Angle resolved X-ray photoelectron spectroscopic (XPS) studies of a series of perfluoropolyethers of general formula F[CF(CF3)CF2O]nCF2CF3, where n = 27, 65, and &gt;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. © 1998 Taylor &amp; Francis Group, LLC.

Pradeep Thalappil

Department Of Chemistry

V. Bindu

K. Smitha

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

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

Molecular Physics

Angle resolved x-ray photoelectron spectroscopic studies of a series of liquid mixtures of a perfluoropolyether (Krytox 16350) and a polyphenylether (Santovac-5) show that the former spreads on the latter and the film thickness could even be smaller than the photoelectron mean free path. The molecular ordering in krytox remain as that of the free liquid which is manifested in the enhancement of the relative intensity of specific features in the photoelectron spectrum as the electron take-off angle is decreased. The preferential ordering is limited to the very top and an upper limit of this thickness is estimated to be about 8 Å. This observation is in accordance with molecular dynamics simulations of long chain hydrocarbon liquids. © 1997 American Institute of Physics.

Fulltext

Journal of Chemical Physics

Investigation of the depth of preferential surface ordering in liquids: A photoelectron spectroscopic investigation of liquid mixtures

Angle resolved X-ray photoelectron spectroscopy from a liquid perfluoropolyether shows that its surface is primarily composed of chain ends oriented nearly perpendicular to the surface. Indirect evidence suggests that the ether oxygens may not be available for any of the surface processes. The results are in agreement with recent atomic beam scattering experiments and molecular dynamics simulations. © 1995 Elsevier Science B.V. All rights reserved.

Chemical Physics Letters

Chemical constitution of a perfluoropolyether liquid surface. A photoelectron spectroscopic study

The Journal of Chemical Physics

Preferential molecular ordering at the surface of a liquid perfluoropolyether revealed by x-ray photoelectron spectroscopy

Low-energy (10-90 eV) atomic ions of group IIIA, IVA, VA, VIA, and VIIA elements (E) undergo reactions with a fluorinated self-assembled monolayer surface to give fluoride cations, EFn+. One, two, or three fluorine atoms can be abstracted. Ion/surface reactions are also observed with polyatomic ions of these elements, but in general, atomic ions are much more reactive and react at lower collision energies than the corresponding polyatomic species. The higher collision energies reflect increased energy consumption needed for fragmentation. Most of the ion/surface reactions investigated in this study are endothermic and are driven by the translational energy of the projectile, although there remains a high degree of thermochemical control over reactivity. Thermochemical control over neutralization of the primary beam is also evident; ions with high recombination energies, like N+ and O+, completely neutralize at the fluorocarbon surface. In addition, certain general trends in behavior have been observed for elements within the same periodic group. The reactions occur in single scattering events, and they are not associated with electron transfer from the surface to the ion, as are the well-known hydrogen and alkyl group abstractions by organic radical ions. In most cases, the ion/surface reaction seems to occur after, or in concert with, dissociation of the polyatomic projectile. When multiple abstractions occur, the fluorine atoms can be lost from the same alkyl chain; evidence for this is the enhanced intensity of specific sputtering products, e.g. C3F3+, upon collisions of ions such as Sb+, which readily abstract more than one fluorine atom. Ion/surface reactions in which new bonds are formed in the surface alkyl group are also observed; such reactions give rise to unusual product ions which are sensitive to the chemical nature of the projectile. Examples include chlorine-for-fluorine atom substitution at the surface and PCF2+ formation in P+ collisions. These processes suggest the possibility of selective chemical modification of the outermost monolayers of surfaces using low-energy reactive ion beams. © 1994 American Chemical Society.

Journal of Physical Chemistry

Low-energy collisions of group IIIA, IVA, VA, VIA, and VIIA ions with fluoroalkyl SAM surfaces: Reactions, chemical sputtering, and mechanistic implications

Low-energy (&lt;100 eV) ion/surface reactions of metal ions, Ti∗+, Cr.+, Fe.+, Mo.+, and W.+, at a fluorinated self-assembled monolayer surface give fluorine-containing scattered ions, MFn+, n &lt; 5. The metal fluorides are the most abundant products in the scattered ion spectra. The single fluorine abstractions are all endothermic and driven by the projectile translational energy. Multiple fluorine abstraction occurs with high efficiency for Mo.+ and W.+ projectiles, where the thermochemistry is most favorable. Polyatomic projectile ions derived by partial fragmentation of the Cr, Mo, and W hexacarbonyls also yield metal fluoride scattered ions, in addition to more complex species formed by fluorine abstraction by ions which retain one or more carbon atoms or carbonyl groups. Fe- and Ti-containing projectile ions, bearing one or two cyclopentadienyl (Cp) groups, also exhibit fluorine abstraction products, with and without retention of a Cp group. TiCln+ (n = 1-4) projectiles exhibit fluorine abstraction, as well. Similar fluorine abstraction products are generated upon collisions at a liquid perfluorinated polyether surface. Gas-phase ion/molecule reactions and thermochemical considerations suggest that multiple fluorine abstractions can occur by a direct reaction mechanism within a single scattering event and that multiple fluorine atoms are probably derived from a single fluorocarbon chain. Angle-resolved scattering data support a single-collision, multiple-atom abstraction mechanism. The scattered ions leave the surface with very low translational energy and collision energy variations suggest that in these ion/surface reactions, projectile dissociation is concerted with or occurs prior to fluorine abstraction. There is no evidence that electron transfer to the surface is involved, as it is in alkyl group abstraction reactions by ions at hydrocarbon surfaces. © 1994, American Chemical Society. All rights reserved.

Journal	Molecular Physics
ISSN	00268976
Open Access	No