Adsorption of carboxylic acids and other chain molecules from n-heptane onto graphite

Abstract

The adsorption from dilute solutions of a series of carboxylic acids (decanoic, dodecanoic, tetradecanoic, and octadecanoic) and of other chain molecules (1-decanol, 1-bromo-octadecane) from n-heptane onto graphitised carbon black has been studied by measuring (i) the surface excess amount and (ii) the enthalpy of displacement.All solute molecules are adsorbed with their major axis parallel to the graphite basal plane and exhibit a tendency to form close-packed monolayers like the higher n-alkanes. 1-Decanol forms a close-packed monolayer phase already at low surface concentrations; the stability of this 2D-phase is attributed to chain association via hydrogen bonds. Carboxylic acids are adsorbed a scyclic dimers and form close-packed 2D arrays only at higher surface concentrations and lower temperatures; this is attributed to a decreased lateral interaction caused by the bulky (COOH)₂ groups. The contribution of lateral solute-solute interaction to the molar enthalpy of displacement is estimated from the dependency of this quantity on the surface concentration of solute. For the carboxylic acids this contribution amounts to roughly one half of the molar enthalpy of freezing.