Chemistry Reference
In-Depth Information
TABLE 4.17. The cmc's of Various Metal Salts
of Dodecylsulfate
Temperature (
C)
Counterion
cmc (mM)
Li
þ
25
8.8
40
10.5
Na
þ
25
8.1
40
8.9
K
þ
40
7.8
Cs
þ
40
6.9
(CH
3
)
4
N
þ
25
5.6
(
2
Ca
2
þ
)
54
2.6
(
2
Mg
2
þ
)
25
1.6
(
2
Zn
2
þ
)
60
2.1
Although within a given valency the size of the hydrated counterion will have
some effect on the micellization of an ionic surfactant, a more significant effect is
produced by changes in valency. As the counterion is changed from monovalent to
di- and trivalent, the cmc is found to decrease rapidly. The cmc's of various salts of
dodecylsulfate are listed in Table 4.17. As discussed earlier, the divalent and higher
salts of carboxylic acid soaps generally have very low water solubility and are not
useful as surfactants in aqueous solution. They have found use in nonaqueous sol-
vents because of their increased solubility in those systems, especially in the pre-
paration of water-in-oil emulsions. As we will see, the presence of ions in aqueous
surfactant solutions beyond the stoichiometric concentration can produce a more
significant effect than changes within a valency group.
4.5.6. The Effects of Additives on the Micellization Process
Most industrial applications of surfactants involve the presence in the solution of
cosolutes and other additives that can potentially affect the micellization process
through specific interactions with the surfactant molecules (thereby altering the
effective activity of the surfactant in solution) or by altering the thermodynamics
of the micellization process by changing the nature of the solvent or the various
interactions leading to or opposing micelle formation. Examples of specific inter-
actions between surfactant molecules and cosolutes are common when the system
contains polymeric materials. Because of the growing importance of such systems,
they are treated as a special topic below.
In the absence of specific interactions, which can be quite complex, it is useful to
be able to rely on laboratory cmc and aggregation number data to predict the char-
acteristics of micellization of a surfactant in use. The use of such data, however,
must be tempered by the knowledge that it is really valid only if the conditions
of use parallel those under which the measurements are made. The reality of sur-
factant life is that in many applications, such parallelism may not, in fact, apply.
