On the hydrogen bond networks in the water-methanol mixtures: topology, percolation and small-world

Statistical mechanics based topological analysis and island (or cluster) statistics were used to study the hydrogen bond (H-bond) networks in the water-methanol mixtures with the following methanol mole fractions (x(m)): 0.00, 0.10, 0.20, 0.25, 0.28, 0.30, 0.32, 0.36, 0.38, 0.42, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00. NPT-Monte Carlo simulations were performed at room conditions using the TIP5P model potential for water and united-atoms (OPLS) for methanol to generate the H-bond networks. We have found evidence for non-ideal behavior of mixtures with x(m) ≈ 0.3. Several structural and topological properties present strong dependence with the mixture composition. Island statistics indicate a change from the percolated to non-percolate regime at x(m) ≈ 0.5. Statistical analysis of the islands' nature (homo-clusters: same type of molecules × hetero-clusters: two types of molecules) yields a preferential formation of homo-clusters that quantifies the local composition and preferential solvation ("microimmiscibility"). The topology of the hydrogen bond networks was characterized by local (clustering coefficients, average degrees), semi-global (path lengths) and global (spectral densities) properties. Small-world patterns (highly clustered and small path lengths) appear for x(m) in the range 0.40-0.70, and the momenta in the spectral densities correlate quite well with previous analysis based on rings, chains and branched chains topologies. It also seems that small quantities of methanol in water cause disruption of the continuous fully connected H-bond networks formed by water molecules.     

Hydrogen bond networks in water and methanol with varying interaction strengths

Metropolis Monte Carlo simulations of hydrogen-bonded liquids (water and methanol) were performed with the well tested effective pair potentials TIP5P and OPLS. The Coulomb contribution for the interaction potential was damped by a factor η varied from 1 to 0.49 for water and 1 to 0.15 for methanol. As a result, the networks formed by the hydrogen-bonded molecules presented interesting properties as a function of η, including small-world patterns and percolation transitions. These complex networks were analyzed by local (clustering coefficients, average degrees), semi-global (path lengths) and global (spectral densities) properties, and islands statistics. From these properties, small-world behavior was found for η in the range 0.60-0.75 for both liquids, interestingly independent of the molecular structure of the liquid. Phase transition behavior was observed for the average degrees and the clustering coefficient curves with critical values at 0.55 for water and 0.34 for methanol. Macroscopic properties such as mass density and vaporization enthalpy were also parametrically dependent on η and they presented phase transition behavior that coincides with the critical values obtained from the topological analysis. This is probably the first time that such phase transitions are observed for these quantities and shows a direct relation between macroscopic properties and topological features of hydrogen bond networks.     

Existence of Solution for an Asymptotically Linear Schrödinger-Kirchhoff Equation

Potential Analysis - We consider the Kirchhoff equation $$-\Big(1+\lambda \int |\nabla u|^{2}\Big){\Delta} u+V(x)u=f(u)\quad\text{in }\quad \mathbb{R}^{N}, $$ where N ∈ {3, 4}, λ...     

Effect of trans‐ and cis‐isomeric defects on the localization of the charged excitations in π‐conjugated organic polymers

We use the long‐range‐corrected hybrid density functional theory models to study the effect of various conformational distortions of weak‐trans and strong‐cis nature on the spatial localization of charged states in poly(p‐phenylene vinylene) (PPV) and its derivative poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene] (MEH‐PPV). The extent of self‐trapping of positive (P⁺) and negative (P^?) polarons is observed to be highly sensitive to molecular conformation that, in turn, controls the distribution of atomic charges within the polymers. It is shown that, to reach good agreement with recent experimental data on lattice distortion for P⁺ and P^? excitations, the polarization of the medium plays a critical role. The introduction of weak‐trans defects along the MEH‐PPV chain breaks the observed symmetry for P⁺ and P^? excitations. The P^? states exhibit more spatial localization owing to lattice relaxation than their vacuum counterparts in contrast to P⁺. These observations suggest higher mobilities of holes than that of electrons in MEH‐PPV, in agreement with the experimental observations. The predicted binding, reorganization, and solvation energies for PPV and MEH‐PPV are analyzed for this difference in the response behavior of holes and electrons for trans and cis distortions. This study allows for a better understanding of charge‐transport and photophysical properties in π‐conjugated organic materials by analyzing their underlying structure–property correlations. © 2013 Wiley Periodicals, Inc. 1 1 This article is a U.S. Government work, and as such, is in the public domain in the United States of America.
J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 935–942     

