Preferential Adsorption from Binary Mixtures of Short Chain n-Alkanes; The Octane−Decane System

Abstract

The competitive adsorption of the alkanes n-octane and n-decane adsorbed from the binary liquid mixture onto graphite has been studied using differential scanning calorimetry (DSC), neutron diffraction, and incoherent quasi-elastic neutron Scattering (IQNS). A solid monolayer is identified at all bulk solution compositions with a melting temperature that varies with bulk composition in a manner resembling the bulk behavior. The IQNS data indicates 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 concentrations. When both alkanes are present on the surface, diffraction indicates that the surface phase is not a mixed crystal, but consists of regions of pure octane and regions of pure decane. The failure of these two compounds to mix on the surface is shown to have its origin in the structures of the monolayers of the pure components.