Preferential adsorption from binary mixtures of short chain n-alkanes; theoctane-decane system

The competitive adsorption of the alkanes n-octane and n- decane adsorbed f rom the binary liquid mixture onto graphite has been studied using differen tial scanning calorimetry (DSC), neutron diffraction, and incoherent quasi- elastic neutron Scattering (IQNS). A solid monolayer is identified at all b ulk solution compositions with a melting temperature that varies with bulk composition in a manner resembling the bulk behavior. The IQNS data indicat es that decane is preferentially adsorbed onto the surface over most of the composition range, as expected. However, octane can coexist on the surface with decane and even becomes the principal component at high octane concen trations. When both alkanes are present on the surface, diffraction indicat es that the surface phase is not a mixed crystal, but consists of regions o f pure octane and regions of pure decane. The failure of these two compound s to mix on the surface is shown to have its origin in the structures of th e monolayers of the pure components.