The development of a method of measuring the resistance of monomolecular films to the evaporation of water
Penley, H. H.
Measurements of the rates of evaporation of water through thin films have been made by Rideal(1) and Langmuir(2). To find the effect of the film alone, the usual method adopted is to measure the rate of evaporation from a pure water surface and then to compare this with the rate from film covered surface. In Rideal's experiments one arm A of a U shaped vessel contained the water to be evaporated. This could be maintained at a constant temperature by means of a thermostat. The other arm B was kept at 0° C. so that after the vessel had been exhausted of air, water from the arm A distilled into the arm B. The rate of evaporation was determined by weighing the cold arm. The experimental rate of evaporation found in this way may be compared with the theoretical rate deduced from the kinetic theory of gases. … Rideal found that the experimental rate of evaporation was only about 0.4 % of that predicted by the kinetic theory and that the presence of a thin film of fatty acid on the water surface reduced the rate of evaporation still further. His experimental arrangements were however by no means ideal and the film resistance deduced therefrom is very unreliable. This will be clear at once when it is noted that he assumed that the pressure above the liquid surface was equal to the saturated vapour pressure at the temperature of the cold arm. Since the pressure above the evaporating surface must be greater than that in the cold arm to which the vapour is flowing, the assumption leads to a value of m which is too large. Rideal takes the temperature of the water surface as being equal to the temperature of the thermostat. Since for rapid evaporation the temperature of the water surface must fall considerably below that of the thermostat this also leads to a value of m which is too large. For the reasons given above it is clear that Rideal’s method is not suitable for the measurement of film resistance and the present work is an attempt to obtain a method which will be more reliable. Numbers in brackets refer to bibliography.