8.4. Self-assembly of amphiphiles[[Student version, January 17, 2003]] 279
Figure 8.5:(Sketch.) A micelle of sodium dodecyl sulfate (SDS). The micelle consists of 60 SDS molecules. The
hydrocarbon chains pack in the core at a uniform density roughly the same as that of liquid oil. [From Israelachvili,
1991.]
several square centimeters of water. In the same way, a small amount of detergent can clean up a
big oily mess by encapsulating oil into stable, hydrophilic droplets small enough to be flushed away
byrunning water.
8.4.2 Micelles self-assemble suddenly at a critical concentration
Amixture of stabilized oil droplets in water may be delicious or useful, but it hardly qualifies as
a“self-assembled” structure. The droplets come in many different sizes (that is, they arepoly-
disperse), and just generally have little structure. Can entropic forces drive the construction of
anything more closely resembling what we find in cells?
Toanswer the above question we begin with another: It may seem from Section 8.4.1 that
surfactant molecules inpurewater (or pure oil) would be stymied: With no interface to go to,
won’t they just have to accept the hydrophobic cost of exposing their tails to the surrounding
water?
Figure 8.5 shows that the answer to the second question is “no.” Surfactant molecules in solution
can assemble into amicelle,asphere consisting of a few dozen molecules. In this way the molecules
can present their nonpolar tails toeach other,and not to the surrounding water. This configuration
can be entropically favorable, even though by choosing to associate in this way each molecule loses
some of its freedom to be located anywhere, oriented in any way (see Section 7.5 on page 240).
Aremarkable feature of Figure 8.5 is that there is a definite “best” size for the resulting micellar
aggregate. If there were too many amphiphile molecules, then some would be completely in the
interior, where their polar heads would be cut off from the surrounding water. But with too few
amphiphiles (for example, just one molecule), the tails would not be effectively shielded. Thus
amphiphilic molecules can spontaneously self-assemble into objects of fixed, limited, molecular-
scale size. The chemical force driving the assembly is not the formation of covalent bonds, but