Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. II. Dynamics

doi:10.1063/1.2464092 (17 pages)The dynamics of monolayer films of the n-alkane tetracosane (n-C24H52) and the branched alkane squalane (C30H62) adsorbed on graphite have been studied by quasielastic and inelastic neutron scattering and molecular dynamics (MD) simulations. Both molecules have 24 carbon atoms along their carbon backbone, and squalane has an additional six methyl side groups symmetrically placed along its length. The authors' principal objective has been to determine the influence of the side groups on the dynamics of the squalane monolayer and thereby assess its potential as a nanoscale lubricant. To investigate the dynamics of these monolayers they used both the disk chopper spectrometer (DCS) and the high flux backscattering spectrometer (HFBS) at the National Institute of Standards and Technology. These instruments made it possible to study dynamical processes such as molecular diffusive motions and vibrations on very different time scales: 1-40--ps (DCS) and 0.1-4--ns (HFBS). The MD simulations were done on corresponding time scales and were used to interpret the neutron spectra. The authors found that the dynamics of the two monolayers are qualitatively similar on the respective time scales and that there are only small quantitative differences that can be understood in terms of the different masses and moments of inertia of the two molecules. In the course of this study, the authors developed a procedure to separate out the low-frequency vibrational modes in the spectra, thereby facilitating an analysis of the quasielastic scattering. They conclude that there are no major differences in the monolayer dynamics caused by intramolecular branching. It remains to be seen whether this similarity in monolayer dynamics also holds for the lubricating properties of these molecules in confined geometries.This work was supported by the U.S. National Science Foundation under Grant Nos. DMR-0109057 and DMR-0411748 and by the U.S. Department of Energy through Grant No. DE-FG02-01ER45912. The neutron scattering facilities in this work are supported in part by the National Science Foundation under Agreement No. DMR-0454672. One of the authors (A.D.E.) thanks the Oticon Foundation, Denmark for financial support

Vibrational Study of 13C-enriched C60 Crystals

The infrared (IR) spectrum of solid C60 exhibits many weak vibrational modes. Symmetry breaking due to 13C isotopes provides a possible route for optically activating IR-silent vibrational modes. Experimental spectra and a semi-empirical theory on natural abundance and 13C-enriched single crystals of C60 are presented. By comparing the experimental results with the theoretical results, we exclude this isotopic activation mechanism from the explanation for weakly active fundamentals in the spectra.Comment: Accepted for Phys. Rev. B, typeset in REVTEX v3.0 in LaTeX. Postscript file including figures is available at http://insti.physics.sunysb.edu/~mmartin/papers/c13twocol2.ps File with figures will be e-mailed by reques

Observation and Assignment of Silent and Higher Order Vibrations in the Infrared Transmission of C60 Crystals

We report the measurement of infrared transmission of large C60 single crystals. The spectra exhibit a very rich structure with over 180 vibrational absorptions visible in the 100 - 4000 cm-1 range. Many silent modes are observed to have become weakly IR-active. We also observe a large number of higher order combination modes. The temperature (77K - 300K) and pressure (0 - 25KBar) dependencies of these modes were measured and are presented. Careful analysis of the IR spectra in conjunction with Raman scattering data showing second order modes and neutron scattering data, allow the selection of the 46 vibrational modes C60. We are able to fit *all* of the first and second order data seen in the present IR spectra and the previously published Raman data (~300 lines total), using these 46 modes and their group theory allowed second order combinations.Comment: REVTEX v3.0 in LaTeX. 12 pages. 8 Figures by request. c60lon

Reconstruction of pre-Illinoian ice margins and glaciotectonic structures from airborne ElectroMagnetic (AEM) surveys at the western limit of Laurentide glaciation, Midcontinent U.S.A.

