Orientation of molecules at a liquid surface revealed by ion scattering and recoiling spectroscopy

Analysis of the kinetic energy of scattered inert gas ions and ejected recoil atoms reveals the identity of atoms in the topmost atomic surface layers of liquids. Spectra from bis(2-ethylhexyl)phthalate (DOP) and bis(2-ethylhexyl)chlorendate (BEHC), two molecules with similar `head–tailsʹ structures, show that the molecules have somewhat different surface orientations. Atom ratios derived from peak intensity ratios show that neither molecule has an exclusively `heads-upʹ orientation at the surface. A `head-downʹ orientation is favored for DOP. For BEHC, part of the ring (head) that includes 2–3 Cl atoms is at the surface.