The amphiphilic nature of surfactant molecules gives them some interesting properties in water. The hydrophobic region is insoluble and the
surfactant molecules are forced to accumulate on the surface and expose
these nonpolar chains away from the water and toward the air. As
Figure 7.14 illustrates, the number density of the molecules at the surface
increases with bulk concentration. As with other adsorbates on surfaces,
each surfactant has a concentration for which no more surfactant molecules can pack at the interface and a saturated monolayer is formed. The
packing density of surfactant molecules within the monolayer will depend
on the intermolecular interactions between neighboring head groups and
the hydrophobic interactions between the tail groups. The formation of
Hydrophobic chain
O
–
Na
+
O
S
O
O
Anionic(SDS)
N(CH 3 ) 2 Br
–
O
–
Cationic (DDAB)
+
Zwitterionic
(DDAPS)
N
OS
O
+
+
Nonionic(C 12 E 3 )
+
Cationic Germini surfactant
O
O
P
O
O
O
O
O
O
N
Hydrophilic head
[–O(CH 2 ) 2 ] 3 OH
Br
–
(CH 3 ) 2 NCH 2
CH 2 N(CH 3 ) 2 Br
–
–
Zwitterionic
(DMPC)
+
C n H 2n+1
C n H 2n+1
Figure 7.13 The molecular
structure of some common surfactants, with the hydrophobic
and hydrophilic moieties indicated. Note that the hydrocarbon chains are shown in
all-trans conformation. This conformation is rarely adopted
in micellar structures or in
adsorbed surfactant films at
interfaces. The surfactants
shown are sodium dodecylsulfate (SDS), didodecyldimethylammonium bromide
N,N-dimethyl-3-ammonio-1proponate-sulfonate (DDAPS),
nonionic C 12 E 3 , cationic. Gemini
surfactant, and the zwitterionic
lipid molecule 1,2-dimyristoylsn-glycero-3-phosphocholine
(DMPC).
CHAPTER 7: Fundamentals of Surface Nanoscience
242