Early and early Middle Pleistocene glaciations in midcontinental USA are poorly understood relative to more recent Illinoian and Wisconsinan glaciations, largely because pre-Illinoian glacial landforms and deposits are eroded and buried. In this paper, we present a new interpretation of buried, pre-Illinoian glacial features along the Laurentide glacial margin in northeastern Nebraska using Airborne ElectroMagnetics (AEM) supplemented with borehole logs and 2 m LiDAR elevation data. We detect and map large-scale (101–102 km) geological features using contrasts in electrical resistivity. The Laurentide glacial limit is marked by a continuous (>120 km) contrast between conductive (40 Ω-m) sandy sediments. Several smaller (102 km2) till salients extend 10s of km westward of this margin. We recognize a lithologically heterogeneous zone characterized by variable resistivity and complex geophysical structures extending as much as 17 km west of the glacial limit. This zone is interpreted as a glaciotectonic thrust complex, and it is analogous to a similar thrust complex in Denmark where structural analysis of co-located seismic and AEM surveys provides a standard for comparison. Our study suggests that the maximum advancement of pre-Illinoian glacial ice into Nebraska involved extensive deformation of sedimentary strata, local overriding of these deformed strata by smaller ice tongues, and emplacement of tills as much as 30 km west of the principal Laurentide ice margin. These insights provide the first glimpse of the large-scale stratigraphic architecture of glacial sediments in Nebraska and point to future clarifications of the geology and geomorphology of the Laurentide glacial limit

Evidence for life in the isotopic analysis of surface sulphates in the Haughton impact structure, and potential applications on Mars

The analysis of sulphur isotopic compositions in three sets of surface sulphate samples from the soil zone in the Haughton impact structure shows that they are distinct. They include surface gypsum crusts remobilized from the pre-impact gypsum bedrock (mean δ34S +31‰), efflorescent copiapite and fibroferrite associated with hydrothermal marcasite (mean δ34S −37‰), and gypsum-iron oxide crusts representing weathering of pyritic crater-fill sediments (mean δ34S +7‰). Their different compositions reflect different histories of sulphur cycling. Two of the three sulphates have isotopically light (low δ34S) compositions compared with the gypsum bedrock (mean δ34S +31‰), reflecting derivation by weathering of sulphides (three sets of pyrite/marcasite samples with mean δ34S of −41, −20 and −8‰), which had in turn been precipitated by microbial sulphate reduction. Thus, even in the absence of the parent sulphides due to surface oxidation, evidence of life would be preserved. This indicates that on Mars, where surface oxidation may rule out sampling of sulphides during robotic exploration, but where sulphates are widespread, sulphur isotope analysis is a valuable tool that could be sensitive to any near-surface microbial activity. Other causes of sulphur isotopic fractionation on the surface of Mars are feasible, but any anomalous fractionation would indicate the desirability of further analysis

Relationship Between Particle Properties and Immunotoxicological Effects of Environmentally-Sourced Microplastics

Background: Concerns on microplastics (MPs) in food are increasing because of our increased awareness of daily exposure and our knowledge gap on their potential adverse health effects. When particles are ingested, macrophages play an important role in scavenging them, potentially leading to an unwanted immune response. To elucidate the adverse effects of MPs on human health, insights in the immunotoxicity of MPs are essential. Objectives: To assess the effect of environmentally collected ocean and land weathered MP particles on the immunological response of macrophages using a state-of-the art in vitro immunotoxicity assay specifically designed for measuring particle toxicity. Methods: Environmentally-weathered macroplastic samples were collected from the North Pacific Subtropical Gyre and from the French coastal environment. Macroplastics were identified using (micro)Raman-spectrometry, FT-IR and Py-GC-MS and cryo-milled to obtain size-fractionated samples up to 300 μm. Physiochemical MP properties were characterized using phase contrast microscopy, gel-permeation chromatography, nuclear magnetic resonance, and differential scanning colorimetry. Macrophages (differentiated THP-1 cells) were exposed to particles (<300 μm) for 48 h before assessment of cell viability and cytokine release. Using both the physiochemical particle properties and biological data, we performed multi-dimensional data analysis to explore relationships between particle properties and immunotoxicological effects. Results: We investigated land-derived polyethylene, polypropylene, polystyrene, and polyethylene terephthalate, water-derived polypropylene macroplastics, and virgin polyethylene fibers and nylon MPs. The different plastic polymeric compositions and MP size classes induced distinct cytokine responses. Macrophages had the largest response to polyethylene terephthalate-particle exposure, including a dose-related increase in IL-1β, IL-8, and TNF-α secretion. Smaller MPs induced cytokine production at lower concentrations. Additionally, a relationship between both physical and chemical particle properties and the inflammatory response of macrophages was found. Discussion: This research shows that MP exposure could lead to an inflammatory response in vitro, depending on MP material and size. Whether this implies a risk to human health needs to be further explored