Selective Adsorption of n‐Fatty Acids (C4–C12–C18) at the Alumina/n‐Heptane Interface: Adsorption Isotherms and Heats of Displacement

Abstract

AbstractThe isotherms and the heats of selective adsorption of n‐fatty acids (C4, C12, C18) at the alumina/n‐heptane interface have been determined at 313 K. All adsorption isotherms show stepwise trend of the L‐4 type isotherm according to Gile's classification. This behaviour has been interpreted assuming progressive rearrangement in the adsorbed layer with partial formation of dimers of n‐fatty acid molecules in correspondence of high surface coverage. – As the monocarboxylic acid orientation is concerned limiting adsorption from n‐heptane is consistent with the hypothesis of the surface structure made up of symmetric bidentate carboxylate ions, with the molecular symmetry axis rotating away from the perpendicular, orientation. – Studying the effects of the sample outgassing temperature upon the differential heats of adsorption, one may assume that: At T ≤ 573 K the differential heats are typical of hydrogen bonding between fatty acids' carboxylic group and isolated surface hydroxyls groups. At higher outgassing temperatures the heats of selective adsorption are representative of dissociative chemisorption on c.u.s. sites. In correspondence of higher surface coverages aspecific Van der Waals interactions become prevalent. – The differential heats of adsorption for all η‐ and α‐alumina samples decrease as the chain length increases. – α‐Al2O3 differential heats are smaller than η‐Al2O3 adsorption heats.