Page 119 - Introduction to Colloid and Surface Chemistry

Liquid-gas and liquid-liquid interfaces 109

groups, fatty acid monolayers form gaseous or liquid-expanded films
at much lower temperatures. Dissolved electrolytes in the substrate
can also have a profound effect on the state of the film; for example,
Ca 2+ ions form insoluble calcium soaps with fatty acid films (unless
the pH is very low), thus making the film more condensed.

Evaporation through monolayers

Water conservation

The annual loss of water from hot country lakes and reservoirs due to
evaporation is usually about 3 m per year. This evaporation can be
reduced considerably by coating the water surface with an insoluble
monolayer; for example, a monolayer of cetyl alcohol can reduce the
rate of evaporation by as much as 40 per cent. Insoluble monolayers
also have the effect of damping out surface ripples.
To attain minimum permeability to evaporation, a close-packed
monolayer under a sufficient state of compression to squeeze out any
surface impurities is required. The monolayer must also be self-
healing in response to adverse meteorological conditions such as
wind, dust and rainfall, so that spreading ability is also needed. To
compromise between these requirements, commercial cetyl alcohol
(which also contains some steryl, myristyl and oleyl alcohol) has been
used successfully.
The monolayer can be spread initially either from solvent or as a
powder, the latter being preferred. Small rafts which allow monolayer
material to seep out slowly and replace losses are usually installed at
points on the water surface. Oxygen diffuses readily through
insoluble monolayers. The oxygen content of the underlying water is
somewhat less (80 per cent saturated rather than the normal 90 per
cent saturated), since the surface is more quiescent but this has no
adverse effect on life beneath the surface.

Monolayers on droplets
Evaporation from small water droplets used to bind fine dust in coal
mines is severe unless the surfaces of the droplets are covered with a
protective insoluble film. By previously dispersing a little cetyl
alcohol in the water the life of the droplets can be increased some