Chemistry Reference
In-Depth Information
TABLE 3.3. Effect of Polyoxyethylene and Hydrocarbon
Chain Length on the Efficiency and Effectiveness of
Surface Tension Lowering for a Number of POE Nonionic
Surfactants with the General Formula R(POE)
n
OH
m
s
min
(x 10
10
mol/cm
2
)
R
n
(mN/M)
C
6
6
2.7
32.8
C
10
6
3.0
30.8
C
12
6
3.7
31.8
C
16
6
4.4
32.8
7
3.8
33.8
9
3.1
36.8
12
2.3
39.8
15
2.1
40.8
21
1.4
45.8
p,t-C
8
H
17
C
6
H
4
7
2.9
30.8
8
2.6
32.8
9
2.5
34.3
10
2.2
35.8
(or surface pressure
p ¼ s
0
s
) is related to the orientation of the adsorbed
molecules at the interface, with maximum lowering resulting from an essentially
perpendicular orientation between hydrophobe and interface (Figure 3.11). For
soluble monolayers such as those in question here, that orientation will be directly
affected by the proximity of the neighboring molecules; thus factors that cause
an increase in molecular separation will also allow the adsorbed molecules to tilt
more relative to the surface, producing a smaller effective surface tension reduction
(Figure 3.11b).
Vapor phase
(a) (b)
Liquid phase
Figure 3.11.
The role of surfactant structure and packing in the determination of packing
efficiency and surface tension: (a) straight-chain hydrophobes, closest packing and maxi-
mum effectiveness—area per molecule determined by head group; (b) branched, unsym-
metric, substituted, and other types of tails reduce packing efficiency—area determined by
tail.
