Evaporation through surface films

Evaporation through surface films. Rideal1 has shown that surface films, both expanded and condensed, retard the rate of evaporation of water considerably. This retardation is not, however, at all easy to observe since the natural evaporation of water is already enormously retarded by the resistance caused by the slow diffusion of the evaporated molecules away from the surface. The real rate of evaporation can only be observed in a perfect vacuum Rideal obtained, by evacuating to just above the pressure at which water boils, a rate from clean water surfaces about 0 4 per cent, of the theoretical rate into a vacuum. Under these conditions, both expanded and condensed films appreciably lowered the rate, the maximum diminution being about 50 per cent. 

Langmuir2 has used the reciprocal of the rate of evaporation as a measure of the resistance to evaporation, and considers the total resistance as the sum of the several partial resistances due to the water itself (a nearly negligible resistance), the film, and the vapour above. That of the water itself was taken as the reciprocal of the theoretical rate of evaporation, calculated from the vapour pressure as in Chap. I, 6. 

Using RideaFs measurements, he found the part of the resistance due to diffusion in the partially evacuated atmosphere to be about 770 units 2 X sec  

The effect of surface films on the evaporation of ether from its aqueous solutions was very marked, but was traced to the film stopping the convection currents, which usually stir up the layers near the surface and eliminate the slow process of diffusion of ether through the water to the surface. There was no evidence of any considerable resistance, offered by the film itself, to the passage of ether molecules. 

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Warehouse drying is normally accomplished by providing an intermediate packaging that breaths. In the case of a rubber bale, the packaging may consist of polyethylene perforated film wrap that enables the surface moisture to evaporate through the film wrap and achieve an acceptable moisture spec (typically less than 0.5 weight percent for products like EPDM or butyl rubber). 

The surface film or sprea ding pressure, FI, is used to account for the change in gas—soHd interaction caused by adsorption of vapor evaporated from the Hquid. A Hquid is called wetting if the contact angle from soHd to Hquid through gas, 65 0 nonwetting if > 90°. Because it is easier to... 

When a polymer film is exposed to a gas or vapour at one side and to vacuum or low pressure at the other, the mechanism generally accepted for the penetrant transport is an activated solution-diffusion model. The gas dissolved in the film surface diffuses through the film by a series of activated steps and evaporates at the lower pressure side. It is clear that both solubility and diffusivity are involved and that the polymer molecular and morphological features will affect the penetrant transport behaviour. Some of the chemical and morphological modification that have been observed for some epoxy-water systems to induce changes of the solubility and diffusivity will be briefly reviewed. 

At the air-water interface, water molecules are constantly evaporating and condensing in a closed container. In an open container, water molecules at the surface will desorb and diffuse into the gas phase. It is therefore important to determine the effect of a monomolecular film of amphiphiles at the interface. The measurement of the evaporation of water through monolayer films was found to be of considerable interest in the study of methods for controlling evaporation from great lakes. Many important atmospheric reactions involve interfacial interactions of gas molecules (oxygen and different pollutants) with aqueous droplets of clouds and fog as well as ocean surfaces. The presence of monolayer films would thus have an appreciable effect on such mass transfer reactions. 

The rates of evaporation of water through unimolecular fi has been examined by Hedestrom and by the writer. It is foi that the rate of evaporation of water through unimolecular fi of fatty acids on the surface of water is relatively slow in compari to the high rate of evaporation calculated with the aid of Herz-Knudsen equation. The rate is approximately inveri proportional to the surface film pressure F as is indicated fi the following figures. 

There are three basic types of devolatilization equipment that have been used for the commercial manufacture of polystyrene wiped film evaporators, devolatilizing extruders and flash evaporators. In wiped film evaporators, the polymer solution is fed into a vessel under vacuum. The solution is moved into thin films along the vessel walls by a set of rotating blades. These blades continue to move the polymer through the vessel while continually renewing the surface area. The tank walls are heated to supply the required energy for devolatilization. These units are typically mounted vertically with the polymer solution fed at the top. At the bottom is a melt pool where a gear pump transfers the melt to the next unit operation, typically pelletization. 

When clear films are applied and dried under humid conditions, frequently a whitish haze called blushing appears in the final film. An electron micrograph of a blushed film is shown in Figure 3 (38). The blushing was caused by the condensation of moisture droplets upon the film in humid air when the film became cold because of solvent evaporation. Liquid lacquer with a relatively low surface tension spontaneously flowed over the water droplets with a higher surface tension, the lacquer then formed a film, and the incompatible encapsulated water droplets eventually evaporated through a partially ruptured film. 

In the preceding section, the case of evaporation of thin films in annular flow was discussed. In this case, the vapor had a dominant role in determining the flow of the film through its influence on the interfacial shear stress. However, there are many situations of industrial importance in which a liquid film is present that falls down the heat transfer surface under the influence of gravity. Here, we can distinguish two cases ... 

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. 

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