Association, Adsorption, and Dielectric Constant

Dielectric constant of water vapor at pressures 3 to 20 mm and at temperatures 23 to 165°C.—Using a heterodyne beat method previously described, a study has been made of the variation of the dielectric constant with pressure and temperature. At room temperature, the curve connecting ($\epsilon -1\mathrm{}$) with pressure consists of two straight parts connected by a transition curve. This change of slope is less marked at higher temperatures and disappears at about 47°C. The rectilinear portion below the transition pressure corresponds to the normal behavior of water vapor. The abnormal behavior above the transition pressure cannot be explained satisfactorily by the association theory suggested by Jona in connection with his results. It is suggested, rather, that the anomaly is due to the formation of an adsorbed layer of water on the condenser plates. As the number of adsorbed molecules increases, the vapor pressure increases, approaching the ordinary vapor pressure as a limit. The final thickness of the layer at saturation pressure is of the order of the radius of molecular attraction and comes out of the order of several microns, corresponding to about 200 molecular layers. The experimental results are in general agreement with this theory.