STUDY OF A FLUID-SOLID INTERFACE OVER A WIDE DENSITY RANGE INCLUDING THE CRITICAL REGION .2. SPREADING PRESSURE AND ENTHALPY OF ADSORPTION OF ETHYLENE-GRAPHITE

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 ehtylene/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.