Study of a Fluid/Solid Interface over a Wide Density Range Including the Critical Region.: II. 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 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.