Molecular layers held together by non-covalent interactions on solid surfaces have gained increasing importance in recent times owing to their diverse commercial and academic applications and the ability to control behaviour by changing the molecular architecture of the adsorbing compounds. High-flux experiments on the powder diffraction beamline I11 at Diamond have enabled very accurate structural characterisation of a particular class of organic compounds, alkyl amides, adsorbed on the surface of graphite. The two-dimensional structures of these molecules reveal a close-packed hydrogen-bonded structure, similar to that seen in their bulk (3D) crystals, and the experiments on I11 have allowed the calculation of very accurate hydrogen bond geometries. This strong hydrogen bond arrangement imparts the layer with exceptional stability compared to other similar organic compounds.
Single molecular layers adsorbed from a liquid onto a solid surface underpin a wide variety of commercial and academic problems, from washing your hands to lubrication, and from mineral separation to food. Although only present as a monomolecular film, these layers can dominate the macroscopic behaviour, hence it is essential to understand their properties and behaviour. Although their importance has long been recognised, many of their features remain to be ascertained. Partly this is because there is not much material in a single monomolecular layer and they are ‘buried’ between two much larger bulk phases; hence any technique required to study the films needs to be very sensitive to the film of interest but insensitive to the bulk phases surrounding them.
Figure 1: A representative alkyl amide molecule: dodecanamide.
Researchers at the Department of Chemistry at the University of Cambridge have recently taken advantage of the high flux of the synchrotron X-rays at Diamond and elsewhere to probe the two-dimensional structure of molecules in these adsorbed layers. This structural information is of high precision, ‘atomic’ and is the key to understanding their behaviour. Beamline I11 at Diamond has recently been demonstrated to be ideal for these kinds of studies. The high intensity of X-rays at I11 is essential as it enables the observation of the very weak scattering from the adsorbed layers to be significant. In addition, I11 has recently installed a new position sensitive detector (PSD) which has been of crucial importance in the detection of the extremely weak signals from adsorbed monolayers in reasonable counting times because it can collect from many different angles simultaneously. The monolayer samples that are often investigated also have a preferred orientation (the crystallites tend to point the same way) which can be exploited with this PSD again enhancing collection times.
Figure 2: Experimental (red) and calculated (blue) diffraction patterns for the monolayers of unsaturated amides on graphite measured at I11.
One particularly important class of molecules recently studied are alkyl amides (Fig. 1) which have a hydrocarbon chain length of 6-12 carbons and a –CONH2 head group, adsorbed on the surface of graphite. These amides are commercially important as friction modifiers in the plastics industry [1]; however there is little understanding of how these molecules work at the interface. Amide monolayers have been studied using synchrotron X-ray diffraction [2], as illustrated in Fig. 2. These patterns have the characteristic saw-tooth line shapes of a 2D monolayer, which can be ‘solved’ for the 2D monolayer structure in an analogous fashion to 3D crystals. The result of this structural solution indicated that the amides form strong hydrogen-bonded dimers, with approximately linear hydrogen bonds. These dimers are in turn hydrogen-bonded into ‘chains’ with comparatively weaker hydrogen bonds but overall imparting the monolayer with exceptional stability. Indeed some diffraction evidence has indicated that even when only a fraction of a monolayer of amide is present, the layer can remain solid well above the temperature (> 50 oC) when the bulk has melted. Other recent experiments on the powder diffraction beamline I11 have helped solve the 2D structure of unsaturated amide molecules (amides which have a double bond in the alkyl chain). These more complex molecules are particularly significant as they are often used commercially. Fig. 3 illustrates the monolayer structures for some representative species and again highlights the formation of dimers and ‘chain’ like arrangements of the hydrogen bonded amide groups.
Figure 3: The two-dimensional crystal structures of unsaturated amides adsorbed on graphite showing the hydrogen-bonded dimers and chains of molecules.This short contribution has illustrated how powder diffraction at instruments such as I11 at Diamond can provide high level insight into the structures of physisorbed monolayers adsorbed from liquids and solutions and how that structural information provides key insight into their behaviour.
References
[1] M.X. Ramirez, K.B. Walters, and D.E. Hirt, Relationship between Erucamide Surface Concentration and Co-efficient of Friction of LLDPE film, J. Vinyl Additive Tech., 11: 9-12 (2005).
[2] T. Bhinde, S.M. Clarke, T.K. Phillips, T. Arnold, and J.E. Parker, Crystalline Structures of Alkylamide Monolayers Adsorbed on the Surface of Graphite. Langmuir, 26(11); 8201-8206 (2010).
Principal Publications and Authors
T. Bhinde, S.M. Clarke, T.K. Phillips, T. Arnold, and J.E. Parker, Crystalline Structures of Alkylamide Monolayers Adsorbed on the Surface of Graphite. Langmuir, 26(11): 8201-8206 (2010).
Funding Acknowledgements
The Nehru Trust for Cambridge University.
Research carried out at Diamond on I11, the ILL and at the Swiss Light Source on X04SA.
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