Molecular dynamics simulation study of polyelectrolyte adsorption on cellulose surfaces

The adsorption of two polyelectrolyte ((carboxy methyl) cellulose and poly(acrylate) in water on crystalline cellulose is studied in this work. The multi-component problem has been splitted up into simulations of solutions of the polyelectrolyte (polyanions including sodium counterions) in water, into simulations of the interface of crystalline cellulose towards water. Finally polyelectrolyte-cellulose systems were studied. Molecular dynamics simulations of diluted (_ 2:5 weight percent) aqueous solutions of two polyelectrolytes, namely sodium (carboxy methyl) cellulose (CMC) and sodium poly(acrylate) (PAA) have been performed. Water and counterions were taken into account explicitly. For CMC the substitution pattern is important. Simulations of CMC oligomers resulted in two different structures: One molecule takes a stretched conformation, while the other one takes a globule-like, collapsed state. In an additional simulation, which starts from a linear state, the second CMC molecules collapses possibly due to solute-solute hydrogen bonding. PAA is stretched during the whole simulation.On a local atomistic scale, CMC and PAA have different hydrogen-bond properties. The COO_ groups of PAA can only act as hydrogen bond acceptors, but due to the high negative charge density, there are still more water molecules assembled around PAA than around CMC. There are 0:029 bonds/amu respectively 0:019 bonds/amu to water for the two CMC oligomers, but more than twice as many for PAA: 0:083 bonds/amu. Beside intermolecular hydrogen bonding, there is a significant amount of intramolecular H-bonding for CMC, which is influenced by the COO-groups, which act as strong H-acceptor. In contrast to hydroxy- and carboxylic groups, ether oxygen are hardly involved in hydrogen bonding.The water-cellulose interface shows common features for both simulations of the 11-0 and 110 surface of cellulose I beta. We have simulated a cellulose-crystal with the 11-0 and 110 surface exposed to water. Both interfaces are stable with respect to surface reconstruction and water penetration, at least on the nanosecond time scale and in the absence of defects. Both show essentially the crystal structure of cellulose I beta, also in the adsorption patterns for water and argon. In spite of the presence of OH groups, both surfaces are non-hydrophilic and lipophilic: Water molecules are not attracted to the surface, as they are equally happy in bulk water. On the other hand, the presence of the surface perturbs the water structure sufficiently to create free volume, in which an argon can dissolve more easily than in the denser bulk water. The wider spacing of cellulose chain allows the 110 surface to expose more of the hydrophobic grooves where lipophilic adsorption takes place. In contrast, the protruding OH groups are less affected by chain packing, so the behavior towards water is similar for both surfaces. We did a direct comparison of the adsorption properties of poly(acrylate) and (carboxy methyl) cellulose on different monoclinic cellulose surfaces. Four systems have been simulated: 110-CMC, 11-0-CMC and 110-PAA and 11-0-PAA. Every