Functional group position directs interfacial adsorption and molecular assembly: A structure-interface relationship established from isomeric fatty acid/Ester pairs.

HYPOTHESIS: Functional group position may represent a long-overlooked structural parameter governing interfacial adsorption behavior of functional molecules. For isomeric saturated fatty acids and esters, terminal functional groups are expected to facilitate higher reactivity, directional anchoring, and compact assembly. In contrast, mid-chain functional groups tend to reduce reactivity and distort molecular conformation, leading to distinctly different adsorbed film structures and interfacial functions.

EXPERIMENTS: Arachidic acid (terminal carboxyl) and decyl decanoate (mid-chain ester) were selected as model isomers. A multi-scale quantitative framework integrating quantum chemical calculations, all-atom molecular dynamics simulations, neutron reflectometry, quartz crystal microbalance with dissipation monitoring, electrochemical impedance spectroscopy, water contact angle measurements, and tribological tests was established to correlate molecular structure with interfacial nanostructure and macroscopic lubrication performance.

FINDINGS: Arachidic acid exhibits a narrower energy gap and higher reactivity, forming an adsorbed film with 77.2% adsorption density and 2.44 nm thickness on iron surfaces, while decyl decanoate shows only 60.2% and 1.81 nm. These differences originate from functional group position: terminal carboxyl directs upright, rigid assembly, whereas mid-chain ester induces V-shaped folding and disordered packing. The ordered film of arachidic acid can be transformed into superior lubrication during friction. Thus, placing functional group at the molecular terminus is key to superior interfacial adsorption and lubrication. This work not only establishes a functional group position-regulated structure-interface relationship for functional molecules, laying theoretical foundation for the rational design of high-performance interfacial molecules, but also provides a generalizable multi-scale framework for study of molecular interfacial adsorption in interface science.