Effects of chain branching on the monolayer structure of alkanes at interfaces: a neutron and helium atom scattering study

A study comparing the monolayer structures of intermediate-length alkanes adsorbed on various substrates using He-atom scattering (HAS) and neutron scattering (NS) reveals significant differences in the ordering of branched and unbranched molecules. HAS measurements of tetracosane (n-C24H50) adsorbed on hydrogen-covered Pt(111) and NS experiments with tetracosane on graphite basal-plane surfaces show the formation of well-ordered monolayers. For squalane monolayers, on the other hand, no diffraction peaks were observed by HAS for adsorption on hydrogen-covered Pt(111) and only a broad diffraction peak was found by NS for adsorption on graphite, corresponding to the nearest-neighbor separation in a direction perpendicular to the principal molecular axis. We conclude that unbranched alkanes form highly ordered monolayers at a close-packed interface whereas the translational order in monolayers of the branched squalane molecule is either shorter range or nonexistent. These results demonstrate the importance of molecular branching for the structure of these organic layers and may have implications for the interfacial ordering of alkane-based lubricants and coatings.