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 article, Monte Carlo and molecular dynamic simulations of the surface of a system of Lennard-Jones (12, 6) molecules have been carried out at temperatures which span most of the liquid range, showing that the density profile, as a function of height, falls monotonically from the density of the bulk liquid to that of gas.
Abstract: The gas–liquid surface of a system of Lennard-Jones (12, 6) molecules has been simulated by Monte Carlo and by Molecular Dynamic methods at temperatures which span most of the liquid range. For systems of 255 molecules the two methods lead to similar results and this agreement confirms that the density profile, as a function of height, falls monotonically from the density of the bulk liquid to that of the gas. The thickness of the surface layer is sensitive to the surface area, and appears to approach its thermodynamic limit for surface areas of 400σ2 for a system of 4080 molecules. The density profile can be represented by a hyperbolic tangent of an appropriately scaled height. The thickness of the surface is of the order of two molecular diameters at temperatures near the triple point and increases rapidly as the critical point is approached. The computed surfacetens ions agree well with those calculated by statistical perturbation theory.Monte Carlo and Molecular Dynamic simulation of a binary mixture shows clearly the adsorption of the component of higher vapour pressure; the amount absorbed agrees with that calculated from Gibbs's isotherm.
330 citations
TL;DR: In this paper, the authors defined the concept of absolute halfcell emf as VMS−φM for the reaction M→M+(solution)+e−(M), where VMS is the electrostatic potential difference between metal electrode and solution and φM the work function of metal in contact with solution.
Abstract: The concept of absolute half‐cell emf is discussed and defined as VMS‐φM for the reaction M→M+(solution)+e−(M), where VMS is the electrostatic potential difference between metal electrode and solution and φM the work function of metal in contact with solution. It is shown that this quantity is equal to VRS‐φR, where VRS is the electrostatic potential difference between a reference electrode in air above the solution and the solution, and φR the work function in air of this reference. The quantity VRS′, the potential difference between reference electrode and the solution surface, was found experimentally by the vibrating condenser method for a number of half‐cells, and φR was determined photoelectrically. It is shown from the variation of VRS′ with electrolyte concentration that the potential difference betwen the bulk of pure H2O and its air interface is ∼0.05 V, the surface being negative relative to bulk, and that this potential is increasingly screened out as electrolyte concentration increases. From ...
274 citations
TL;DR: In this paper, the authors used molecular dynamics simulations to predict the equilibrium liquid-vapor interface structure and surface tension of n-alkanes decane and eicosanes, C 10 H 22 and C 20 H 42, respectively.
Abstract: Molecular dynamics simulations are used to predict the equilibrium liquid-vapor interface structure and surface tension of n-alkanes decane and eicosanes, C 10 H 22 and C 20 H 42 , respectively. The model treats each methyl and methylene group as one united Lennard-Jones atom. Realistic bond lengths, bond angle potentials, and torsional potentials are included in the simulation. The simulations predict that although the total mass density profile is monotonic, the density profile of the chain centers of mass and central segments is strongly peaked. The outer edge of the surface is dominated by chain ends
193 citations
TL;DR: This is the first vibrational spectrum ever observed from a neat liquid surface and it is concluded that the surface methanol molecules are polar oriented with the CH{sub 3} group pointing away from the liquid with a very broad orientational distribution.
Abstract: Using infrared-visible sum-frequency generation we have obtained the vibrational spectra of CH stretches of methanol molecules at the interface between methanol vapor and liquid. This is the first vibrational spectrum ever observed from a neat liquid surface. The measured polarization dependence and phase of the nonlinear susceptibility allow us to conclude that the surface methanol molecules are polar oriented with the ${\mathrm{CH}}_{3}$ group pointing away from the liquid with a very broad orientational distribution.
169 citations
TL;DR: In this article, the authors measured the mean free path of poly(p-xylylene) polymers as a function of the kinetic energy of the poly(poly(p)-polymer.
Abstract: Electron mean free paths as a function of kinetic energy have been measured by the substrate overlayer technique for in situ-polymerized films of poly(p-xylylene) and the monochloro- and monobromo-substituted derivatives. The results are compared with previous estimates of mean free paths available in the literature for organic materials. Comparison is also drawn with corresponding experimental data for typical metals and semiconductors, and it is shown that organic polymers fit into a consistent picture which may be rationalized on the basis of existing theory. For electrons of kinetic energy ∼969 eV, ∼1170 eV, 1202 eV, and 1403 eV, mean free paths of ∼14 A, ∼22 A, ∼23 A, and ∼29 A, respectively, are obtained for the poly(p-xylylene) polymer films studied in this work.
136 citations