Tilted orientation of molecular chains at the surface of a series of perfluoropolyether liquids: an X-ray photoelectron spectroscopic investigation

Abstract

Angle resolved X-ray photoelectron spectroscopic (XPS) studies of a series of perfluoropolyethers of general formula F[CF(CF₃)CF₂O]_n CF₂CF₃, where n = 27, 65, and >70, show that there is preferential ordering of-CF₃ groups at the liquid surfaces. The C 1s intensity corresponding to the -CF₃ groups increases with decrease in the electron take-off angle. No measurable change in the oxygen or fluorine intensity is observed upon varying the electron take-off angle. However, experiments show that no adsorption site exists at the surfaces, indicating that the ether oxygens are not available for any of the surface processes. Relative enhancement of the C 1s intensity of the -CF₂/CF₃ group with decreasing take-off angle is different for different liquids, with smaller chain length liquids showing a more rapid change. This is explained as due to an increase in helicity and tilt of molecular chains from the surface normal. Computational studies have been performed at the semiempirical level to understand the molecular structure. XPS data along with the computational studies suggest that at the liquid-air interface the molecular chains are ordered nearly perpendicular to the surface with a tilt which increases with chain length. The conclusions are in agreement with recent atomic and molecular beam scattering, ion/surface scattering and theoretical modelling studies.