Investigating adsorption of organic molecules at challenging metal/oil interfaces

Buried interfaces, pertinent to realistic environments such as those found in a car engine, are notoriously difficult to investigate due to the challenges of accessing information concerning the interface itself without being swamped by the much greater signal from the bulk materials; here, we describe the combination of a suite of sophisticated surface study techniques to characterise small molecules adsorbing at key metal surfaces from an oil phase. The metal surfaces themselves have been extensively investigated using such analytical techniques as X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) in order to establish the exact nature of the chemical groups present pre-adsorption.

Polarised neutron reflectometry has been used to quantify the thicknesses and compositions of the films formed when the metal surfaces are exposed to solutions of organic additives, such as long-chain fatty acids or amines. This has proven to be most powerful when used in conjunction with infrared-based surface techniques, most particularly the non-linear sum frequency generation spectroscopy (SFG) which gives information concerning the functional groups present uniquely at the interface, as well as the orientation and tilt of any adsorbed molecule, and overall order or disorder of the film packing. Depletion isotherms have also been used to substantiate these findings by providing the adsorption parameters and pointing to the likely mechanisms of adsorption (Langmuir, BET etc.).

Wood, M. H.; Casford, M. T.; Steitz, R.; Zarbakhsh, A.; Welbourn, R. J. L.; Clarke, S. M. Langmuir, 2016, 32, 534.