Determination of the nitrogen content of nitrocellulose from smokeless gunpowders and collodions by alkaline hydrolysis and ion chromatography

In this work, a method to determine the nitrogen content of nitrocellulose from gunpowders and collodions is proposed. A basic hydrolysis of nitrocellulose with 1.0% (m/v) NaOH at 150 °C during 30 min was carried out for nitrocellulose from gunpowders (after its previous isolation by a protocol optimized by our research group) and from collodion samples. The concentration of nitrate and nitrite ions in the hydrolysate was determined by ion chromatography with suppression and conductimetric detection. The nitrogen content of nitrocellulose was calculated from the values of the concentration of both ions. The quantitative method was evaluated in terms of selectivity, sensitivity, robustness, limits of detection and quantification, and precision, measured as repeatability and intermediate precision. These parameters were good enough to demonstrate the validity of the method and its applicability to the determination of the nitrogen content of nitrocellulose contained in different types of gunpowders (single- and double-base gunpowders, manufactured from 1944 to 1997) and in commercial collodion samples. For gunpowders, the nitrogen content determined with the optimized method was compared with the values reported by the official label of the ammunition (obtained by a digestion/titration method) and errors, by defect, ranging from 1% to 15.2% (m/m) were calculated. The highest errors were obtained for the oldest gunpowders and could be attributed to the loss of nitro groups in the nitrocellulose molecule during aging. For collodion samples, errors could not be calculated since the real nitrogen content for these samples was not given in the label. In addition, the analysis time (2 h for nitrocellulose isolation, 1.5 h for nitrocellulose hydrolysis, and 0.2 h for chromatographic separation) was about 10 times lower than in the digestion/titration method nowadays used for gunpowder samples.     

The influence of the curvature dependence of the surface tension on the geometry of electrically charged menisci

We evaluate how the curvature dependence of surface tension affects the shape of electrically charged interfaces between a perfectly conducting fluid and its vapour. We consider two cases: i) spherical droplets in equilibrium with their vapour; ii) menisci pending in a capillary tube in presence of a conducting plate at given electric potential drop. Tolman-like dependence of surface tension on curvature becomes important when the “nucleation radius” is comparable with the interface curvature radius. In case i) we prove existence of the equilibrium minimal radius and estimate its dependence on the electric fields and Tolman-like curvature effects. In case ii) the menisci are subject to the gravitational force, surface tension and electrostatic fields. We determine the unknown surface of the menisci to which the potential is assigned using an iterative numerical method and show that Tolman-like corrections imply: 1) a variation of the height (up to 10% in some cases) of the tip of the menisci; 2) a decrease of the maximum electrical potential applicable to the menisci before their break-down amounting to 40V over 800V in the considered cases. We conjecture that these effects could be used in new experiments based on electric measurements to determine the dependence of the equilibrium surface tension on curvature. Received January 19, 1998     

Kinetic and Molecular-Modelling Studies of Reactions of a Class-A β-Lactamase with Compounds Bearing a Methoxy Group on the β-Lactam Ring

The interactions between Staphylococcus aureus PC1 enzyme and compounds bearing a methoxy group on the α-face of the β-lactam ring (cefmetazole ( 1 ), moxalactam ( 2 ) and cefoxitin ( 3 )) were studied. With these compounds, a partitioning of the acyl-enzyme between deacylation and a transiently inactivated form of the enzyme were observed. The individual microscopic rate constants were determined, indicating for 1 and 3 that k′₃>k₃ (Scheme 1), whereas for 2 k′₃≈k₃. This different behavior could be attributed to the presence of the α-carboxy group at the C(7) side chain of 2 , which is able to act as a general-base catalyst in the deacylation step. Molecular-modelling studies allowed correlation of K_S and k₂ with the structures of the Henri-Michaelis complexes that these compounds formed with the S. aureus enzyme. The acylation rate constant (k₂) for these `β-lactamase-stable' compounds was lower than that observed with substrates lacking the methoxy group. Molecular-modelling studies indicated that the methoxy group increases the displacement of the crystallographically observed water molecule (Wat81), which is involved in the acylation mechanism. On the other hand, an average of the most important interactions in the Henri-Michaelis complexes was related to K_S. An increase of 0.2 – 0.5 Å in this average value was found to result in an increase in K_S by about one order of magnitude.     

