Chemistry Reference
In-Depth Information
multilayers from the solid substrate was used. Multilayers of arachidic acid and octa-
decylamine were analyzed with respect to Cd
++
and HPO
4
−.
Interest in the dielectric studies of deposited LB films of fatty acids and their
metal salts was one of the parameters of main investigation in the early stages of
research on LB films, for example, capacitance, resistance, and dielectric constant.
In early investigations, measurements on impedance of films and related phenomena
were carried on Cu- and Ba-stearate, and Ca-stearate using both X- and Y-type films.
Initially, Hg droplets were used for small-area probe measurements and an AC bridge
was used for impedance measurements. The resistance of the films was found to be
very low (<1 Ω), with high signal voltages, whereas it was of the order of megaohms
with signals of 1 or 2 V. The results were not satisfactory, and later measurements
were carried out after replacing the AC bridge by a radiofrequency bridge. The mea-
surements at frequencies of 1 MHz and 0.244 MHz and for films containing 7-41
deposited monolayers determined dielectric constant values ranging between 1.9 and
3.5 with an average value of 2.5. In both types of films, the capacity decreased with
thickness, as can be expected from the following relation:
Capacitance of the deposited LB films:
C
=
A
ε/4 π
N d
(4.35)
where
C
is the capacity, ε denotes the dielectric constant,
N
is the number of layers,
d
the layer thickness, and
A
the area of contact between drop and film. On the other
hand, the values of the resistance per layer showed a definite increase with the thick-
ness of the film. The specific resistance of the films, thus determined from their values
of the resistance per layer, was ca. 10
13
ohms. This was based on the results of capaci-
tance measurements on some 75 samples. The measurements thus performed on stear-
ate films (1-10 layers) led to ε values between 2.1 and 4.2, with a bulk value of 2.5.
In many of the measurements reported in the literature, the organic film was sand-
wiched between evaporated aluminum electrodes. The fact that an oxide layer grows
on the base of the aluminum electrode was present, and its effect on the capacitance
values of the device was neglected considering that the resistivity of oxide film is
small compared to that of the organic LB layers. The presence of such a thin oxide
layer between metal electrodes and fatty acids can be analyzed, and the capacitance
has been reported to be a linear function of [1/C] with respect to the number of
transferred monolayers (Figure 4.12). LB films of Ba-salts of fatty acids deposited at
Π = 50 mN/m (Birdi, 2002a) gave the following relation between [1/C] and
N
:
Ba-stearate:
1/C = 15.9
N
+ 1.13, [10
6
F
−1
]
(4.36)
Ba-behenate:
1/C = 17.2
N
+ 8, [10
6
F
−1
]
(4.37)
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