DFT study of n-alkyl carboxylic acids on oxidized aluminum surfaces: From standalone molecules to self-assembled-monolayers

Abstract

We report on a systematic density-functional-theory (DFT) study of adsorption of n-alkyl carboxylic acids (CA) on oxidized aluminum surfaces, where we address the roles of the adsorption mode, molecular coverage, tilt angle of alkyl chain, and alkyl chain size—from CA-2 to CA-18, where the suffix represents the number of C atoms in the molecule—on the stability of the formed CA monolayers. Adsorption was modeled on two models of the oxidized aluminum surface: both consist of a hydroxylated ultrathin-oxide film on top of Al(111). The two models differ in OH coverage, types of adsorption sites, and their lateral distribution. Two different adsorption modes were considered: (i) plain adsorption mode, where the CA binds to the surface with H-bonds, and (ii) acid-base condensation adsorption mode, where CA replaces a surface OH group by forming a water molecule as a co-product. Carboxylates bind considerably stronger to the surface than intact CAs, but due to bond-breaking energy cost, the adsorption free energy of the condensation mode is by about 0.15 eV less exergonic than that of the plain mode. While on the fully hydroxylated surface we only identified monodentate bonded carboxylates, bidentates are by about 0.7 eV more stable than monodentates near OH vacancies. Molecular packing within the monolayer is driven by lateral interchain interactions and the optimum tilt angle is achieved when the interchain distances are similar to those in the polyethylene crystal. This implies that the tilt angle depends on the coverage, i.e., it decreases with increasing coverage. At full monolayer coverage, the tilt angles are found to be about 40°–50° vs. the surface-normal, depending on the oxide/hydroxide composition on Al, and do not depend on chain size for longer chains (e.g., for CA-8 and beyond). The adsorption energy can be decomposed into the headgroup–surface and lateral interchain components, the former being largely independent on the chain size and the latter being proportional to the number of C atoms in the chain. Consequently, the adsorption is stabilized by about 1 eV/molecule at full monolayer coverage as passing from CA-2 to CA-18. Finally, we propose a simplified way to calculate the vibrational contribution to the free energy of the adsorbed monolayers.