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COO
-
COO
-
COO
-
+
H
3
N
CH
+
H
3
N
CH
COO
-
+
H
3
N
CH
H
C H
3
CH
2
+
H
3
N
CH
CH
2
C
CH
2
CH
CH
CH
3
CH
3
H
3
C
H
3
C
CH
3
VAL
LEU
ILEU
PHE
(a)
COO
-
+
H
3
N
CH
COO
-
COO
-
C
CH
2
COO
-
+
H
3
N
CH
+
H
3
N
CH
CH
2
+
H
3
N
CH
CH
2
CH
2
C
CH
2
H
+
N
CH
2
CH
2
C
CH
2
CH
C
C
HC
H
NH
+
O
O
H
2
N
H
2
N
ASP
GLN
LYS
HIS
COO
-
+
H
3
N
CH
CH
2
CH
2
COO
-
GLU
(b)
FIGure 3.4
Polar (a) and apolar (b) amino acids.
Surfactants exhibit surface activity, which means these molecules will adsorb
preferentially at interfaces:
Air-water
Oil-water
Solid-water
The magnitude of surface tension is reduced since the hydrophobic alkyl chain or
group is energetically more attracted to the surface than being surrounded by water
molecules inside the bulk aqueous phase. Figure 3.5 shows the monolayer formation
of the surface-active substance at a high bulk concentration. Since the closely packed
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