Spectroscopic study and local coordination of polyphosphate colloidal systems

The interaction between metaphosphate chains and the metal ions Ca2+ and Eu3+ has been studied in water by Eu3+ luminescence, infrared absorption, and 31P NMR spectroscopy. Two main families of sites could be identified for the metal ions in the aqueous polyphosphate colloidal systems: (1) cagelike sites provided by the polyphosphate chain and (2) a family which arises following saturation of cagelike sites. Occupation of this second family leads to supramolecular interactions between polyphosphate chains and the consequent destabilization of the colloidal system. In the polyphosphate-Ca2+ system, this destabilization appears as a coacervation process. Equilibrium existing between colloidal species as a function of the compositions could be reasoned based on the spectroscopic measurements. The determination of coordination numbers and the correlation of the results with the observation of coacervates show that Eu3+ luminescence properties can be used to probe in a unique way the coacervation process.     

Ultrasound speckle and equivalent scatterers

B-mode ultrasound images are characterised by the speckle artefact, which introduces fine-false structures whose apparent resolution is beyond the imaging system capabilities. Speckle presence is due to interference effects between overlapping echoes and its occurrence is related to a great number of randomly distributed structure scatterers within a resolution cell. Basing our analysis on linear system theory, we show that a dense random set of scatterers can be substituted by an equivalent one with a much smaller number of periodic scatterers. This new structure with regularly distributed scatterers is able to give rise to the same B-mode image and the same speckle pattern, for a given ultrasound pulse. This new approach helps the understanding of the deterministic nature of speckle and may reduce drastically the computing time in numerical simulations. Additionally, it can contribute to periodicity analysis used in tissue characterisation.     

Hydroxyl radical generation via photoreduction of a simple pyridine N-oxide by an NADH analogue

Photoreduction of pyridine N-oxide, which has a key structure of antitumor agents for hypoxic solid tumors, by 1-benzyl-1,4-dihydronicotinamide in deaerated aprotic media resulted in generation of hydroxyl radical, leading to the oxidation of salicylic acid to 2,3- and 2,5-dihydroxybenzoic acids, and catechol.     

Note on scheduling with general learning curves to minimize the number of tardy jobs

Several research studies have confirmed that people and organizations become better at their tasks as the tasks are repeated. The effect of this learning phenomenon on classical scheduling problems has been studied recently. One of the single-machine scheduling problems which seems to become nontrivial when learning effects are introduced is that of minimizing the number of tardy jobs. In this note, we study the special case where all jobs share a common due-date. We show that even when the learning process is assumed to be general and job-dependent, the problem remains polynomially solvable.     

Kinetics and equilibria in the system Br + t-BuO2H ⇆ HBr + t-BuO2. OH bond dissociation energy in t-BuO2-H

The kinetics and equilibria in the system Br + t-BuO₂H ? HBr + t-BuO₂· have been measured in the range of 300–350 K using the very low pressure reactor (VLPR) technique. Using an estimated entropy change in reaction (1) ΔS₁ = 3.0 ± 0.4 cal/mol·K together with the measured ΔG₁, we find ΔH₁ = 1.9 ± 0.2 kcal/mol and DHº (t-BuO₂-H) = 89.4 ± 0.2 kcal/mol ΔH_f·(tBuO₂·) = 20.7 kcal/mol and DH^º (t-Bu-O₂) = 29.1 kcal/mol. The latter values make use of recent values of ΔH_f·(t-Bu) = 8.4 ± 0.5 kcal/mol and the known thermochemistry of the other species. The activation energy E₁ is found to be 3.3 ± 0.6 kcal/mol, about 1 kcal lower than the value found for Br attack on H₂O₂. It suggests a bond 1 kcal stronger in H₂O₂ than in tBuO₂H.     

Self-association of thiols. The baggy fit problem in weak complexes

The PMR technique has been used to obtain thermodynamic data for hydrogen bonding of alkanethiols (RSH) in 1:1 dimers in carbon tetrachloride. At ca. 303°K these are (R, 10⁴K(M^?1), ?ΔH°(kcal/mole), ?ΔS°(eu)): n-C₃H₇, 51 ± 5, 0.9 ± 0.15, 13 ± 1; i-C₃H₇, 50 ± 10, 0.8 ± 0.3, 13 ± 1; n-C₄H₉, 35 ± 2, 0.8 ± 0.15, 14 ± 1; t-C₄H₉, 14 ± 4, 1.1 ± 0.7, 16 ± 2; C₆H₁₁, 1.3 ± 2, 0.7 ± 0.3, 15 ± 1. Alkanethiol self-association is weak, and although an exact expression [Eqn. (5)] reproduces spectral data precisely, the fit is sufficiently ‘loose’ or ‘baggy’ so that values of K, ΔH° and ΔS° are uncertain. The methodology of the treatment of self-association data and their errors is examined and Deranleau's useful approach is extended. The impossibility of obtaining reliable data for very weak (< 10 %) or very strong (> 90 %) associations by techniques equivalent to ours is emphasized. The possibility of cyclic thiol dimers is discussed. It is suggested that the PMR method cannot give trustworthy self-association data for aryl or arylalkylthiols because of the relatively large anisotropy effects introduced into the dilution shift.     

Van hove self-correlation function of a hard-disk fluid: Enskog theory and computer simulation

The van Hove self-correlation function in a hard-disk fluid is analyzed using the Lorentz-Enskog kinetic equation and the kinetic model method of solution. Numerical convergence of the model solutions is demonstrated and accurate model results are used to interpret molecular dynamics simulation data at finite wave numbers. It is found that at about 60% of freezing density the error in the Enskog theory can be mainly attributed to an underestimate of the effective self-diffusion coefficient, but at 90% freezing density a theory which treats correlated collisions is needed to describe the width behavior of the singleparticle density fluctuation spectrum.     

Kinetics of polymerization of gaseous formaldehyde

The polymerization of gaseous monomeric formaldehyde has been studied. The effects of the following variables have been investigated: temperature, initial monomer pressure, surface-to-volume ratio of reaction vessel, thickness of polymer deposit, and partial pressure of added oxygen. A three-step mechanism is proposed which successfully accounts for the results of experiments in which monomer is allowed to deposit on the bare wall of the reaction vessel. When a sufficient amount of polymer has been laid down, the termination reaction becomes negligible and a limiting two-step mechanism holds. The present results combined with literature values from work done on the depolymerization reaction give the activation energy of the propagation and termination reactions to be 14.2 ± 0.6 and 7.8 ± 0.9 kcal./mole, respectively. Although the reaction is heterogeneous, the activation energies are independent of surface-to-volume ratios within the experimental error. The nature of the polymer active sites is considered and these are thought to be hydroxyl groups. The results of work done with added oxygen show a small inhibitory effect.