The surface excess amount of ethylene adsorbed on graphitised carbon black has been measured over a pressure range from 1 to 120 bar using a high pressure microbalance technique. The data span a range of temperatures from 263 to 323 K (0.93 ≤ T/Tc ≤ 1.14) and a range of bulk densities up to ρ/ρc=1.7. The results show the transition from the low‐temperature behavior (T < Tc) with multilayer adsorption on approaching the saturation pressure, to the situation above the critical temperature Tc when the surface excess isotherms Γσ(P) exhibit a pronounced maximum at a pressure Pmax > Pc. The dependence of Pmax on temperature, and the variation of Γσ with temperature along the critical isochore, is determined quantitatively. The surface excess is discussed in terms of an operationally defined exponential density profile of the fluid at the interface. The dependence of the decay length of this profile on the bulk density and on temperature is shown.

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A gravimetric method for the determination of surface excess isotherms of compressed gases on solid adsorbents is described. Buoyancy corrections are minimized by using a symmetric‐beam microbalance and by compensating the volume of the adsorbent. — Isotherms of argon and methane onto Graphon (a graphitized carbon black) are determined up to 150 bar at −20, 0, 25, and 50°C, corresponding to bulk densities up to the critical density for methane, and more than half the critical density for argon. Surface excess isotherms exhibit a maximum at about 0.4 to 0.5 of the critical density. The range of applicability of simple isotherm equations for higher gas pressures is discussed briefly.

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The thermodynamic properties of a solid/fluid interface are studied over a wide range of densities of the fluid around its critical temperature. Starting from experimental surface excess isotherms of the system ethylene/graphite the spreading pressure and the integral enthalpy of adsorption is derived. In the one-phase region of the fluid the spreading pressure increases with increasing density and with decreasing temperature; along the critical isochore it increases from the one-phase region into the two-phase region, with a discontinuity in slope at the critical point. The integral enthalpy of adsorption per unit area contains two terms: an adsorption term which is large at low densities and near the critical point, and a term related to the bulk density of the fluid which dominates at high densities when the surface excess concentration is small.Die thermodynamischen Eigenschaften der Grenzflache zwischen einem Festkorper und einer Fluiden werden uber einen weiten Dichtebereich in der Umgebung der kritischen Temperatur der Fluiden untersucht. Ausgehend von den experimentellen Adsorptionsdaten des Systems EthylenIGraphit werden der Spreitungsdruck und die integrale Adsorptionsenthalpie berechnet. Der Spreitungsdruck n nimmt im Einphasengebiet der Fluiden mit steigender Dichte und sinkender Temperatur zu; entlang der kritischen Isochore nimmt er beim Ubergang vom Einphasengebiet ins Zweiphasengebiet zu, wobei am kritischen Punkt eine Diskontinuitat der Steigung (Bn/BT), beobachtet wird. Die integrale Adsorptionsenthalpie bezogen auf die Flacheneinheit des Festkorpers besteht aus zwei Anteilen: einem Adsorptionsbeitrag, der bei geringen Dichten und in der Nahe des kritischen Punktes mafigebend ist; und einem Beitrag, der proportional zur Dichte der fluiden Phase ist und bei hohen Dichten iiberwiegt, wo die Grenzflacheniiberschufikonzentration der Fluiden gering ist.