Facile Regioselective Synthesis of a Novel Chitosan–Pexiganan Conjugate with Potential Interest for the Treatment of Infected Skin Lesions

A pexiganan–chitosan conjugate was designed to combine the exceptional bioadhesion and tissue-regenerating abilities of chitosan with the excellent antibiotic properties of pexiganan. We herein report our first results on the successful synthesis, including Fourier transform infrared (FT-IR) and amino acid analysis of such conjugate, which was prepared by regioselective covalent attachment of a Cys-containing pexiganan analog to the chitosan's amino groups. Further results from ongoing research will be reported.     

Evaluation of thermal stability and parameters of dissolution of nifedipine crystals

Nifedipine is a calcium channel blocker as well as a powerful vasodilator used to treat ischemic heart disease and hypertension. Its photosensitivity and very low solubility in water have been widely acknowledged as important properties deserving improvements. The main thrust of this study is to characterize the nature and the solid-state of nifedipine crystals obtained using different solvents as well as assess the stability by thermal methods (TG and DSC) and crystals structure by means of spectroscopic techniques (MID FTIR and XRD) and assess the dissolution parameters for such crystals. The calculated kinetic parameters activation energy (E _a = 123.3 kJ mol^?1 ± 0.1), the factor frequency (A = 25.93 ± 0.9 min^?1), and the reaction order (n = 0.2) of the main stage of thermal decomposition of nifedipine raw material were performed according to the Ozawa model. The data showed a zero-order kinetic behavior for all crystals despite the different values of E _a and A. The dissolution profiles were obtained for such crystals in three dissolution media with different pH values. After 1 h of dissolution, the higher amount of nifedipine dissolved was observed for crystals obtained in isopropyl alcohol (52.5 %, pH 4.5), followed by those in chloroform (48.1 %, pH 1.2) and subsequently in acetone (32.5 %, pH 6.8). Results showed different thermal stabilities and significant variations in the solubility of the crystals.     

Real-time determination of CO2 production and estimation of adsorbate coverage on a proton exchange membrane fuel cell under oscillatory operation

A number of fuel cell relevant reactions are known to undergo kinetic instabilities under certain conditions. The majority of the experiments in such systems have been performed in liquid electrolyte systems on half-cell setups. Results for proton exchange membrane fuel cells fed with H₂/CO mixtures at the anode show that there can be a range of gas flow rate and current density where spontaneous potential oscillations take place. Despite the recent developments in this area, there are still many mechanistic aspects underlying the emergence of electrochemical oscillations that remain unknown. In the present contribution, we report results on the CO₂ production during the oxidation of carbon monoxide-containing hydrogen in a proton exchange membrane fuel cell, as measured by online mass spectrometry. By extensive fitting and careful consideration of the proposed mechanism, we were able to estimate the coverage of hydrogen and CO during the oscillations. As no other approach seems capable to probe the adsorbate coverage in an operando fuel cell, our analyses access experimental hidden information that can be of high value for fuel cell research.     

Itinerant electron-driven chiral magnetic ordering and spontaneous quantum Hall effect in triangular lattice models

We study the Kondo Lattice and the Hubbard models on a triangular lattice. We find that at the mean-field level, these rotationally invariant models naturally support a noncoplanar chiral magnetic ordering. It appears as a weak-coupling instability at the band filling factor 3/4 due to the perfect nesting of the itinerant electron Fermi surface. This ordering is a triangular-lattice counterpart of the collinear Neel ordering that occurs on the half-filled square lattice. While the long-range magnetic ordering is destroyed by thermal fluctuations, the chirality can persist up to a finite temperature, causing a spontaneous quantum Hall effect in the absence of any externally applied magnetic field.