Chemistry Reference
In-Depth Information
TABLE 10.2. Adsorption Efficiency (as log
C
0
) of Some Typical Surfactants
on Various Substrate Types
pH (I.S.
a
)
Temp. (
C)
log C
o
Surfactant
Substrate
n-C
8
H
17
SO
4
Na
AgI
20
3 (0.001 M)
2.60
n-C
10
H
23
SO
4
Na
AgI
20
3 (0.001 M)
3.89
n-C
12
H
25
SO
4
Na
AgI
20
3 (0.001 M)
4.50
n-C
14
H
29
SO
4
Na
AgI
20
3 (0.001 M)
5.15
n-C
10
H
23
SO
4
Na
AgI
20
3 (0.0015 M)
3.40
n-C
12
H
25
SO
4
Na
AgI
20
3 (0.0015 M)
4.38
n-C
14
H
29
SO
4
Na
AgI
20
3 (0.0015 M)
4.78
C
12
H
25
C
6
H
4
SO
3
Na
AgI
20
3 (0.0015 M)
4.84
n-C
10
H
23
SO
4
Na
a
-Al
2
O
3
25
7.2 (0.002 M)
2.75
a
-Al
2
O
3
n-C
12
H
25
SO
4
Na
25
7.2 (0.002 M)
3.55
a
-Al
2
O
3
n-C
14
H
29
SO
4
Na
25
7.2 (0.002 M)
4.25
n-C
16
H
23
SO
4
Na
a
-Al
2
O
3
25
7.2 (0.002 M)
5.00
n-C
10
H
23
NH
3
OAc
SiO
2
25
6.7
1.75
n-C
12
H
25
NH
3
OAc
SiO
2
25
6.7
2.60
n-C
14
H
29
NH
3
OAc
SiO
2
25
6.7
3.45
n-C
16
H
33
NH
3
OAc
SiO
2
25
6.7
4.30
n-C
18
H
37
NH
3
OAc
SiO
2
25
6.7
5.15
a
I.S.
ΒΌ
ionic strength.
increase, decrease, or remain unchanged as a result of those same changes. The
effect of an increase in the chain length of the hydrophobe, for example, will
depend on the orientation of the adsorbed molecules on the surface. If adsorption
is perpendicular to the substrate surface, in an approximately close-packed array,
an increase in the chain length of a normal alkane will not result in any significant
change in the number of moles of surfactant adsorbed per unit area of surface at
saturation. Since the cross-sectional area of a perpendicularly adsorbed, straight-
chain molecule does not change much with an increase in the number of units in
the chain, the absence of a change in adsorption effectiveness is not a surprising
result. In addition, the cross-sectional area of most adsorbed hydrophilic groups
is greater than that of normal-chain hydrophobic groups, and is therefore the limit-
ing factor in determining the number of surfactant molecules that can be adsorbed
per unit area of surface.
If the mode of adsorption is less than perpendicular (L or train-shaped), or if the
molecules are slightly tilted, there may be some increase in the effectiveness of
adsorption as the length of the alkyl chain is increased. Again, such an effect is
not surprising when one considers that greater dispersion force interaction resulting
from a larger alkyl group can lead to greater lateral interactions among surfactant
molecules, making possible a greater packing density for the longer chains. A simi-
lar effect may be seen when one considers the density, surface tension, cohesive
energy density, and many other properties of the homologous series of n-alkanes
from C
8
to C
18
(Figure 10.13).
