Speciation and Structural Properties of Hydrothermal Solutions of Sodium and Potassium Sulfate Studied by Molecular Dynamics Simulations

Aqueous solutions of salts at elevated pressures and temperatures play a key role in geochemical processes and in applications of supercritical water in waste and biomass treatment, for which salt management is crucial for performance. A major question in predicting salt behavior in such processes is how different salts affect the phase equilibria. Herein, molecular dynamics (MD) simulations are used to investigate molecular‐scale structures of solutions of sodium and/or potassium sulfate, which show contrasting macroscopic behavior. Solutions of Na?SO₄ exhibit a tendency towards forming large ionic clusters with increasing temperature, whereas solutions of K?SO₄ show significantly less clustering under equivalent conditions. In mixed systems (Na_xK_2?xSO₄), cluster formation is dramatically reduced with decreasing Na/(K+Na) ratio; this indicates a structure‐breaking role of K. MD results allow these phenomena to be related to the characteristics of electrostatic interactions between K⁺ and SO₄^2?, compared with the analogous Na⁺?SO₄^2? interactions. The results suggest a mechanism underlying the experimentally observed increasing solubility in ternary mixtures of solutions of Na?K?SO₄. Specifically, the propensity of sodium to associate with sulfate, versus that of potassium to break up the sodium–sulfate clusters, may affect the contrasting behavior of these salts. Thus, mutual salting‐in in ternary hydrothermal solutions of Na?K?SO₄ reflects the opposing, but complementary, natures of Na?SO₄ versus K?SO₄ interactions. The results also provide clues towards the reported liquid immiscibility in this ternary system.     

Quickest online selection of an increasing subsequence of specified size

Given a sequence of independent random variables with a common continuous distribution, we consider the online decision problem where one seeks to minimize the expected value of the time that is needed to complete the selection of a monotone increasing subsequence of a prespecified length n. This problem is dual to some online decision problems that have been considered earlier, and this dual problem has some notable advantages. In particular, the recursions and equations of optimality lead with relative ease to asymptotic formulas for mean and variance of the minimal selection time. © 2016 Wiley Periodicals, Inc. Random Struct. Alg., 49, 235–252, 2016     

Elastic Light Tunable Tissue Adhesive Dendrimers

Development of bioadhesive formulations for tissue fixation remains a challenge. The major drawbacks of available bioadhesives are low adhesion strength, toxic byproducts, and complexity of application onto affected tissues. In order to address these problems, this study has developed a hydrogel bioadhesive system based on poly amido amine (PAMAM) dendrimer, grafted (conjugated) with UV‐sensitive, 4‐[3‐(trifluoromethyl)‐3H‐diazirin‐3‐yl] benzyl bromide (PAMAM‐g‐diazirine). This particular diazirine molecule can be grafted to the surface amine groups of PAMAM in a one‐pot synthesis. Diazirine functionalities are carbene precursors that form covalent crosslinks with hydrated tissues after low‐power UV activation without necessity of free‐radical initiators. The rheological properties and adhesion strength to ex vivo tissues are highly controllable depending on diazirine grafting, hydrogel concentration, and UV dose intensity fitting variety types of tissues. Covalent bonds at the tissue/bioadhesive interface provide robust adhesive and mechanical strength in a highly hydrated environment. The free flowing hydrogel conversion to elastic adhesive after UV activation allows intimate contact with the ex vivo swine tissue surfaces with low in vitro cytotoxicity observed, making it a promising bioadhesive formulation toward clinical applications.

     

Measuring coexisting densities from a two-phase molecular dynamics simulation by voronoi tessellations

A new algorithm is presented that allows for the determination of bulk liquid and vapor densities from a two-phase Molecular Dynamics (2phiMD) simulation. This new method does not use any arbitrary cutoffs for phase definitions; rather it uses single-phase simulations as a self-consistency check. The method does not use any spatial bins for generating histograms of local properties, thereby avoiding the statistical issues associated with bins. Finally, it allows one to approach very close to the critical point. The new method utilizes Voronoi tessellations to determine the molecular volume of every point at every instance in a molecular dynamics simulation. Since the molecular volume is calculated throughout the simulation, statistical parameters such as the average molecular volume and average molecular variance are easy to obtain. To define the phases, the normalized variance of the molecular volume from 1phiMD and 2phiMD is used as a self-consistency check. The new method gives new insight into the nature of the near-subcritical fluid. The critical properties from this analysis are T(c) = 1.293 and rho(c) = 0.313. Direct simulation of the two-phase system was performed up to a temperature of 1.292. The results show excellent agreement to experimental results and Gibbs Ensemble Monte Carlo for coexisting densities. We see that well below the critical temperature, some particles are neither liquid nor vapor. These interfacial particles are primarily, but not exclusively, concentrated at the bulk interface. However, as we approach the critical point, some particles are considered both liquid and vapor. These interfacial particles are distributed through the system.     

