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The influence of fatty acid monolayers on the rate of evaporation of water has been studied by measuring the rate of absorption of water vapor by a solid desiccant supported above the water surface. The data are reported as the specific resistance of the monolayer to evaporation which is equal to the reciprocal of the specific rate constant for condensation through the film. These data establish a contamination effect upon which the retardation of evaporation is strongly de-pendent and which has introduced serious error in all previous work. Our results give a description, of the rate of evaporation through monolayers, which is fundamentally different from earlier work. The resistance to evaporation of monolayers of the four saturated fatty acids Cu, Cis, Cis and C20 was measured as a function of surface pressure, chain length, monolayer phase, subphase composition and surface temperature. In the liquid condensed phase, monolayer resistance is independent of surface pressure ana subphase pH; its logarithm is a linear function of chain length. On the other hand, in the solid phase on an alkaline subphase containing calcium ions, the logarithm of resistance is a linear function of both chain length and surface pressure. The logarithm of the resistance in the liquid condensed phase of Ci2 acid monolayers is a linear function of the reciprocal of the absolute temperature, which substantiates the description of evaporation resistance in terms of an exponential energy barrier. A theory is proposed for the source of the energy barrier; calculation of its magnitude from heats of vaporization data agrees well with the experimental values. All of the monolayers studied decrease the rate of evaporation of water by a factor of about 104.

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In a previous communication, the rate of evaporation was investigated by employing monolayers of the C1,, Clel CIO and C~O members of the saturated fatt acid series. The rate was independent of the surface pressure of these monolayers in the range 10-24 dyne/cm., provigd the film was spread initially under pressure to prevent the entrance of impurities. This finding has been confirmed using a similar technique. The present investigation extends the study to nine pure substances, esters, acids, alcohols and one fluorinated alcohol, The surface area, the resistance to eva oration and the surface vwcosity were measured for each substance as a function of the surface ressure. Certain of these &ms are highly compressible in the liquid phase, as compared to the incom ressible saturated z t t y acids. The compressible films are poor retardants of evaporation, whereas the incom ressible I l m s retard by a large factor. The s ecific conductances (reciprocal resistances to evaporation) are additive &r a mixed film composed of substances of s i d a r compressibility, for example, a 50-50 mol mixture of stearic and arachidic acids. A film composed of 80 mol arachidic acid (incompressible) and 20 mol ethyl palmitate (compressible) reduces the rate of evaporation. Additivity of the conductances is approached at very low pressures, whereas under high pressures the resistance approaches that of the incompressible component. No direct relationship was found between surface ressure, surface viscosity and s ecific resistance to evaporation. The correlation depends upon the compressibility of the Elm except for the fluorinated aEohol which is exceedingly viscous.