phase (LE) occurs as monolayer compression begins and the tail starts to
lift away from the surface. The transition states form a plateau because the
existence of two phases gives the monolayer only one degree of freedom
and constant temperature gives constant pressure, P (Knobler, 1990).
The liquid-condensed phase (LC) forms with further compression. The
hydrocarbon chains are highly aligned and a high number of trans conformations appear, indicating the achievement of long-range order due to
van der Waals interactions. Finally, application of great external pressure
leads to the ordered solid (S) phase, and the area per molecule is comparable to the closely packed chains of the amphiphile’s 3D crystal.
Further compression leads to the collapse pressure, P c , where the molecules are ejected randomly throughout the film.
Molecular dimensions can be obtained through the P–A isotherm by
extrapolating the slope of the S phase to zero pressure to get the molecular
area per molecule at zero pressure, A 0 . A 0 can be compared to theoretical
values of the cross-sectional area of hydrocarbon chains, and a close
correlation indicates that a closely packed monolayer has been formed
with the hydrophobic tails oriented normal to the surface. Furthermore,
the P–A isotherm gives information on the stability of the monolayer at
high pressures.
Example 10.1 Determining the Limiting Surface
Area per Molecule
Use Figure 10.2 to determine the area per molecule for the
amphiphile described by the isotherm.
Solution By extrapolating the slope of the S phase to zero pressure (the dashed line in Figure 10.2), one obtains an area of 0.2 nm
2
for each molecule.
10.1.2 Langmuir–Blodgett films
The LB film is subsequently formed by the immersion of a substrate into
the water to break the Langmuir film and transfer the monolayer onto the
substrate. Additional immersions result in the fabrication of the multilayer film, whose thickness depends largely on the molecular chain length
and number of dippings. The LB deposition technique is illustrated in
Figure 10.3.
Deposition of tightly packed molecular monolayers with unilateral
molecular directionality normal to the substrate can be induced through
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
386
lift away from the surface. The transition states form a plateau because the
existence of two phases gives the monolayer only one degree of freedom
and constant temperature gives constant pressure, P (Knobler, 1990).
The liquid-condensed phase (LC) forms with further compression. The
hydrocarbon chains are highly aligned and a high number of trans conformations appear, indicating the achievement of long-range order due to
van der Waals interactions. Finally, application of great external pressure
leads to the ordered solid (S) phase, and the area per molecule is comparable to the closely packed chains of the amphiphile’s 3D crystal.
Further compression leads to the collapse pressure, P c , where the molecules are ejected randomly throughout the film.
Molecular dimensions can be obtained through the P–A isotherm by
extrapolating the slope of the S phase to zero pressure to get the molecular
area per molecule at zero pressure, A 0 . A 0 can be compared to theoretical
values of the cross-sectional area of hydrocarbon chains, and a close
correlation indicates that a closely packed monolayer has been formed
with the hydrophobic tails oriented normal to the surface. Furthermore,
the P–A isotherm gives information on the stability of the monolayer at
high pressures.
Example 10.1 Determining the Limiting Surface
Area per Molecule
Use Figure 10.2 to determine the area per molecule for the
amphiphile described by the isotherm.
Solution By extrapolating the slope of the S phase to zero pressure (the dashed line in Figure 10.2), one obtains an area of 0.2 nm
2
for each molecule.
10.1.2 Langmuir–Blodgett films
The LB film is subsequently formed by the immersion of a substrate into
the water to break the Langmuir film and transfer the monolayer onto the
substrate. Additional immersions result in the fabrication of the multilayer film, whose thickness depends largely on the molecular chain length
and number of dippings. The LB deposition technique is illustrated in
Figure 10.3.
Deposition of tightly packed molecular monolayers with unilateral
molecular directionality normal to the substrate can be induced through
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
386