Two-dimensional supramolecular assembly of phenylmercury(II) and cadmium(II) complexes with a tridentate thiohydrazone NNS donor ligand: Synthesis,coordination behavior and crystal structure

The reaction of phenylmercury(II) acetate and cadmium(II) acetate with a refluxed solution of diacetylmonoxime and morpholine N-thiohydrazide formed a novel phenylmercury(II) complex, [PhHg(Hdammthiol)] (1) and a cadmium(II) complex, [Cd(Hdammthiol)₂] (2), respectively (where H₂dammthiol is the thiol form of diacetylmonoximemorpholine N-thiohydrazone (Hdammth) formed by the condensation of diacetylmonoxime and morpholine N-thiohydrazide in the presence of phenylmercury(II) and cadmium(II) ions). The complexes were characterised by elemental analyses and spectral data (electronic, infrared and ¹H NMR) and also by X-ray crystal structure analysis. The X-ray crystallography shows that the phenylmercury(II) complex attained a tricoordinated distorted T-shaped structure, while the cadmium(II) complex attained a trapezoidal bipyramidal geometry. The phenylmercury(II) complex forms a two-dimensional sheet via C–H?O and O–H?N hydrogen bonding and also forms a two-dimensional supramolecular dimer, having C–H?π synthons. Intermolecular C–H?O and O–H?O hydrogen bonding of the cadmium(II) complex forms a two-dimensional supramolecular sheet along the bc plane and posses an impressively short intermolecular C(sp³)?O(sp³) contact.     

Development of a reader for track etch detectors based on a commercially available slide scanner

NRPB has operated a routine neutron personal dosimetry service based on the electrochemical etch of PADC elements since 1986. Since its inception it has used an automated reader based on a video camera and real time analysis. A new and more powerful replacement system has been developed using a commercially available photographic slide scanner. This permits a complete image of the dosemeter to be grabbed in a single scan, generating a 2592x3888 pixel file which is saved for subsequent analysis. This gives an effective pixel size of 10x10 μm with an image of the entire dosemeter in one field of view. Custom written software subsequently analyses the image to assess the number of etched pits on the dosemeter and read the detector identification number (code). Batch scanning of up to 40 detectors is also possible using an autofeed attachment. The system can be used for electrochemically etched tracks for neutron detectors and chemically etched tracks for radon detectors.     

Nucleophilic displacements of non-racemic α-trifluoromethyl benzylic triflates

Effective protocols for the introduction of chiral α-trifluoromethyl benzyl moieties by nucleophilic displacement of enantiomerically enriched α-trifluoromethyl benzylic triflates are presented. The effects of substrate electronics, solvent polarity, temperature, and base are studied by measuring the diastereomeric or enantiomeric excesses of the displacement products formed by coupling a variety of α-trifluoromethyl benzylic triflates with a range of nucleophiles including amines, carboxylates, thiols, and malonates. Preliminary investigations to elucidate the mechanism(s) involved in the loss of stereochemical integrity at the benzylic center in the nucleophilic displacement reactions are also reported.     

Convexity Bias in Eurodollar Futures Prices: A Dimension-Free HJM Criterion

In the theory of interest rate futures, the difference between the futures rate and forward rate is called the “convexity bias,” and there are several widely offered reasons why the convexity bias should be positive. Nevertheless, it is not infrequent that the empirical the bias is observed to be negative. Moreover, in its most general form, the benchmark Heath–Jarrow–Morton (HJM) term structure model is agnostic on the question of the sign of the bias; it allows for models where the convexity bias can be positive or negative. In partial support of the practitioner’s arguments, we develop a simple scalar condition within the HJM framework that suffices to guarantee that the convexity bias is positive. Moreover, when we check this condition on the LIBOR futures data, we find strong empirical support for the new condition. The empirical validity of the sufficient condition and the periodic observation of negative bias, therefore leads one to a paradoxical situation where either (1) there are arbitrage possibilities or (2) a large subclass of HJM models provide interest rate dynamics that fail to capture a fundamental feature of LIBOR futures.