Applications of Monolayers and Monolayer Concepts

At the surface of water, amphipathic molecules are oriented in such a way as to interact extensively, at least for ordinary surface concentrations. This results in the formation of the various two-dimensional condensed phases with the attendant effect on surface viscosity. In this section we consider some situations for which monolayers or the concepts involved in their discussion find application. 

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. 

EXAMPLE 7.3 Suppression of Evaporation by Monolayers. The rate of evaporation is quantified by a parameter called the transport resistance r. For water with octadecanol monolayers at surface pressures of 10, 20, 30, and 40 mN m -1, ris about 1, 2, 3, and 4 s cm 1, respectively. This resistance drops off rapidly at lower pressures and approaches 2 10 3 s cm -1 for pure water. By considering the rate of water uptake as a diffusion problem, suggest how these r values are calculated from data collected in an experiment like that described above. Use the fact that Mr is dimensionally equivalent to the diffusion coefficient D divided by a length. 

Solution To be collected by the desiccant, molecules evaporating from a surface of area A must diffuse across a gap of width Ax between the water surface and the desiccant. The gap contains the monolayer as well as the air space, so the diffusion coefficient used is an effective value rather than the actual D value for a homogeneous region. According to Equations (2.20) and (2.22), the rate at which the desiccant increases in weight is given by 

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