Selective Adsorption of Binary Mixtures on Gold: The Composition of the Adsorbate

The selective adsorption behavior of binary mixtures
of carbon
dioxide (CO2) with propane (C3H8)
or ethane (C2H6) on a gold-coated surface is
investigated using molecular dynamics simulations. These gold surfaces
mimic surfaces used in highly accurate gravimetric density and sorption
analyzers, where sorption affects density measurements, in particular
near saturation. Several mixture compositions are studied to characterize
how each component preferentially adsorbs onto the surface at different
temperatures. The results show that as the state of the bulk vapor
phase approaches the saturated-vapor line, the adsorbed phase becomes
increasingly dominated by a liquid-like film whose composition systematically
converges toward that of a saturated-liquid phase. Hence, near the
saturated-vapor phase boundary, the adsorbate composition closely
approaches that of the saturated liquid phase. This indicates that
the condensate forming on the surface effectively mirrors the equilibrium
liquid phase. This provides the physical basis for correction models
in gravimetric densimetry, validating the use of the saturated liquid
composition as an approximation for the adsorbate near the phase boundary.