Kinetic and Molecular‐Modelling Study of the Interaction between Staphylococcus aureus PC1 Enzyme and Imipenem

The interactions between imipenem ( 3 ), a clinically significant carbapenem antibiotic, and Staphylococcus aureus PC1 enzyme, were studied in detail. Imipenem behaves as a slow substrate that reacts by a branched pathway, which suggests the formation of a second acyl‐enzyme intermediate. The individual microscopic rate constants for the process were determined. The results were analysed in the light of molecular‐modelling considerations. Based on the analysis, the Ser‐70(O^γ) group in the Michaelis‐Menten complex formed between 3 and PC1 is very distant from the carbonyl group of the β‐lactam ring of 3 , which is consistent with the decreased value of k₂ (Model 2, see Scheme 2) for imipenem relative to an appropriate substrate such as benzylpenicillin ( 2 ). The deacylation is the rate‐determining step of the turnover process. This can be ascribed to the fact that in the deacylation of the second acyl‐enzyme, the H₂O molecule lying closest to the ester group, Wat81, is in an unfavorable orientation to hydrolyse the intermediate.     

Broom fibers as reinforcement for thermoplastic matrices

Vegetable fibers deriving from the basts of broom plants have been used as reinforcement for two polypropylenic matrices: a conventional isotactic polypropylene (iPP) and a maleate modified one (iPPMA). Before mixing the fibers were subjected to two different extraction procedures: a conventional alkaline treatment and an innovative steam explosion process. Afterwards, the composites obtained by compression moulding were thermally, morphologically, and mechanically characterized. Moreover, water absorption tests, to examine the behaviour of these materials in wet conditions, were also performed.     

Tailoring the electroactive area of carbon screen-printed electrodes by simple activation steps towards rutin determination

Journal of Solid State Electrochemistry - Screen-printed electrodes (SPEs) have the advantage of being considered electrochemical cells that can be implemented in portable sensor applications. With...     

All-Aqueous,Surfactant-Free,and pH-Driven Nanoformulation Methods of Dual-Responsive Polymer Nanoparticles and their Potential use as Nanocarriers of pH-Sensitive Drugs

All-aqueous, surfactant-free, and pH-driven nanoformulation methods to generate pH- and temperature-responsive polymer nanoparticles (NPs) are described. Copolymers comprising a poly(methyl methacrylate) (PMMA) backbone with a few units of 2-(dimethylamino)ethyl methacrylate (DMAEMA) are solubilized in acidic buffer (pH 2.0) to produce pH-sensitive NPs. Copolymers of different molar mass (2.3–11.5 kg mol⁻¹) and DMAEMA composition (7.3–14.2 mol%) are evaluated using a “conventional” pH-driven nanoformulation method (i.e., adding an aqueous polymer solution (acidic buffer) into an aqueous non-solvent (basic buffer)) and a robotized method for pH adjustment of polymer dispersions. Dynamic light scattering, zeta-potential (ζ), and sedimentation-diffusion analyses suggest the formation of dual-responsive NPs of tunable size (from 20 to 110 nm) being stable for at least 28 days in the pH and temperature intervals from 2.0 to 6.0 and 25 to 50 °C, respectively. Ultraviolet-visible spectroscopic experiments show that these NPs can act as nanocarriers for the pH-sensitive dipyridamole drug, expanding its bioavailability and potential controlled release as a function of pH and temperature. These approaches offer alternative strategies to prepare stimuli-responsive NPs, avoiding the use of harmful solvents and complex purification steps, and improving the availability of biocompatible polymer nanoformulations for specific controlled release of pH-sensitive cargos.