Chemistry Reference
In-Depth Information
ionic materials in the presence of a large excess of neutral electrolyte,
¼
1; for
fully ionized ionic surfactants,
2, since one counterion must be adsorbed for
each surfactant molecule, giving a total of two species.
Equation (3.20) shows that the effectiveness of a surfactant at lowering the
surface tension of a solution is related to three main factors: (1) the cmc of the sur-
factant; (2) the surfactant concentration required to attain the surface tension at
which Gibbs equation linearity begins, C
1
; and (3) the maximum surface excess
concentration of the surfactant,
¼
m
, at surface saturation. The effectiveness of a sur-
factant can be conveniently quantified by using a value of C
1
at which the surface
tension has been reduced by 20 mN/m, assuming
20
&
m
, so that the two con-
cepts of efficiency and effectiveness can be directly linked quantitatively.
Application of Eq. (3.20) allows for the calculation of a standard quantity,
cmc/C
20
, which serves as a useful measure of overall surfactant effectiveness.
Some representative values that illustrate the effects of well-controlled changes
in surfactant structure are given in Table 3.2.
It is often found that the efficiency and effectiveness of surfactants at lower-
ing the surface tension of a solution do not run parallel; in fact, it is commonly
observed that more efficient materials that produce significant lowering of the
surface tension at low concentrations will be less effective or will have a smaller
TABLE 3.2. Experimental Values of cmc/C
20
,
20
, and s
min
for Some Typical
Surfactants in Aqueous Solution
Temperature
20
s
min
(
10
10
mol/cm
2
)
(
C)
Surfactant
cmc/C
20
(mN/m)
C
12
H
25
SO
4
Na
þ
25
2.0
3.3
40.3
C
12
H
25
SO
3
Na
þ
25
2.3
2.9
40.8
C
12
H
25
SO
4
Na
þ
60
1.7
2.6
44.8
C
12
H
25
SO
3
Na
þ
60
1.9
2.5
43.9
C
16
H
33
SO
4
Na
þ
60
2.5
3.3
37.8
C
12
H
25
C
6
H
4
SO
3
Na
þ
70
1.3
3.7
47.0
p-C
12
H
25
C
6
H
4
SO
3
Na
þ
75
1.6
2.8
48.8
C
16
H
33
C
6
H
4
SO
3
Na
þ
70
1.9
1.9
45.0
C
12
H
25
C
5
H
5
N
þ
Br
30
2.1
2.8
42.8
C
14
H
29
C
5
H
5
N
þ
Br
30
2.2
2.8
41.8
C
12
H
25
N(CH
3
)
þ
Br
30
2.1
2.7
41.8
C
10
H
21
(POE)
6
OH
25
17.0
3.0
30.8
C
12
H
25
(POE)
6
OH
25
9.6
3.7
31.8
C
16
H
33
(POE)
6
OH
25
6.3
4.4
32.8
C
12
H
25
(POE)
9
OH
23
17.0
2.3
36.8
C
16
H
33
(POE)
9
OH
25
7.8
3.1
36.8
C
12
H
25
(POE)
12
OH
23
11.8
1.9
40.8
C
16
H
33
(POE)
12
OH
25
8.5
2.3
39.8
C
16
H
33
(POE)
15
OH
25
8.9
2.1
40.8
C
16
H
33
(POE)
12
OH
25
8.0
1.4
45.8
p,t-C
8
H
17
C
6
H
4
(POE)
7
OH
25
22.9
2.9
30.8
