Surface Tension of Organophosphorus Compounds: Sarin and its Surrogates.

While the production and stockpiling of organophosphorus chemical warfare agents (CWAs), such as sarin, was banned three decades ago, CWAs have remained a threat. New approaches for decontamination and destruction of CWAs require detailed knowledge of their various physicochemical properties. In particular, surface tension is needed to describe the formation and evolution of hazardous aerosols when CWA liquids are dispersed in the air. Due to the extreme toxicity of sarin, most experimental studies are carried out using its surrogates─organophosphorus compounds which, while having similar structures, are much less toxic, e.g., dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP). However, not only for sarin, but also for its surrogates, literature data on the surface tension are scarce. Here we present experimental measurements and computational predictions of the surface tension of DMMP and DIMP. Classical molecular dynamics simulations using the Transferable Potentials for Phase Equilibria (TraPPE) force field produced an excellent agreement with the experimental results in the temperature range from 3 to 60 °C, validating the predictive capability of TraPPE. Consequently, we applied the TraPPE force field to sarin. Our modeled values for the sarin surface tension cover the range of temperatures from 0 to 85 °C, and the four experimental data points from the literature measured between 20 and 35 °C agree perfectly with our predictions. The temperature-dependent surface tension values for sarin and its surrogates obtained in our study can be used in models predicting the formation and evolution of aerosols made of these chemicals. Furthermore, our results justify the use of the TraPPE force field to derive the thermodynamic properties of other organophosphorus compounds with structures similar to the ones studied here.

Removal of diisopropyl methyl phosphonate (DIMP) from heated metal oxide surfaces.

Diisopropyl methyl phosphonate (DIMP) is an organophosphorus compound used as a surrogate of sarin, a chemical weapon agent. Thermal decomposition of DIMP and similar liquids may be affected by added inorganic solids. Understanding such effects is needed to guide decontamination and environmental mitigation work. Here, liquid DIMP mixed with powders of γ-Al2O3 or SiO2, was heated to 350 °C in a thermogravimetric analyzer while observing effluent gas using a mass spectrometer. For both powders, evaporation of DIMP occurred between 50 and 200 °C, followed by a second mass loss step up to 350 °C. The amount of DIMP evaporated in the first step varied; however, the size of the second, mass loss step was consistent between experiments for each solid used. For γ-alumina, 2-propanol and propene were released below the DIMP boiling point and mostly propene at higher temperatures. Calcining alumina prior to exposure to DIMP reduced the release of 2-propanol. For silica, the second mass loss step was smaller and only propene was released. Powders exposed to DIMP and recovered at different temperatures showed FTIR peaks corresponding to the individual bond vibrations of DIMP. At higher temperatures, only the P-CH 3 stretching vibration was observed.

Atomic Scale Insights into the First Reaction Stages Prior to Al/CuO Nanothermite Ignition: Influence of Porosity.

This theoretical work aims to understand the influence of nanopores at CuO-Al nanothermite interfaces on the initial stage of thermite reaction. ReaxFF molecular dynamics simulations were run to investigate the chemical and structural evolution of the reacting interface between the fuel, Al, and oxidizer, CuO, between 400 and 900 K and considering interfaces with and without a pore. Results show that the initial alumina layer becomes enriched with Al and grows primarily into the Al metal at higher temperatures. The modification of alumina is driven by simultaneous Al and O migration between metallic Al and the native amorphous Al2O3 layer. However, the presence of a pore significantly affects the growth kinetics and the composition of this alumina layer at temperatures exceeding 600 K, which impacts the initiation properties of the nanothermite. In the system without a pore, where Al is in direct contact with CuO, a ternary aluminate layer, a mixture of Al, O, and Cu, is formed at 800 K, which slows Al and O diffusion, thus compromising the nanothermite reactivity in fully dense Al/CuO composites. Conversely, the presence of a pore between Al and CuO promotes Al enrichment of the alumina layer above 600 K. At that temperature, any free oxygen molecules in the pore become attached to the reactive alumina surface resulting in a rapid oxygen pressure drop in the pore. This is expected to accelerate the reduction of the adjacent CuO as observed in experiments with Al/CuO composites with porosity at the CuO-Al interfaces.

Preparation and Characterization of Silicon-Metal Fluoride Reactive Composites.

