Equilibrium Film Pressure on a Flat, Low-Energy Solid

Introduction The equilibrium film pressure of an adsorbate, π e, is defined as π e = Y S - Y SV (1) where Y S is the surface free energy of the solid in vacuum, and Y SV is the surface free energy of the solid in contact with the saturated vapor of the adsorbate. The value of this property for an adsorbate which, as a liquid, forms a non-zero contact angle on the solid, has been a matter of uncertainty for some time ( 1 - 9 ). This fact has detracted from the usefulness of measurements of contact angle, θ, for the estimation of solid surface free energies ( 2, 3 ). See refs. ( 5 ) and ( 6 ) for an important discussion of the problem as it has existed in recent years. The measurement of π e is not particularly easy; and up to very recently ( 8 ) the only determinations that had been reported for low-energy solids were made on powders ( 4, 5, 6 ), while reported contact angle measurements were made