92168.pdf

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,

Ca2+ 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