The amphiphilic nature of surfactants is the reason why these molecules
self-assemble into nanostructures, both as micelles and microemulsions,
and as monolayers. The ability to adsorb at interfaces and lower surface
tension is the main reason why surfactants are used in almost all detergent and cleaning products. Microemulsions, with their ability to solubilize oil, have shown promise in tertiary oil recovery. Furthermore,
manipulating the molecular structure while still preserving the amphiphilic nature allows the possibility of introducing a desirable functionality
into the surfactant molecules and then exploiting its ability to selfassemble into novel materials.
7.3.2 The determination of surface excess: The CMC
and the cross-sectional area per molecule
Surface tension measurements of aqueous amphiphilic solutions allow
one to determine the bulk concentration at which a saturated monolayer
is reached. This concentration also corresponds to the CMC if a surfactant
is used. Furthermore, one can use surface tension measurements to
determine the cross-sectional area occupied by each molecule on the
surface. Let’s first consider a series of aqueous solutions of the cationic
surfactant cetyltrimethylammonium bromide [(C 16 H 33 )N(CH 3 ) 3 Br], or
CTAB. CTAB is an effective antiseptic agent against bacteria and fungi and
is used in hair conditioning products. After measuring the concentration
of each solution, we can generate a surface tension versus concentration
plot, as shown in Figure 7.16. The surface tension values drop from pure
water (~72 mN m
−1 ) to ~40 mN m
−1 when the bulk aqueous concentration
of CTAB is 1 mM. The decrease in surface tension in this range is due to
the increase in packing density as the bulk concentration is increased.
Beyond 1 mM, the surface tension remains constant at 40 mN m
−1 , which
indicates that the packing density of CTAB molecules at the air–water
interface is not increasing as the bulk concentration increases. At 1 mM
and beyond, the surface has been saturated with CTAB molecules and a
close-packed, highly oriented monolayer is present on the surface.
What happens to the excess surfactant molecules in the bulk phase when
the concentration exceeds that required to form a complete monolayer on
the surface (1 mM in the case of CTAB)? They are unable to remain as
individual molecules in solution and instead self-assemble into nanoscale
micelles. This is shown in Figure 7.16, where a break point in a plot of the
surface tension versus surfactant concentration marks the CMC. The
CMC of an aqueous solution of CTAB is 1 mM.
CHAPTER 7: Fundamentals of Surface Nanoscience
246
self-assemble into nanostructures, both as micelles and microemulsions,
and as monolayers. The ability to adsorb at interfaces and lower surface
tension is the main reason why surfactants are used in almost all detergent and cleaning products. Microemulsions, with their ability to solubilize oil, have shown promise in tertiary oil recovery. Furthermore,
manipulating the molecular structure while still preserving the amphiphilic nature allows the possibility of introducing a desirable functionality
into the surfactant molecules and then exploiting its ability to selfassemble into novel materials.
7.3.2 The determination of surface excess: The CMC
and the cross-sectional area per molecule
Surface tension measurements of aqueous amphiphilic solutions allow
one to determine the bulk concentration at which a saturated monolayer
is reached. This concentration also corresponds to the CMC if a surfactant
is used. Furthermore, one can use surface tension measurements to
determine the cross-sectional area occupied by each molecule on the
surface. Let’s first consider a series of aqueous solutions of the cationic
surfactant cetyltrimethylammonium bromide [(C 16 H 33 )N(CH 3 ) 3 Br], or
CTAB. CTAB is an effective antiseptic agent against bacteria and fungi and
is used in hair conditioning products. After measuring the concentration
of each solution, we can generate a surface tension versus concentration
plot, as shown in Figure 7.16. The surface tension values drop from pure
water (~72 mN m
−1 ) to ~40 mN m
−1 when the bulk aqueous concentration
of CTAB is 1 mM. The decrease in surface tension in this range is due to
the increase in packing density as the bulk concentration is increased.
Beyond 1 mM, the surface tension remains constant at 40 mN m
−1 , which
indicates that the packing density of CTAB molecules at the air–water
interface is not increasing as the bulk concentration increases. At 1 mM
and beyond, the surface has been saturated with CTAB molecules and a
close-packed, highly oriented monolayer is present on the surface.
What happens to the excess surfactant molecules in the bulk phase when
the concentration exceeds that required to form a complete monolayer on
the surface (1 mM in the case of CTAB)? They are unable to remain as
individual molecules in solution and instead self-assemble into nanoscale
micelles. This is shown in Figure 7.16, where a break point in a plot of the
surface tension versus surfactant concentration marks the CMC. The
CMC of an aqueous solution of CTAB is 1 mM.
CHAPTER 7: Fundamentals of Surface Nanoscience
246