Chemistry Reference
In-Depth Information
4.3.3.1 transport of Ions in biological cell membranes
The most important biological cell-membrane function is the transport of ions
(such as Na, K, Li, Mg) through the hydrophobic lipid part of the bilipid membrane
(BLM) (Birdi, 1989, 1999). This property is related to many conditions, such as
infection, and thus to the activity of antibiotics. These complicated biological pro-
cesses have been studied using monolayer model systems. For instance, valinomy-
cin monolayers have been extensively investigated. They exhibited K-ion specificity
exactly as found in biological cells. It is well known that cell membranes inhibit the
free transport of ions (the alkyl chains of the lipids hinder such transport). However,
molecules such as valinomycin assist in specific ion (K-ion) transport through bind-
ing (Birdi, 1989).
4.4
eFFect oF lIPId monolayerS on
evaPoratIon rateS oF lIquIdS
In arid and semiarid areas, the amount of water lost from reservoirs by evapora-
tion frequently exceeds the amount beneficially used. Evaporation of water from
lakes and other reservoirs is a very important phenomenon in those parts of the
world where water is not readily available. Further, from ecological considerations
(rain and temperature), the evaporation phenomenon has much importance for global
temperature. Cloud formation depends on the evaporation of water from lakes and
oceans. Clouds are thus isolators for sunlight reaching Earth. This cycle is related to
the global heating process, and has been extensively discussed with regard to CO
2
and global warming. The reduction of even a part of the evaporation losses would
therefore be of incalculable value.
In general, the fall in water level would arise from the following parameters:
Fall in water level
+
rainfall
= evaporation + seepage + abstractions
At the air-water interface, water molecules are constantly evaporating and condens-
ing in a closed container. In an open container, water molecules at the surface will
desorb and diffuse into the gas phase. It is therefore important to determine the
effect of a monomolecular film of amphiphiles at the interface. The measurement of
the evaporation of water through monolayer films was found to be of considerable
interest in the study of methods for controlling evaporation from great lakes. Many
important atmospheric reactions involve interfacial interactions of gas molecules
(oxygen and different pollutants) with aqueous droplets of clouds and fog as well
as ocean surfaces. The presence of monolayer films would thus have an appreciable
effect on such mass transfer reactions.
In the original procedure for measuring the evaporation of water, the box contain-
ing the desiccant is placed over the water surface, and the amount of water sorbed is
determined by simply removing the box and weighing it (La Mer, 1962; Langmuir,
1943). The results are generally expressed in terms of specific evaporation resistance,
r
. The methods for calculating
r
from the water uptake values, together with the
assumptions involved, are described in detail in the aforementioned references. The
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