Adsorption of Surfactants on alpha-Fe2O3(0001): A Density Functional Theory Study

From corrosion inhibition to lubrication, a detailed understanding of the interactions between surfactants and iron oxide surfaces is critical for a range of industrial applications. However, there is still limited understanding of this behavior at the atomic-level, which hinders the design of improved surfactant molecules. In this study, the adsorption of three surfactants which are commonly employed as lubricant additives (carboxylic acid, amide, monoglyceride) on a alpha-Fe2O3(0001) surface is studied with density functional theory. The nature and strength of the adsorption for the different surfactants, as well as their propensity to deprotonate on the surface, is studied at a range of surface coverages. In agreement with the available experiments, strong chemisorption on alpha-Fe2O3(0001) is observed for all cases considered. Dissociation is energetically favorable for carboxylic acid and glyceride surfactants through the formation of a surface hydroxyl group, whereas this is not the case for amides. Glycerides form the most strongly adsorbed films at both low and high surface coverage due to the presence of multiple functional groups, which can all act as binding sites. However, the large size of the glyceride headgroup also means that adsorption is stronger at low coverage, where the formation of multiple bonds with the surface is possible, than at high coverage. Conversely, carboxylic acid films have similar stability at low and high coverage, where van der Waals forces between proximal tailgroups stabilize the adsorption structures. The results of this study provide atomic-level insights which help to explain friction results from previous macroscopic tribology experiments and classical molecular dynamics simulations. They also facilitate the molecular design of new surfactants to maximize the adsorption energy, surface coverage, and ultimately friction reduction on iron oxide surfaces.