Chemistry Reference
In-Depth Information
When substitution occurs internally, leaving the terminal CF
3
- intact, the effects
are much less significant. This is in agreement with general observations that indi-
cate that the surface energies of polymers and adsorbed monolayers are determined
primarily by the top atomic layers, as they are less dependent on the chemical
nature of succeeding layers. The fact that all CF
3
-terminated commercial surfac-
tants do not produce the same low surface tension is a reflection of molecular pack-
ing defects introduced by fluorocarbon chain branching, the presence of homologs
and other impurities, and the steric demands of the various linking groups and head
groups employed. In essentially all cases, however, the fluorinated materials will be
significantly more surface-active than the analogous hydrocarbon.
4.5.4. The Hydrophilic Group
As seen from the preceding section, the nature of the hydrophobic group has a
major effect on the critical micelle concentration of a surfactant. The effect of the
hydrophilic head group on the cmc's of a series of surfactants with the same hydro-
carbon chain may also vary considerably, depending on the nature of the change. In
aqueous solution, for example, the difference in cmc for a C
12
hydrocarbon with an
ionic head group will lie in the range of 0.001 M while a nonionic material with the
same hydrocarbon chain will have a cmc in the range of 0.0001 M.
The cmc's of several ''model'' surfactants are given in Table 4.15. It is evident
from the data that the nature of the ionic head group has a rather small effect
TABLE 4.15. Effects of Hydrophilic Group on the cmc's of Surfactants with Common
Hydrophobes
Temperature (
C)
Hydrophobe
Hydrophile
cmc (mM)
COO
K
þ
C
12
H
25
25
12.5
SO
3
K
þ
25
9.0
SO
3
Na
þ
25
8.1
H
3
N
þ
Cl
30
14
(CH
3
)
3
N
þ
Cl
30
20
(CH
3
)
3
N
þ
Br
25
16
H
3
N
þ
Cl
C
16
H
23
55
0.85
(CH
3
)
3
N
þ
Cl
30
1.3
(CH
3
)
3
N
þ
Br
60
1.0
(CH
3
)
2
C
2
H
4
OH N
þ
Cl
30
1.2
(CH
3
)(C
2
H
4
OH)
2
N
þ
Cl
30
1.0
(C
2
H
4
OH)
3
N
þ
Cl
30
1.0
C
8
H
17
OCH
2
CH
2
OH
25
4.9
(OCH
2
CH
2
)
2
OH
25
5.8
C
9
H
19
COO(CH
2
CH
2
O)
9
CH
3
27
1.0
COO(CH
2
CH
2
O)
16
CH
3
27
1.8
C
10
H
21
O(CH
2
CH
2
O)
8
CH
3
30
0.6
O(CH
2
CH
2
O)
11
CH
3
30
0.95
O(CH
2
CH
2
O)
12
CH
3
30
1.1
