Chemistry Reference
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β = X(R
2
Me)/(XR
−
)
2
(M
2
+(Me))
(4.39)
where R− = RCOO-. Furthermore, the following can be derived from these:
β = [X(R
2
Me) X
2
(H
+
)]/(X
2
(RH) X (Me
2
+) Kd
2
)
(4.40)
where K
d
= [X(R
−
) X(H
+
)]/X(RH)
The foregoing expression represents the Langmuir adsorption isotherms, in which
the Me
2+
ion occupies two adsorption sites, the number of the lattice sites being
dependent on the subphase pH. The interactions in the monolayer and the subphase
are neglected. The magnitude of X (R
2
Me) can be estimated from the experimental
known ratio (Me/RCOOH).
The data for various binding constants have been reported in the literature. These
data were found to be in good agreement with other literature studies.
4.6.4 p
h y S I c a l
p
r o p e r T I e S
o f
lb f
I l m S
The surface property of a solid changes even after one layer of lipid is formed as an
LB film, such as, the contact angle decreases after LB deposition. Similarly, many
other physical methods have shown that LB films change the surface characteristics
(Gaines, 1966; Adamson and Gast, 1997; Birdi, 2002a).
Fourier transform infrared (FTIR)-attenuated total reflection (ATR) spectra have
been measured for LB films of stearic acid deposited on a germanium plate with
1,2,3,5, and 9 monolayers. In this study, special attention was given to obtaining
accurate and unequivocal examinations of changes in structure, subcell packing, and
molecular orientation as a function of the number of monolayers deposited. C=O
stretching band at 1702 cm
−1
was missing for the monolayer. The intensity increased
linearly for multilayer samples. The CH scissoring band at 1468 cm
−1
appeared as
a singlet in the case of the 1 monolayer. A doublet at 1473 cm
−1
and 1465 cm
−1
was
observed for films containing more than 3 monolayers. Band progression due to
-CH
2
- wagging vibration of the trans-zigzag hydrocarbon chains is known to appear
between 1400 cm
−1
and 1180 cm
−1
. The intensities increased in this region with the
number of layers. Monolayers of a surface-active dye (1-methyl-1′-octyadecyl-2,2′
cyanine perchlorate, S-120) incorporated into inert matrix material (e.g., methyl
arachidate + arachidic acid) were transferred onto structurally defined silver films
by the LB method. The dye-containing monolayers were spaced from the surface by
accurately known increments by the deposition of inert spacer monolayers.
Surface-enhanced resonance Raman spectra were observed from dye molecules
spaced as distant as six spacer increments (ca. 16 nm = 16 Å) from the silver sur-
face. These studies suggested that an electromagnetic mechanism is operative in
this assembly in contradistinction to a chemical mechanism that would require
direct contact between the Raman-active species and the metal surface. These
studies are of relevance in the study of chromophoric species in biological mem-
branes (e.g., enzymes, redox proteins, and chlorophylls).
In order to compare the monolayer properties at the air-water interface and the
spectral characteristics in the visible range, purple membrane films were transferred
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