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 a dsorbed on various substrates using He-atom scattering (HAS) and neutron sc attering (NS) reveals significant differences in the ordering of branched a nd unbranched molecules. HAS measurements of tetracosane (n-C24H50) adsorbe d on hydrogen-covered Pt(1 1 1) and NS experiments with tetracosane on grap hite basal-plane surfaces show the formation of well-ordered monolayers. Fo r squalane monolayers, on the other hand, no diffraction peaks were observe d by HAS for adsorption on hydrogen-covered Pt(1 1 1) and only a broad diff raction peak was found by NS for adsorption on graphite, corresponding to t he nearest-neighbor separation in a direction perpendicular to the principa l molecular axis. We conclude that unbranched alkanes form highly ordered m onolayers at a close-packed interface whereas the translational order in mo nolayers of the branched squalane molecule is either shorter range or nonex istent. These results demonstrate the importance of molecular branching for the structure of these organic layers and may have implications for the in terfacial ordering of alkane-based lubricants and coatings. (C) 2001 Elsevi er Science B.V. All rights reserved.