Chemistry Reference
In-Depth Information
evidence to confirm the occurrence of premicellar aggregation as a general rule in
conventional aqueous surfactant solutions.
The discussions of micelle formation given in Chapter 4 indicated that surfactant
properties such as the cmc and aggregation number can be reasonably well corre-
lated with the size and nature of the hydrophobic group. In each case, as the hydro-
phobic group increases in size for a given head group, the cmc decreases and the
aggregation number increases within an homologous series. No comparable rela-
tionships have so far been determined that can accurately relate surfactant structure
and solubilizing power, mainly because the structure of the additive can play such
an important role in the overall aggregation process.
As already mentioned, hydrocarbons and polar organic compounds with low
water solubility are usually found to be solubilized in the interior of the micelle
or deep within the palisades layer. It has generally been shown that, not surpris-
ingly, the amount of such materials solubilized increases as the size of the micelle
increases. As a result, any of the factors discussed in Chapter 4 that cause an
increase in micelle size might also be expected to increase the solubilizing
power of the system. An increase in the length of the hydrocarbon chain, for exam-
ple, leads to a lower cmc and larger aggregation number, so that more of a nonpolar
additive can be incorporated into the micellar core per mole of surfactant in the
system (Table 6.1).
In a study of the solubilization of ethylbenzene in a series of potassium carbox-
ylates ranging from C
8
to C
16
, it was found that as the concentration of surfactant
increased, the amount of ethylbenzene solubilized increased, and that as the length
of the carbon chain increased, the quantity of material incorporated per mole of sur-
factant increased with the carbon chain length. Such results have been criticized
because of the assumption that the activity of the monomeric surfactant remained
TABLE 6.1. Effects of Surfactant Hydrocarbon Chain Length
on cmc and Solubilization of Ethylbenzene in Potassium Soap
Solutions
a
Surfactant (M)
cmc (M)
Solubilized
C
7
H
15
COO
(0.48)
0.66
0.141
(0.83)
0.152
C
9
H
19
COO
(0.44)
0.17
0.197
(0.72)
0.233
C
11
H
23
COO
(0.20)
0.046
0.364
(0.50)
0.407
(0.86)
0.522
C
13
H
27
COO
(0.24)
0.012
0.745
(0.50)
0.866
(0.57)
0.888
C
15
H
31
COO
(0.15)
0.0032
1.15
(0.29)
1.48
a
Moles solubilized per mole of soap in micelles.
