Dean S. Venables

Spectroscopy and dynamics of mixtures of water with acetone, acetonitrile, and methanol

3t o 55 cm 21 , and from 400 to 1200 cm 21 . The far-infrared absorption of the mixtures is substantially less than that for ideal mixtures, and Debye time constants calculated from the spectra are longer for the real than for the ideal mixtures. Significant composition dependence is observed in the high frequency librational spectra of the mixtures, and is reproduced by the MD simulations. Single dipole and angular velocity spectra are also reported, as are detailed changes in the hydrogen bonding environment in the mixtures. There is a loss of tetrahedral water structure on mixing, yet water molecules have a strong tendency to aggregate, especially in the acetone and acetonitrile mixtures. Spatial distribution functions are reported for the acetone/water system.

Far-infrared spectra and associated dynamics in acetonitrile-water mixtures measured with femtosecond THz pulse spectroscopy

We report the frequency-dependent absorption coefficient and index of refraction in the far-infrared region of the spectrum for mixtures of acetonitrile and water. The mixtures do not behave ideally, and deviate from ideality most noticeably for mixtures that are between 25% and 65% acetonitrile by volume. Two implementations of the Debye model for describing the dielectric relaxation behavior of mixtures are compared, and we show that these mixtures are better treated as uniform solutions rather than as two-component systems. We find an enhanced structure in the mixtures, relative to ideal mixtures, but we do not find direct evidence for microheterogeneity. The Debye time constant for the primary relaxation process for the mixtures is up to 25% longer than that for an ideal mixture.

Structure and dynamics of nonaqueous mixtures of dipolar liquids. II. Molecular dynamics simulations

In the preceding paper, 1 we discussed how a combination of experimental intermolecular spectra and molecular dynamics ~MD! simulations is needed to arrive at a detailed understanding of the dynamics of liquids. The combination of experimental and simulation techniques is a two-way procedure: the experimental spectra are required to evaluate the accuracy of the simulations, which serve in turn to interpret the experimental spectra. Our focus in these two papers is on the nonaqueous dipolar liquid mixtures of acetone/methanol, acetonitrile/methanol, and acetone/acetonitrile. In the preceding paper, we measured the experimental infrared spectra of these mixtures below 120 cm 21 , and from 400 to 1000 cm 21 for the methanolic mixtures. 1 We observed that the low frequency spectra of all the mixtures behave ideally ~in contrast to aqueous mixtures at these frequencies!. However, at high frequencies there is a dramatic change in the librational band of the methanol molecules as co-solvent is added. In this paper, we report the results of MD simulations of these mixtures. Computer simulation has proven to be an indispensable tool for investigating the behavior of liquids. Simulations have successfully reproduced many of the macroscopic and microscopic properties of liquids. 2 In particular, MD simulations allow us to probe both the structure and the dynamics of the system in unprecedented detail. The interaction potentials of liquids used in simulations are optimized to describe the experimental properties of neat liquids. It does not necessarily follow that the same models will also describe the behavior of mixtures satisfactorily. Before we can infer details about molecular behavior with confidence, we must first verify that simulations reproduce experimental results. However, we are not aware of any studies comparing the experimental intermolecular spectra of dipolar mixtures to spectra calculated using MD simulations. Our purpose in this study is to use MD simulations to interpret the experimental spectra of acetone/methanol, acetonitrile/methanol, and acetone/acetonitrile mixtures that were reported in the preceding paper. MD simulations of the same mixtures were performed to calculate dynamic properties related to the spectra. The structure and dynamics of the mixtures, particularly the effects of hydrogen bonding, will be examined. Since we find good agreement between the simulations and experiment, we are able to extract information about the microscopic behavior of the mixtures from the simulations.

Structure and dynamics of nonaqueous mixtures of dipolar liquids. I. Infrared and far-infrared spectroscopy

Mixtures of acetone/methanol, acetonitrile/methanol, and acetone/acetonitrile over their whole composition range were studied with infrared and far-infrared (THz) spectroscopy. Experimental spectra of all mixtures were obtained below 120 cm−1, and spectra of methanolic mixtures were also measured from 400 to 1000 cm−1. The mixtures display ideal absorption spectra below 120 cm−1, contrasting with significant nonidealities in the absorption of aqueous mixtures in the same frequency range. Dramatic changes as a function of composition were found in methanolic mixtures at higher frequencies. The high frequency librational band of methanol, centered at 670 cm−1, shifts to substantially lower frequencies upon dilution, indicating marked changes in the librational motion of the hydroxyl hydrogen of methanol. This band is a sensitive probe of the hydrogen bonding environment experienced by methanol molecules.

Long optical cavities for open-path monitoring of atmospheric trace gases and aerosol extinction

An incoherent broadband cavity-enhanced absorption spectroscopy setup employing a 20 m long optical cavity is described for sensitive in situ measurements of light extinction between 630 and 690 nm. The setup was installed at the SAPHIR atmospheric simulation chamber during an intercomparison of instruments for nitrate (NO(3)) radical detection. The long cavity was stable for the entire duration of the two week campaign. A detection limit of approximately 2 pptv for NO(3) in an acquisition time of 5 s was established during that time. In addition to monitoring NO(3), nitrogen dioxide (NO(2)) concentrations were simultaneously retrieved and compared against concurrent measurements by a chemiluminescence detector. Some results from the campaign are presented to demonstrate the performance of the instrument in an atmosphere containing water vapor and inorganic aerosol. The spectral analysis of NO(3) and NO(2), the concentration dependence of the water absorption cross sections, and the retrieval of aerosol extinction are discussed. The first deployment of the setup in the field is also briefly described.

