On physical adsorption XVII. Experimental verification of the two-dimensional van der Waals equation of state above and below the critical temperature

Abstract

Adsorption isotherms of carbon tetrachloride, chloroform and fiuorotrichloromethane on a substrate of graphitized carbon are reported at temperatures between 200 and 300 °K. Evidence is presented that at these temperatures the residual heterogeneity of the substrate is not observed: under these conditions the true equation of state of the adsorbed film can be deduced directly from the measured adsorption isotherms. All the data reported are described by the adsorption isotherm equation corresponding to a two-dimensional van der Waals gas; this description continues to apply at temperatures where the isotherms show discontinuities characteristic of first-order phase changes. The two-dimensional critical temperature of each of the adsorbed films is rather less than the value predicted by the two dimensional van der Waals equation; this is taken as evidence for the polarization of the adsorbate molecules by an electric field present at the graphite surface. The results obtained with the isotropic carbon tetrachloride molecule indicate a surface field of 1 x 10 5 e. s. u./cm 2 ; we deduce that the anisotropic adsorbates should be oriented at the interface, with the axis of the permanent dipole alined with the surface field.