Retardation of Evaporation by Monolayers

Monolayers can be used to retard evaporation (water reservoirs), a related effect is the use of an oil film to dampen waves (surface viscosity), and monolayer interfacial films are often the stabilizing agent in emulsions (lower y and increase i/ ). The first scientific studies of spread monolayers appear to have included those by Benjamin Franklin in 1774 and Agnes Pockels in 1891. Experimental methods for determining the retardation of evaporation by monolayers have been reviewed by Barnes [52]. 

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V. K. LaMer, ed.. Retardation of Evaporation by Monolayers Transport Processes, Academic Press, New York, 1962. 

G.T. Bames and V.K. La Mer, The evaporation resistances of monolayers of long-chain acids and alcohols and their mixtures, in V.K. La Mer (Ed.), Retardation of Evaporation by Monolayers Transport Processes, Academic Press, New York, 1962, pp. 9-33. 

One area in which monolayers have been successfully employed is the retardation of evaporation. Particularly in arid regions of the world, evaporation of water from lakes and reservoirs constitutes an enormous loss of a vital resource. Under some conditions the water level of such bodies may change as much as 1 ft per month due to evaporation. The usual unit for water reserves is the acre-foot, a volume of water covering an acre of surface to the depth of 1 ft. It equals about 1/3 million gallons for each acre of water surface. Considerable research has been conducted both in the laboratory and in the field on the effectiveness of insoluble monolayers in reducing evaporation. An American Chemical Society Symposium in 1960 dealt exclusively with this topic the proceedings of that symposium are given by LaMer (1962). 

Laboratory research in this area is conducted by suspending a porous box of desiccant very close to the surface of a film balance. The rate of water uptake is determined by weighing at various times. This way the retardation of evaporation may be measured as a function of film pressure and correlated with other properties of the monolayer determined by the same method. As might be expected, the resistance to evaporation that a monolayer provides is enhanced by those conditions that promote the most coherent films, most notably high film pressures and straight-chain compounds. To see how this is quantified, consider the Example 7.3. 

Under natural conditions, Ri may be reduced by wind, so that spread monolayers of hexadecanol or similar materials may significantly retard evaporation. In the last few years they have found commercial application in reducing evaporation from lakes and reservoirs in hot, arid regions where the amount of water lost by evaporation may be so great as to exceed the amount usefully used. Another application is to reduce the evaporation from heated swimming-pools here it is important to save the latent heat of evaporation rather than the water. 

The spreading of an insoluble monolayer is a process analogous to adsorption with a number of specialized applications. Thus, cetyl alcohol is spread as a monoloaycr on reservoirs to retard the evaporation of water. Some antifoaming agents act by spreading as monolaveis. 

Hexadecyl and octadecyl alcohol have been extensively studied and shown to be highly effective in evaporation retardation. Scattering powdered samples of commercial-grade alcohols by boat on lake surfaces or the continuous addition of alcohol slurries from floating dispensers are two of the methods that have been employed to apply these monolayers. Wind conditions and the activity of aquatic birds have a considerable effect on the stability of the monolayer and therefore on the rate at which the monolayer chemicals must be reapplied. Rates of application rarely exceed 0.5 lb acre- day-1, however, so that the cost of the materials used is not excessive. 

On the more technical side, monolayers have been used to retard evaporation and in this way help to conserve water in arid regions, particularly in the dry season where otherwise several tens of a cm could be lost by evaporation. 

The study of surface films of the Langmuir type covers an extremely diverse group of phenomena. Measurements of film viscosity, diffusion on the surface, diffusion through the surface film, surface potentials, the spreading of monolayers, and chemical reactions in monolayers are just a few of the topics that have been studied. One interesting application is the use of long-chain alcohols to retard evaporation from reservoirs and thus conserve water. The phrase to pour oil on the troubled waters reflects the ability of a mono-molecular film to damp out ripples, apparently by distributing the force of the wind more evenly. There are also several different types of surface films only the simplest was discussed in this section. 

Hexadecanol and octadecanol have been used extensively in the southwestern United States to produce a monolayer on the top of lakes and other nonflowing bodies of water to retard evaporation. These compounds also have potential agricultural applications (Taylor et al., 1964). Germination of bluegrass seed was increased by application of these materials to soil (Atsatt and Bliss, 1963). 

Monolayers demonstrate well how interactions on a molecular level affect macroscopic phenomena. For instance, a monolayer of oil on the surface of an ocean damps the waves, and precious water supplies in ponds and lakes may be conserved by a monolayer that retards evaporation of the water underneath. 

In 1765, Benjamin Franklin made a variety of experiments on the spreading of oils on the pond in Clapham Common, London. Franklin estimated the thickness of the oil layers to be about 25 A. Many more recent scientists have tried to use similar layers of fatty acids and alcohols to retard evaporation from ponds and reservoirs in arid regions. The monolayers used today usually are characterized by a resistance around 2 sec/cm. Assuming that they are the thickness of Franklin s layer and that they can dissolve up to 1.8% water, estimate the diffusion coefficient across the monolayers. Answer 7 10 cm /sec. 

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