The quantitative analysis of some IR absorption modes may become more
complex. In anisotropic films disorder is expressed by two factors:
(i) The conformational disorder introduced by the presence of gauche conformations in the chain and;
(ii) Molecular disorder caused by different orientations of adsorbed molecules.
Order parameter (S) represents both types of disorder in a molecular film. S was
described by Seelig and Seelig and it may be calculated for the asymmetric and
symmetric methylene stretching modes: [75, 76].
0.000
0.001
0.002
0.003
0.004
0.005
0.006
3050
3000
2950
2900
2850
0.000
0.002
0.004
0.006
0.008
0.010
FR
FR
as
(CH 3
)
as
(CH 2
)
s
(CH 3
)
lipid bilayer at the
air|gold interface
a)
as
(CH 3
)
random distribution
Wavenumber / cm
-1
0.000
0.001
0.002
0.003
0.004
0.005
0.006
3050
3000
2950
2900
2850
0.000
0.002
0.004
0.006
0.008
0.010
FR
FR
lipid bilayer at the
D 2 O|gold interface
b)
as
(CH 3
)
as
(CH 2
)
s
(CH 3
)
as
(CH 3
)
Wavenumber / cm
-1
random distribution
Fig. 2.20 PM IRRA spectra of POPE:Kdo2-lipidA bilayer recorded at the (a) air|gold and (b) D 2 O|
gold interfaces with corresponding calculated spectra of randomly distributed molecules in these
films. FR—Fermi resonance
Table 2.3 The integral intensities of the methylene stretching modes of the anisotropic
(Anisotropic)
exp
A
and isotropic A
Isotropic
ð
Þ
Reference
POPE:Kdo2-lipidA bilayers and the θ angles of the asymmetric and
symmetric methylene stretching modes in bilayers on the air|gold and D 2 O|gold interfaces
Experiment
ν s (CH 2 )
ν s (CH 2 )
A
Anisotropic
ð
Þ
exp
(cm
À1
)
A
Isotropic
ð
Þ
Reference
(cm
À1
)
θ νs(CH2)
(
)
A
Anisotropic
ð
Þ
exp
(cm
À1
)
A
Isotropic
ð
Þ
Reference
(cm
À1
)
θ νas
(CH2) (
)
Ex situ
0.123
0.142
57
0.036
0.060
63
In situ
0.045
0.134
70
0.008
0.043
76
40
2 Polarization Modulation Infrared Reflection Absorption Spectroscopy: From. . .