as micelles, which were briefly mentioned in Chapter 6. The shape and
size of these micelles depends on the structure of the surfactant. For
example, SDS forms spherical micelles; each micelle is 4 nm in diameter
and is composed of about 60 molecules. The number of molecules
making a micelle is known as the aggregation number.
The concentration at which a full monolayer is present at the interface
and micelles begin to form in the bulk solution is known as the critical
micelle concentration, or CMC. The process of micellization is in some
ways similar to precipitation, but the precipitate itself has a very narrow
size distribution and is stable and soluble in water. This property is
because the micelle structure is such that the hydrophobic chains are
aggregated in the core while the polar head groups form the exterior part
of the structure (Figure 7.14d). The delicate balance between inter-head
group interactions and the geometry of the surfactant molecule result in a
micelle with a particular shape, size, and aggregation number.
The CMC of a particular surfactant solution is very sensitive to impurities
and other physical conditions. For example, increasing the temperature
of an aqueous surfactant solution increases the CMC. Thermal agitation
makes it more difficult for the molecules to self-assemble into micelles, so
a higher concentration is required to reach the CMC. The addition of salt
decreases the CMC of ionic surfactant solutions because the added ions
screen the charged head groups of the surfactant, thus making it easier to
form micelles. As the chain length of the surfactant increases, the CMC
decreases due to the reduction in solubility of the surfactant. Table 7.2
lists the CMCs of some common surfactants.
In a nonpolar solvent, surfactant molecules can self-assemble to form
“reverse micelles.” In this situation, the molecules are aggregated in such
a way that the hydrophobic moieties form the exterior of the micelle
and the polar regions of molecules form the core of the micelle.
These micelles are thermodynamically stable in a nonpolar solvent such
as hexane.
Micelles in aqueous solution are capable of solubilizing small amounts of
oil added to the aqueous surfactant solution above its CMC. As a result,
the micelles swell with oil and increase in size (Figure 7.15). This
swollen micellar phase is thermodynamically stable and is known as a
microemulsion. In essence, tiny oil droplets are solubilized in water.
Microemulsion phases are generally made by putting an aqueous surfactant phase in contact with an oil phase. The two phases don’t mix
completely, but a small equilibrium amount of the oil will enter the
CHAPTER 7: Fundamentals of Surface Nanoscience
244
size of these micelles depends on the structure of the surfactant. For
example, SDS forms spherical micelles; each micelle is 4 nm in diameter
and is composed of about 60 molecules. The number of molecules
making a micelle is known as the aggregation number.
The concentration at which a full monolayer is present at the interface
and micelles begin to form in the bulk solution is known as the critical
micelle concentration, or CMC. The process of micellization is in some
ways similar to precipitation, but the precipitate itself has a very narrow
size distribution and is stable and soluble in water. This property is
because the micelle structure is such that the hydrophobic chains are
aggregated in the core while the polar head groups form the exterior part
of the structure (Figure 7.14d). The delicate balance between inter-head
group interactions and the geometry of the surfactant molecule result in a
micelle with a particular shape, size, and aggregation number.
The CMC of a particular surfactant solution is very sensitive to impurities
and other physical conditions. For example, increasing the temperature
of an aqueous surfactant solution increases the CMC. Thermal agitation
makes it more difficult for the molecules to self-assemble into micelles, so
a higher concentration is required to reach the CMC. The addition of salt
decreases the CMC of ionic surfactant solutions because the added ions
screen the charged head groups of the surfactant, thus making it easier to
form micelles. As the chain length of the surfactant increases, the CMC
decreases due to the reduction in solubility of the surfactant. Table 7.2
lists the CMCs of some common surfactants.
In a nonpolar solvent, surfactant molecules can self-assemble to form
“reverse micelles.” In this situation, the molecules are aggregated in such
a way that the hydrophobic moieties form the exterior of the micelle
and the polar regions of molecules form the core of the micelle.
These micelles are thermodynamically stable in a nonpolar solvent such
as hexane.
Micelles in aqueous solution are capable of solubilizing small amounts of
oil added to the aqueous surfactant solution above its CMC. As a result,
the micelles swell with oil and increase in size (Figure 7.15). This
swollen micellar phase is thermodynamically stable and is known as a
microemulsion. In essence, tiny oil droplets are solubilized in water.
Microemulsion phases are generally made by putting an aqueous surfactant phase in contact with an oil phase. The two phases don’t mix
completely, but a small equilibrium amount of the oil will enter the
CHAPTER 7: Fundamentals of Surface Nanoscience
244