Investigation of the depth of preferential surface ordering in liquids: A photoelectron spectroscopic investigation of liquid mixtures
15 Jan 1997-Journal of Chemical Physics (American Institute of Physics)-Vol. 106, Iss: 3, pp 1231-1233
TL;DR: In this paper, 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 photoelectram mean free path.
Abstract: 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 A. This observation is in accordance with molecular dynamics simulations of long chain hydrocarbon liquids.
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TL;DR: 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 as discussed by the authors.
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 mo...
5 citations
TL;DR: In this article , the coordination structures and diffusion in PFPEs terminated with hydroxyl or carboxyl moieties and poly(ethylene oxide) (PEO) embedded with lithium and bis(trifluoromethylsulfonyl)imide ions were characterized using molecular dynamics.
Abstract: Perfluoropolyether (PFPE) polymers potentially improve the thermodynamic stability, compatibility, and ionic conductivity of sulfide-based solid-state electrolyte (SSE)/Li-metal interfaces. However, ion-transport kinetics of PFPE polymers remain inadequately understood. Using molecular dynamics, we characterized the coordination structures and diffusion in PFPEs terminated with hydroxyl or carboxyl moieties and poly(ethylene oxide) (PEO) embedded with lithium and bis(trifluoromethylsulfonyl)imide ions. The mechanism underlying Li+ diffusion in the PFPE polymers was found to differ from that in PEO, and their respective ion–polymer contact strengths and durations were analyzed. Furthermore, the terminal structure of PFPE polymers was shown to significantly alter the cation–polymer and anion–polymer coordination structures, which in turn accounts for the different extents of the Li+ coordination strength in PFPE-diol and PFPE-dimethylcarbonate (DMC). Finally, PFPE-DMC favors longer durations of contact with cations and shorter ones with anions, whereas this relationship is reversed in PFPE-diol. Thus, we provide a novel way to explain experimentally the observed differences in their ionic conductivities, and we illustrate a general principle for increasing Li+ diffusivity in PFPE polymers, providing an avenue for understanding ionic migration in PFPE–SSE composite electrolytes.
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References
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TL;DR: In this paper, the results of molecular dynamics simulations of the liquid-vapor interface of water were studied and the density profile of the interfacial transition region between liquid and vapor is monotone with a 10% to 90% width of 3.45 A, in good agreement with the value determined from x-ray reflectance measurements, namely 3.30 A.
Abstract: We report the results of molecular dynamics simulations of the liquid–vapor interface of water. Two different water–water interactions were studied. The density profile of the interfacial transition region between liquid and vapor is monotone with a 10%–90% width of 3.45 A, in good agreement with the value determined from x‐ray reflectance measurements, namely 3.30 A. The water molecules in the interface tend to lie with the HOH bisector in the plane of the surface and one OH bond pointing out of the surface. As the density falls in the transition region the tetrahedral structure of the bulk liquid breaks up and there is a tendency towards dimerization. The diffusion constant in the interfacial region is 58% larger than that in the bulk region.
125 citations
TL;DR: In this article, the inelastic mean-free pathlengths λ, of 1042 − 1402 eV electrons through cadmium arachidate (C19H39COO)2Cd, monolayers have been determined from attenuation measurements of the core level photoemission (XPS) signals from gold, silver and indium substrates.
Abstract: The inelastic mean‐free pathlengths λ, of 1042–1402 eV electrons through cadmium arachidate (C19H39COO)2Cd, monolayers have been determined from attenuation measurements of the core level photoemission (XPS) signals from gold, silver and indium substrates. Both fixed angle‐variable overlayer thickness experiments and variable angle‐fixed overlayer thickness experiments were performed. We derive a preferred value of 3.6 nm at 1117 eV and an E0.5 energy dependence over the narrow energy range considered. Values determined for bulk cadmium arachidate and arachidic acid from absolute intensity comparisons with gold and silver are close to the monolayer values. The results are discussed in the light of the controversy existing concerning other values of λ’s determined for organic and polymeric materials.
97 citations
TL;DR: In this paper, computer simulations with molecular dynamics technique were carried out to investigate properties of liquid-vapor interface of methanol for a wide temperature range of 160-350 K. The estimated surface excess thermodynamic quantities, especially surface entropy, are characteristic of strongly hydrogen bonding liquids, but the density profile resembles that of simple fluids.
Abstract: Computer simulations with molecular dynamics technique were carried out to investigate properties of liquid–vapor interface of methanol for a wide temperature range of 160–350 K. The estimated surface excess thermodynamic quantities, especially surface entropy, are characteristic of strongly hydrogen‐bonding liquids, but the density profile resembles that of simple fluids. Orientational structuring near the surface was also studied, and one apparent tendency, much stronger than that of water, was observed; the methanol molecule projects its methyl group toward the vapor phase. This orientational ordering can be understood as a result of putting a hydrophobic methyl group outside of liquid phase to maximize the number of hydrogen bonds near the surface for energetical stabilization. A considerable part of the thermodynamic anomaly is due to this orientational ordering. Since the electric dipole is almost parallel to the surface in this orientation, the calculated surface potential is quite small, about −30 mV at room temperature.
96 citations
TL;DR: In this paper, a molecular dynamics study of the planar vapour-liquid interface for mixtures of Ar and Kr modelled by truncated Lennard-Jones potentials at 115·77 K and at two compositions is presented.
Abstract: We report a molecular dynamics study of the planar vapour-liquid interface for mixtures of Ar and Kr modelled by truncated Lennard-Jones potentials at 115·77 K and at two compositions. The simulations yield the density profiles, surface tension, surface of tension, adsorption, and normal and transverse components of the pressure tensor. Both the Irving-Kirkwood (IK) and Harasima (H) forms of the pressure tensor are calculated. The values of the surface tension calculated by the thermodynamic and mechanical (for both the IK and H pressure tensors) routes are in agreement, but the IK and H pressure tensors yield different values for the surface of tension, as expected. These results are compared with predictions of the mean field theory (MFT) of the interface. The agreement is generally good, the principal differences being due to the fact that the MFT predicts too low a liquid density. The MFT is also used to predict properties of the mixture interface for the full Lennard-Jones potential. For low concentr...
80 citations
78 citations