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.