Chemistry Reference
In-Depth Information
4.5. EXPERIMENTAL OBSERVATIONS OF MICELLAR SYSTEMS
The preceding sections presented a brief review of some of the basic theoretical
concepts pertaining to the formation of surfactant micelles. The following sections
are devoted to the presentation of some experimental results from the literature that
illustrate many of the various effects of surfactant chemical structure and solution
environment on the aggregation process. While the previously described models of
micelle formation serve as a useful basis for the interpretation of the results given
below, it must be remembered that in all cases they represent a very simplified pic-
ture and involve a number of assumptions, many of which are not fully justified by
experiment.
4.5.1. Micellar Aggregation Numbers
Several references have been made to the number of surfactant monomers aggre-
gating to form a micelle—the aggregation number, n. The classical method for
determining n is to use elastic light scattering. It is possible to determine a
weight-average molecular weight (Mw) for a micellar solution, and therefore the
average number of surfactant molecules in the structure, from the intensity of
light scattered at a given angle at surfactant concentrations above the cmc relative
to that of the pure solvent (or solvent plus surfactant below the cmc). Newer tech-
niques such as laser light scattering and fluorescence quenching produce data that
allow for the determination of the aggregation number and the distribution of
micellar sizes, as well as giving some idea of their approximate shapes. Typical
aggregation numbers for various surfactant types are given in Table 4.4.
Because the size and size distribution of micelles are sensitive to many internal
(hydrophobic structure, head group type, etc.) and external (temperature, pressure,
TABLE 4.4. Aggregation Numbers for Some
Surfactants in Water
Temperature
Aggregation
Surfactant
(
C)
Number
C
10
H
21
SO
3
Na
þ
30
40
C
12
H
25
SO
3
Na
þ
40
54
(C
12
H
25
SO
3
)
2
Mg
2
þ
60
107
C
12
H
25
SO
4
Na
þ
23
71
C
14
H
29
SO
3
Na
þ
60
80
C
12
H
25
N(CH
3
)
3
þ
Br
23
50
C
8
H
17
O(CH
2
CH
2
O)
6
H
30
41
C
10
H
21
O(CH
2
CH
2
O)
6
H
35
260
C
12
H
25
O(CH
2
CH
2
O)
6
H
15
140
C
12
H
25
O(CH
2
CH
2
O)
6
H
25
400
C
12
H
25
O(CH
2
CH
2
O)
6
H
35
1400
C
14
H
29
O(CH
2
CH
2
O)
6
H
35
7500