systems consists of the water solvent, a cellulose-crystal and the polyelectrolyte. As found in experiment, CMC does adsorb onto the cellulose surface, and PAA does not. This is due to a high number of hydrogen bonds from the cellulose surface to the CMC-oligomers. There are hardly any hydrogen bonds between cellulose and PAA. One (carboxy methyl) cellulose is at on the surface, whereas other oligomers take a end-on-the-surface conformation. The anhydroglucose unit which is in contact with the cellulose crystal aligns parallel to the cellobiose-units of the crystal (200-plane).Es wurden verdünnte (2.5 Gewichtsprozent) Lösungen von Polyelektrolyten ((Carboxy methyl) Zellulose und Polyacrylat) in Wasser simuliert. Wasser und Natrium-Gegenionen waren Teil des Simulationsmodells.Unsere Molekulardynamik Simulationen von CMC führen in Abhängigkeit vom Substitutionsmuster zu zwei unterschiedlichen CMC-Strukturen: Ein CMC-Oligomer behält seine gestreckte Konformation, ein zweites nimmt eine geknäulte Form an. Diese globulartige Struktur ist unabhängig von der Anfangskonformation (gestreckt oder schon geknäult). Polyacrylsäure (PAA) ist gestreckt.CMC und PAA haben verschiedene inter- und intramolekulare Wasserstoffbrückenbindungen. Die COO-Gruppe der Polyacrylsäure ist ein starker H-Akzeptor. Durch die negative Ladung ist an PAA mehr Wasser (pro Monomer, pro Masse) gebunden als an CMC. Die beiden simulierten CMC-Oligomere haben 0:029 H-Bindungen/amu (gestreckte Konformation) bzw. 0:019 H-Bindungen/amu (geknäulte). PAA hat 0:083 Bindungen/amu mit Wasser.Beide Zelluloseoberflächen (110 und 11-0) eines monoklinen Kristalls haben gemeinsame Eigenschaften. Beide Oberflächen sind gegen Wasser stabil. Es findet keine Umstrukturierung in den Simulationen mit reinem Wasser statt. Die Kristallstruktur überträgt sich auf das Adsorptionsmuster von Argon und Wasser über der Grenzschicht. Trotz der OH-Gruppen auf der Oberflächen ist Cellulose lipophil und hydrophob. Wasser wird nicht von der Oberfläche angezogen. Jedoch stört die Gegenwart des Kristalls die Wasserstruktur und es bildet sich ein freies Volumen, in dem sich Argon-Atome aufhalten. Die breiteren Lücken zwischen den Zellbioseketten der 110-Oberfläche legen dort stärker lipophilen Bereiche frei: Auf der 110 Oberfläche adsorbiert mehr Argon. Vorstehenden OH-Gruppen auf der Oberfläche haben ähnliche Eigenschaften auf beiden - der 'breiten' 110 und der 'schmalen' 11-0 - Oberfläche.Es wurden vier verschiedene Kombinationen von Oberflächen und Polyelektrolyten simuliert. Der direkte Vergleich der Adsoption von CMC und PAA auf der Zelluloseoberfläche gibt das Experiment wieder: CMC adsorbiert, PAA nicht. CMC wird durch viele Wasserstoffbrückenbindungen von der Zelluloseoberfläche, besonders von der O2 und O6 Gruppe gebunden. Zwischen PAA und Zellulose gibt es kaum H-Brücken.Die adsorbierten CMC-Oligomere stehen mit einem Ende auf der Oberfläche. Der Glucosering in Kontakt mit der Oberfläche richtet sich parallel zur 200 Kristallebene aus. Nur ein CMC-Oligomer liegt flach auf der Kristalloberfläche.Der 11-0-Zellulosekristall ist in der Gegenwart von CMC oder PAA nicht stabil. Es bildet sich ein Defekt auf der Oberfläche in der Form eines Knicks