Fuel-rich composite powders combining elemental Si with the metal fluoride oxidizers BiF3 and CoF2 were prepared by arrested reactive milling. Reactivity of the composite powders was assessed using thermoanalytical measurements in both inert (Ar) and oxidizing (Ar/O2) environments. Powders were ignited using an electrically heated filament; particle combustion experiments were performed in room air using a CO2 laser as an ignition source. Both composites showed accelerated oxidation of Si when heated in oxidizing environments and ignited readily using the heated filament. Elemental Si, used as a reference, did not exhibit appreciable oxidation when heated under the same conditions and could not be ignited using either a heated filament or laser. Lower-temperature Si fluoride formation and oxidation were observed for the composites with BiF3; respectively, the ignition temperature for these composite powders was also lower. Particle combustion experiments were successful with the Si/BiF3 composite. The statistical distribution of the measured particle burn times was correlated with the measured particle size distribution to establish the effect of particle sizes on their burn times. The measured burn times were close to those measured for similar composites with Al and B serving as fuels.

Inactivation of aerosolized Bacillus atrophaeus (BG) endospores and MS2 viruses by combustion of reactive materials.

Accidental release of biological agents from a bioweapon facility may contaminate large areas, possibly causing disastrous environmental consequences. To address this issue, novel halogen-containing reactive materials are being designed with the added capability to inactivate viable airborne microorganisms. This study determined the efficiency of combustion products of such materials to inactivate aerosolized bacteria and viruses. Spores of Bacillus atrophaeus and MS2 viruses dispersed in dry air were exposed for subsecond time intervals to hydrocarbon flames seeded with different reactive powders so that bioaerosol particles interacted with the combustion products in a controlled high-temperature environment. The experiments were designed to quantify differences in the biocidal effects of different reactive material powders including Al and Mg, a B•Ti nanocomposite, an 8Al•MoO(3) nanothermite, and a novel Al•I(2) nanocomposite. Compared to pure hydrocarbon flame, powder-seeded flame (with no iodine) produced about an order of magnitude greater inactivation of bacterial spores. The iodine-containing material increased the spore inactivation by additional 2 orders of magnitude. The aerosolized MS2 viruses (generally not as stress-resistant as spores) were fully inactivated when exposed to combustion of either the iodinated or noniodinated powders. Overall, the study suggests a great biocidal potential of combustion products generated by novel iodine-containing nanocomposite materials.

Oxidation of aluminum particles in the presence of water.

Oxidation of spherical aluminum powder was investigated in mixed argon-oxygen-steam atmospheres by thermogravimetric measurements at heating rates between 1 and 20 K/min and up to 1100 degrees C. The observed oxidation behavior in the presence of steam differs markedly from oxidation in dry oxygen. Oxidation in steam is complete near 1000 degrees C vs 1500 degrees C in dry oxygen. Furthermore, in steam, a stepwise weight change is observed at the melting point of aluminum, while no such step can be distinguished in dry oxygen. The complete oxidation observed at a lower temperature in steam as compared to dry oxygen is explained by the stabilization of the gamma polymorph of the surface oxide in the presence of water so that a denser and slower growing alpha-alumina does not form until higher temperatures. Experiments in mixed oxygen/steam oxidizers showed that the size of the oxidation step observed upon aluminum melting only correlates with the concentration of steam in the atmosphere. This may be interpreted as the effect of transient porosity, the degree of which is controlled by the steam concentration, or the surface oxide stressed by the expanding melting metal core may behave as a semipermeable membrane where hydrous species have significantly higher diffusion rates than oxygen. A clear distinction cannot be drawn, and further research is warranted. Preliminary results on isoconversion processing of the oxidation kinetics are presented.

Oxidation and melting of aluminum nanopowders.

Recently, nanometer-sized aluminum powders became available commercially, and their use as potential additives to propellants, explosives, and pyrotechnics has attracted significant interest. It has been suggested that very low melting temperatures are expected for nanosized aluminum powders and that such low melting temperatures could accelerate oxidation and trigger ignition much earlier than for regular, micron-sized aluminum powders. The objective of this work was to investigate experimentally the melting and oxidation behavior of nanosized aluminum powders. Powder samples with three different nominal sizes of 44, 80, and 121 nm were provided by Nanotechnologies Inc. The particle size distributions were measured using small-angle X-ray scattering. Melting was studied by differential scanning calorimetry where the powders were heated from room temperature to 750 degrees C in an argon environment. Thermogravimetric analysis was used to measure the mass increase indicative of oxidation while the powders were heated in an oxygen-argon gas mixture. The measured melting curves were compared to those computed using the experimental particle size distributions and thermodynamic models describing the melting temperature and enthalpy as functions of the particle size. The melting behavior predicted by different models correlated with the experimental observations only qualitatively. Characteristic stepwise oxidation was observed for all studied nanopowders. The observed oxidation behavior was well interpreted considering the recently established kinetics of oxidation of micron-sized aluminum powders. No correlation was found between the melting and oxidation of aluminum nanopowders.