the monolayer represents a thermodynamically favorable situation: polar
head groups are buried in the aqueous phase and the hydrophobic tails
are avoiding contact with water. This scenario explains why surfactants
lower the surface tension of the air–water interface. It is easy to see that
surface tension decreases as the surfactant concentration increases, but
then levels off to a constant value when a saturated monolayer is formed.
In Chapter 8, we see how the surface tensions of surfactant solutions are
measured.
7.3.1 Micelle and microemulsion formation
What happens if we continue to increase the surfactant concentration in
an aqueous solution beyond that required to form a monolayer? The
molecules are unable to adsorb to the air–water interface. Instead, they
self-assemble in the bulk phase into nanostructures or aggregates known
Air−water
interface
(a)
(b)
Increasing surfactant concentration
(c)
(d)
Figure 7.14 Surfactant molecules in the bulk aqueous
phase are in equilibrium with a
monolayer at the air–water
interface. The population of
the molecules increases with
increasing bulk concentration
(a)–(c). At the CMC (d) a saturated monolayer is formed and
micelles are present in the bulk
phase.
SURFACTANT CHEMISTRY 243