on- THE ISFLUENCE
OF THIN SURFACE FILMS ON THE EVAPORATION OF WATER ERIC K. RIDEAL
In a recent communication Hedestrandl, as a result of a series of experiments, arrives at the conclusion tha,t a unimolecular film of oil molecules on the surface of mater is without influence on the rate of evaporation of Ihe water from the surface. This conclusion being a t variance with eome experimental results obtained some time ago, it was considered of interest to repeat and enlarge upon these preliminary experiments. As a result it may be definitely stated that the rate of evaporation of water from a surface is very considerably diminished by the presence of a unimolecular film of fatty acid upon the surface and that lhis diminution in rate is materially affected by the compression or surface concentration of the film. Hedestrand has noticed that, in the air-streaming method employed by him, a stagnant layer of air and vapour may be assumed to exist in close proximity to the liquid surface, through which layer the water vapour has to diffuse before removal by the air current. He assumes in addition, however, that the concentration gradient of water vapour in this staanant layer is always sufficiently small so that any real change in the rate of evaporation of t'he water from the liquid surface would be detected by a change in the rate a t which water vapour would be swept from the surface of the stagnant layer by the air stream. A simple calculation, however, indicates that this is by no means the case. When equilibrium between a water surface and the vapour above it has been established, the rates of evaporation and condensation are equal. On the assumption that there is no reflection of water molecules striking the surface from the gas phase the rate of condensation, and thus the rate of evaporation may readily be calculated from the effusion equation of Herz and Knudsen.
where p is rate of evaporation in gm. mols. per sq. cm. per second. P the vapour pressure, M the molecular weight and T the absolute temperature. Hedestrand employed a water surface of 18.25 sq.cm. at a temperature of zo°C. R e should thus anticipate, according to the above considerations, an evaporation rate of no less than 4.032 gms. per second; his observed maximum rate a t which the water was swept from the quiescent layer never exceeded 0.331 gm. per hour or 0 . 0 0 2 3 ~ of~the true rate of evaporation. Evidently the number of molecules of water vapour swept away from the stagnant layer per J. Phys. Chern. 38, 1244 (1924).
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