Coarse Graining of Nonbonded Inter-particle Potentials Using Automatic Simplex Optimization to Fit Structural Properties

We implemented a coarse-graining procedure to construct mesoscopic models of complex molecules. The final aim is to obtain better results on properties depending on slow modes of the molecules. Therefore the number of particles considered in molecular dynamics simulations is reduced while conserving as many properties of the original substance as possible. We address the problem of finding nonbonded interaction parameters which reproduce structural properties from experiment or atomistic simulations. The approach consists of optimizing automatically nonbonded parameters using the simplex algorithm to fit structural properties like the radial distribution function as target functions. Moreover, any mix of structural and thermodynamic properties can be included in the target function. Different spherically symmetric inter-particle potentials are discussed. Besides demonstrating the method for Lennard--Jones liquids, it is applied to several more complex molecular liquids such as diphenyl carbonate, tetrahydrofurane, and monomers of poly(isoprene).Comment: 24 pages, 3 tables, 14 figures submitted to the Journal of Chemical Physics (JCP

Bremen’s and Hamburg’s port position: Transport infrastructure and hinterland connections within the North Range

The quality and efficiency of the northern German ports’ hinterland connections define their competitive potential. The assessment of gross domestic product achieved within a certain travel period via rail, road, inland waterways and intermodal freight transport provides a major indicator for the competitive positioning of the northern German ports. To measure this potential, we use simple travel time matrices for different modes of transport. The achievable purchasing power of the hinterland territories is thereby a prime determinant of port competitiveness. The European Union’s Trans-European Transports Network programme (TEN-T) may advance Hamburg and Bremen/Bremerhaven ports’ competitive situation by improving their hinterland accessibility, removing cross-border bottlenecks and upgrading infrastructure and streamline transport operations throughout the EU

What are the Red flags to aid the early detection of metastatic bone disease as a cause of back pain?

Existing high performance work system (HPWS) research has rarely considered cultural influences. This study investigates the relationships between guanxi, HPWS and employee attitudes in China. A data set consisting of 226 employees in a Chinese state-owned enterprise in the railway sector was used to test the hypotheses. Using structural equation modelling as an analytical tool, we found that guanxi was positively related to HPWS and trust. Similar to research in the Western context, HPWS was found to be positively related to trust and job satisfaction. Moreover, the results also revealed that HPWS mediated between guanxi and both trust and job satisfaction. Theoretical and practical implications are both discussed

A Study of Cosmic Ray Composition in the Knee Region using Multiple Muon Events in the Soudan 2 Detector

Deep underground muon events recorded by the Soudan 2 detector, located at a depth of 2100 meters of water equivalent, have been used to infer the nuclear composition of cosmic rays in the "knee" region of the cosmic ray energy spectrum. The observed muon multiplicity distribution favors a composition model with a substantial proton content in the energy region 800,000 - 13,000,000 GeV/nucleus.Comment: 38 pages including 11 figures, Latex, submitted to Physical Review

A Review of the Tools Used for Marine Monitoring in the UK: Combining Historic and Contemporary Methods with Modeling and Socioeconomics to Fulfill Legislative Needs and Scientific Ambitions

Marine environmental monitoring is undertaken to provide evidence that environmental management targets are being met. Moreover, monitoring also provides context to marine science and over the last century has allowed development of a critical scientific understanding of the marine environment and the impacts that humans are having on it. The seas around the UK are currently monitored by targeted, impact-driven, programmes (e.g., fishery or pollution based monitoring) often using traditional techniques, many of which have not changed significantly since the early 1900s. The advent of a new wave of automated technology, in combination with changing political and economic circumstances, means that there is currently a strong drive to move toward a more refined, efficient, and effective way of monitoring. We describe the policy and scientific rationale for monitoring our seas, alongside a comprehensive description of the types of equipment and methodology currently used and the technologies that are likely to be used in the future. We contextualize the way new technologies and methodologies may impact monitoring and discuss how whole ecosystems models can give an integrated, comprehensive approach to impact assessment. Furthermore, we discuss how an understanding of the value of each data point is crucial to assess the true costs and benefits to society of a marine monitoring programme

Measurement of the cosmic ray spectrum above 4×10184{\times}10^{18} eV using inclined events detected with the Pierre Auger Observatory

A measurement of the cosmic-ray spectrum for energies exceeding $4{\times}10^{18}$ eV is presented, which is based on the analysis of showers with zenith angles greater than $60^{\circ}$ detected with the Pierre Auger Observatory between 1 January 2004 and 31 December 2013. The measured spectrum confirms a flux suppression at the highest energies. Above $5.3{\times}10^{18}$ eV, the "ankle", the flux can be described by a power law $E^{-\gamma}$ with index $\gamma=2.70 \pm 0.02 \,\text{(stat)} \pm 0.1\,\text{(sys)}$ followed by a smooth suppression region. For the energy ($E_\text{s}$) at which the spectral flux has fallen to one-half of its extrapolated value in the absence of suppression, we find $E_\text{s}=(5.12\pm0.25\,\text{(stat)}^{+1.0}_{-1.2}\,\text{(sys)}){\times}10^{19}$ eV.Comment: Replaced with published version. Added journal reference and DO