 mN/m, so that
G
trn
-CH
2
-), and the hydrophilic group (
n
G
trn
þ
G
trs
log
ð
C
Þ
20
¼
pC
20
¼
3RT
þ
K
ð
3
:
18
Þ
2
:
3RT
2
:
¼
where K
G
trm
. For a given head group S under constant conditions of tem-
perature, pressure, solvent composition, and so on, the equation reduces to a direct
dependence of efficiency on the length of the hydrocarbon chain n.
Since the surfactant efficiency is directly related to the thermodynamics of chain
transfer from bulk to interface, it is reasonable to expect that chain modifications
that alter that characteristic, such as changes in the hydrophobic character of the
surfactant, will produce corresponding changes in the value of pC
20
. The linear
relationship between the number of -CH
2
- linkages in a chain and the adsorption
efficiency for a variety of hydrophilic groups illustrated in Figure 3.8.
Branching in the hydrophobic group will result in a reduction in the hydropho-
bicity of a surfactant chain relative to that of a related straight-chain material with
the same total carbon content. It is found, for example, that carbon atoms located on
branch sites will contribute approximately two-thirds as much to the surface acti-
vity of a surfactant molecule as one located in the main chain. Similar results are
observed for surfactants with two or more shorter-chain hydrophobes of equal total
carbon content (e.g., internal substitution of the hydrophilic group) and for the pre-
sence of unsaturation in the chain. The phenyl group, -C
6
H
4
-, will usually contri-
bute an effect equivalent to approximately 3.5 methylene groups.
Adsorption efficiency (× 10
10
)
4,0
3,5
3,0
2,5
2,0
1,5
1,0
8
9
10
11
12
13
14
15
16
N
methylene groups
°
Figure 3.8.
The effect of hydrocarbon chain length (as -CH
2
- groups) on the adsorption
efficiency of sodium sulfonate surfactants.
