Chemistry Reference
In-Depth Information
rates of evaporation are measured both without (Rw) and with (Rf) the film. The
resistance
r
is given as
r
= A (
vw
−
vd
) (Rf − Rw)
(4.34)
where A is the area of the dish, and
vw
and
vd
are the water vapor concentrations for
water and desiccant, respectively. The condensed monolayers gave a much higher
r
value than the expanded films, as expected.
A more advanced arrangement was described that made use of recording film
balances, in which the temperature difference between two cooled sheet metal
probes was measured. If both probes are over clean water and the rate of mois-
ture condensation is the same, then there is no difference in the temperature.
However, if monolayer is present, the retardation of evaporation gives rise to a
temperature difference.
Other techniques for the diffusion of gas through monolayers at the liquid inter-
face have also been investigated (Blank, 1970). In these methods, the differential
manometer system was used to measure the adsorption of gases such as CO
2
and O
2
into aqueous solutions with and without the presence of monolayers. The Geiger-
Mueller counter with a suitable sorbent and a radioactive tracer gas was used to
measure the reduction of evolution of H
2
S and CO
2
from the surface solution when
a monolayer was present.
Many decades ago, it was found that the evaporation rates of water were reduced
by the presence of monomolecular lipid film. This observation resulted in important
applications in reducing the loss of water from great lakes during summer in such
continents as Australia and Africa. The simplest method is to measure the loss of
weight of a water container with or without the presence of a monomolecular lipid
film. La Mer investigated the effect of long-chain films (C
14
H
29
OH, C
16
OH, C
18
OH,
C
20
OH, C
22
OH), and found that the resistance to evaporation increased with chain
length. For instance, the resistance increased by a factor 40 for C
14
OH to C
22
OH.
This indicates that evaporation takes place through the alkyl chains. Since the attrac-
tion between alkyl chains increases with the number of carbon atoms, the resistance
to evaporation also increases.
4.4.1 m
I x e d
m
of n of l a y e r S
o f
l
I p I d S
The spread mixed lipid monolayer studies provide information about the packing
and orientation of such molecules at the water interface. These interfacial character-
istics affect many other systems. For instance, mixed surfactants are used in froth
flotation. The monolayer surface pressure of a pure surfactant is measured after the
injection of the second surfactant. From the change in Π, the interaction mechanism
can be measured. The monolayer method has also been used as a model biological
membrane system. In the latter BLM, lipids are found to be mixed with other lipid-
like molecules (such as cholesterol). Hence, mixed monolayers of lipids + cholesterol
have been found to provide much useful information on BLM. The most important
BLM and temperature melting phenomena is the human body temperature regula-
tion. Normal body temperature is 37°C (98°F), at which all BLM function efficiently.
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