Chemistry Reference
In-Depth Information
TABLE 4.8. Effects of Sulfate Substitution and Total
Carbon Content on Cmc's of Sodium Alkyl Sulfates
at 40
C
Total Carbon Number
Sulfate Position
cmc (mM)
8
1
136
8
2
180
10
2
49.5
11
3
28.9
11
6
83
14
1
2.4
14
2
3.3
14
3
4.3
14
4
5.15
14
5
6.75
14
7
9.70
15
2
1.71
15
3
2.20
15
5
3.4
15
8
6.65
16
1
5.8
16
4
1.72
16
6
2.35
16
8
4.25
18
1
1.65
18
2
2.6
18
4
4.5
18
6
7.2
of carbon atoms ranged from 8 to 18 and the position of the sulfate substitution
changed from the terminal to the middle carbons are listed in Table 4.8. The
data show that, except for the lower carbon number samples, the homologous series
follows a linear relationship similar to the Klevens equation.
The values of the constants A and B for the different series are given in Table 4.9.
From the data it can be seen that the value of A decreases slightly as the sulfate
group is moved toward the interior of the hydrocarbon chain. Such an effect may
be interpreted as indicating a smaller thermodynamic contribution to micelle
formation from added methylene groups with internal sulfation. The values of B
remain relatively constant within a series with different sulfate locations. For a
series of alkyl sulfates with the same number of carbon atoms, the cmc increases
as the sulfate group is moved internally. In the example of sodium tetradecylsulfate,
the measured cmc varies from 9.7 mM for the 7-sulfate to 2.4 mM for the terminal
sulfate. By analogy to straight-chain sulfates, such a change would correspond to a
decrease of about two carbon atoms.
Many surfactants of commercial interest have nonterminal hydrophilic substitu-
tion. However, because such materials are seldom of sufficient purity to warrant
