Adsorption from solution of chain molecules onto graphite: Evidence for lateral interactions in ordered monolayers

Abstract

Isotherms of the specific surface excess and of the enthalpy of displacement (enthalpy of adsorption from solution) are reported for dilute solutions of n-docosane (n-C₂₂H₄₆) in heptane onto graphitized carbon black, for tempertures from 15 to 45 °C.

As already noted by previous authors long-chain n-alkanes are strongly preferentially adsorbed onto the graphite basal plane. The present work shows that the isotherms for such a system exhibit a positive initial curvature and a point of inflection before approaching a plateau corresponding to a complete adsorbed monolayer. This deviation from ideal Langmuir-type behaviour is attributed to strong lateral interactions between long-chain molecules adsorbed side-by-side on the flat graphite basal plane. There are indications for a phase separation in the adsorbed layer at the lower temperatures, involving the formation of close-packed ordered arrays of n-docosane molecules on the surface. — The experimental results can be accounted for quantitatively by the parallel layer model of adsorption from non-athermal solutions of chain molecules.