Near-ultraviolet absorption cross sections of nitrophenols and their potential influence on tropospheric oxidation capacity.

Nitrophenols and methylnitrophenols have been identified as photolytic precursors of nitrous acid, HONO, but their gas-phase absorption has not previously been reported. In this study, the absorption cross sections of 2-nitrophenol, 3-methyl-2-nitrophenol, and 4-methyl-2-nitrophenol were measured from 320 to 450 nm using incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS). The benzaldehyde absorption spectrum was measured to validate the approach and was in good agreement with literature spectra. The nitrophenol absorption cross sections are large (ca. 10(-17) cm(2) molecule(-1)) and blue-shifted about 20 nm compared to previously measured solution spectra. Besides forming HONO, nitrophenol absorption influences other photochemistry by reducing the available actinic flux. The magnitudes of both effects are evaluated as a function of solar zenith angle, and nitrophenol absorption is shown to lower the photolysis rates of O(3) and NO(2).

Reduction of tungsten oxides with carbon monoxide

Abstract The reduction of WO 3 with CO was studied using thermogravimetry, evolved gas analysis, X-ray powder diffraction, and scanning electron microscopy. The intermediate phases W 20 O 58 , W 18 O 49 , and WO 2 were observed in the reduction. The final product of the reduction with CO was WC, compared with the tungsten formed when hydrogen and/or carbon was used. The reactant-to-product gas ratio has a considerable influence on the reactions taking place. The morphology of the sample was characterised at different stages of the reduction. The kinetics and mechanism of the reduction of WO 3 with CO were studied in isothermal experiments, from 650 to 900°C. Reduction occurred at a phase boundary with an activation energy of 40 kJ mol −1 . The reduction of WO 2 was studied under similar conditions. The reaction also occurred at a phase boundary and had an activation energy of 62 kJ mol −1 .

Reduction of tungsten oxides with hydrogen and with hydrogen and carbon

Abstract The reductions of WO 3 and of WO 3 -graphite mixtures with hydrogen were studied in isothermal experiments in a tube furnace from 575 to 975°C, using evolved gas analysis, X-ray powder diffraction, and scanning electron microscopy. The intermediate phases W 20 O 58 , W 18 O 49 , and WO 2 were observed in the reductions. The final product of the reductions with hydrogen and carbon was tungsten. The reactant/product gas ratio had a considerable influence on the reactions occurring. The morphology of the sample was characterised at different stages of the reduction. Some particle growth was observed in the reduction with hydrogen and was attributed to the formation of WO 2 (OH) 2 (g), but the sizes and shapes of the tungsten particles produced were not greatly affected by the presence of carbon. The reactions were controlled by mass-transfer under the conditions investigated. The addition of carbon increased the rate of the reduction process, but did not affect the phases formed in the system. CO 2 was evolved mainly at the start, and CO mainly at the end of the process. The reaction mechanisms were determined on the basis of the evolved gas analyses. Results are compared with results obtained in other studies of the reduction using carbon alone and carbon monoxide.

Enhanced Volatile Organic Compounds emissions and organic aerosol mass increase the oligomer content of atmospheric aerosols

Secondary organic aerosol (SOA) accounts for a dominant fraction of the submicron atmospheric particle mass, but knowledge of the formation, composition and climate effects of SOA is incomplete and limits our understanding of overall aerosol effects in the atmosphere. Organic oligomers were discovered as dominant components in SOA over a decade ago in laboratory experiments and have since been proposed to play a dominant role in many aerosol processes. However, it remains unclear whether oligomers are relevant under ambient atmospheric conditions because they are often not clearly observed in field samples. Here we resolve this long-standing discrepancy by showing that elevated SOA mass is one of the key drivers of oligomer formation in the ambient atmosphere and laboratory experiments. We show for the first time that a specific organic compound class in aerosols, oligomers, is strongly correlated with cloud condensation nuclei (CCN) activities of SOA particles. These findings might have important implications for future climate scenarios where increased temperatures cause higher biogenic volatile organic compound (VOC) emissions, which in turn lead to higher SOA mass formation and significant changes in SOA composition. Such processes would need to be considered in climate models for a realistic representation of future aerosol-climate-biosphere feedbacks.

Reduction of tungsten oxides with carbon. Part 1: Thermal analyses

The kinetics and mechanism of the reduction of WO3 with carbon (in the form of graphite and of lamp black) were studied using isothermal thermogravimetry of small sample masses (< 50 mg) in the temperature range 935 to 1100°C. Two stages were observed in the reduction. The first stage corresponds approximately to the formation of WO2 and the final product of the reduction was tungsten. The COCO2 ratio in the gaseous products had a considerable influence on the reactions occurring. The rate of the first stage of the reduction under isothermal conditions could be described by diffusion models, and is proposed to involve diffusion of CO(g) and CO2(g) through the pores of the reacting tungsten oxides. The activation energies of the graphite and lamp black systems differed significantly for this first stage of reduction (386 compared to 465 kJ mol−1). These activation energies are high for a diffusion process and may be inflated by changes in the structure of the product and the COCO2 equilibrium ratio as the temperature increases. The rate of the second stage of reaction can be described by a first-order rate equation, and it is proposed that the second stage of reaction is limited by the reaction of carbon with carbon dioxide, rather than by the reduction of a tungsten oxide. The measured activation energy of 438 kJ mol−1 is slightly higher than the reported values for the carbon-carbon dioxide reaction (up to 400 kJ mol−1).