(Dis)Similarities of adsorption of diverse functional groups over alumina and hematite depending on the surface state

To accelerate the conversion to more sustainable lubricants, there is a need for an improved understanding of the adsorption at the solid/liquid interface. As a first step, the density functional theory computed adsorption energies can be used to screen the ability of additives to cover a surface. Analogously to what has been found in catalysis with the universal scaling relations, we investigate here if a general universal ranking of additives can be found, independently of the surface considered. We divided our set of 25 diverse representative molecules into aprotic and protic molecules. We compared their adsorption over alumina and hematite, which are models of surface oxidized aluminum and steel, respectively. The adsorption energy ranking of our set is not strongly affected by alumina hydration. In contrast, adsorption on hematite is more strongly affected by hydration since all exposed Fe Lewis acid sites are converted into hydroxylated Brønsted basic sites. However, the ranking obtained on hydrated hematite is close to the one obtained on dry alumina, paving the road to a fast screening of additives. In our library, protic molecules are more strongly adsorbed than non-protic molecules. In particular, methyl and dimethyl phosphates are the most strongly adsorbed ones, followed by N-methyldiethanolamine, succinimide, and ethanoic acid. Additives combining these functional groups are expected to strongly adsorb at the solid/liquid interface and, therefore, likely to be relevant components of lubricant formulations.