Shock Tube Study of High-Temperature Reactions of Cyclopentadiene

The thermal decomposition of cyclopentadiene has been studied in the temperature range 1260–1530 K behind reflected shocks. The total pressure ranged from 1.5 to 2.3 bar. Resonance absorption was used to record the temporal concentration profiles of H atoms. This sensitive technique allowed the study of the reaction systems under favorable conditions by applying very low initial concentrations (0.5–8 ppm). For cyclopentadiene decomposition R1, C₅H₆ → C₅H₃-c + H, a rate expression of k₁ = 1.1 × 10¹⁵ exp(–38760/T) s⁻¹ was deduced. In a separate series of experiments the consumption by cyclopentadiene of H atoms, which had been generated by the thermal decay of ethyl iodide, was investigated. A preliminary value of k₂ = 1.4 × 10¹⁴ exp(–2739/T) cm³ mol⁻¹ s⁻¹ was deduced for the total rate of H-atom consumption by cyclopentadiene R2, C₅H₆ + H → products.     

A Shock Tube Study of the Reactions of H Atoms with COS,CS2, and H2S

Reactions of H atoms with COS, CS₂, and H₂S were studied behind reflected shock waves at temperatures between 1170 K and 1830 K and pressures around 1.0 bar by applying atomic resonance absorption spectroscopy (ARAS) for time-resolved measurements of H atoms at L_α. The thermal decomposition of a few ppm ethyl iodide (C₂H₅I) was used as a H-atom source. In the presence of a large excess of the molecular reactant COS, CS₂, or H₂S, a consumption of H was observed which follows a pseudo first-order rate law. Rate coefficients for the reactions: were determined to be: k₁ = 2.4 × 10¹⁴exp(–3415 K/T) cm³mol⁻¹s⁻¹ k₂ = 1.4 × 10¹⁵exp(–9250 K/T) cm³mol⁻¹s⁻¹ k₃ = 2.5 × 10¹⁴exp(–2890 K/T) cm³mol⁻¹s⁻¹     

Thermal Isomerization of Azulene. Single-Pulse Shock Tube Investigation

The thermal isomerization of azulene was studied behind reflected shocks in a pressurized driver single-pulse shock tube. The temperature range covered was 1050–1400 K at overall densities of ∼2.5 × 10⁻⁵ mol/cm³. The main reaction of azulene under these conditions is a unimolecular isomerization to naphthalene, but it also isomerizes, although at a much lower rate, to another isomer. The suggested “tetracyclic triene” intermediate structure for the azulene-naphthalene isomerization can lead also to transition states that can describe isomerizations to 1-methylene-1H-indene and 1, 2,3-metheno-1H-indene,2,3-dihydro. Small quantities of C₂H₂, C₄H₂, C₆H₆, and C₆H₅-C≡CH were also found in the post-shock samples, particularly at high temperatures. The Arrhenius parameters of the two high pressure limit rate constants for the isomerization processes are: azulene → naphthalene, k₁ = 10^12.93 exp(–62.8 × 10³/RT) s⁻¹ azulene → second isomer, k₂ = 10^12.42 exp(–69.5 × 10³/RT) s⁻¹ A discussion of the mechanism for these isomerization processes is presented.     

Temperature Jump Measurements in Gas Kinetics

The temperature jump relaxation method is applied to kinetic measurements in the gas phase. The system N₂O₄⇄2NO₂ is studied. Fast T-jumps are induced by IR absorption from a TEA CO₂ laser. Temperature jumps of less than 1 K lead to readily detectable concentration changes which are monitored by UV absorption in a simple single shot experiment. k_rec(Ar) is found to be 1.05 × 10¹⁰ cm³ mol⁻¹ s⁻¹ at P_Ar = 1 bar and T = 258 K, in agreement with recent experimental data.     

Temperature effect on diffusion of carbon dioxide through upper layers of Yucca Mountain

The effective diffusivities of carbon dioxide through the Tiva Canyon tuff and the lower lithophysal zone of the Topopah Spring tuff (outcrop samples of the layer above the proposed nuclear repository site layer) were determined using a steady-state method (counter diffusion). The diffusivity of carbon dioxide through the Tiva Canyon and lithopysal zone tuffs increased with temperature. The following correlation was obtained to estimate the effective diffusivity of carbon dioxide through the Tiva Canyon tuff in cm² s⁻¹ as a function of temperature (K). D_e = 1.22 × 10⁻² − 3.77 × 10⁻⁵T + 9.95 × 10⁻⁸T²The effective diffusivity of carbon dioxide through the lower lithophysal zone of the Topopah Spring tuff (layer right above the proposed repository site) also increased with temperature. For this layer, the following correlation was obtained to estimate the effective diffusivity of carbon dioxide in cm² s⁻¹ as a function of temperature (K). D_e = − 1.11 × 10⁻³ + 1.25 × 10⁻⁵T + 1.83 × 10⁻⁹T²     

Membrane transport of organics. II. Permeation of some carboxylic acids through strongly basic polymer membrane

The permeability characteristics of the strongly basic polymer membrane Neosepta® AFN‐7, (Tokuyama Soda) have been studied for acetic, propionic, lactic, tartaric, oxalic, and citric acid. The results were interpreted by using the model of transport in reactive membranes. The specific constants, that is, the maximum flux J_max, the reactivity constant K, and the permeability coefficient (P), were calculated using the experimental quasi‐stationary fluxes and the equation derived as a sum of reaction–diffusion (Michaelis–Menten‐type), and the solution–diffusion transport equation. The constants K and J_max were found to range from 0.1 to 5 dm³ mol⁻¹ and from 0.4 × 10⁻⁷ to 2.5 × 10⁻⁷ mol cm⁻² s⁻¹ depending, on the acid properties. The values of K and J_max were correlated with the dissociation constants K_dis.acid, and the diffusion coefficients D_aq.acid in aqueous media, respectively. It was found that the reaction–diffusion flux is predominating for all acids, except for the lactic one, when the feed concentration is lower than 0.5 mol dm⁻³. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2179–2190, 1999     

Structural and Thermal Characterization of A Novel High Nitrogen Energetic Material: (NH4)2DNMT

1,4‐Dihydro‐5H‐(dinitromethylene)‐tetrazole ammonium salt ((NH₄)₂DNMT), a high nitrogen energetic compound, was synthesized and structurally characterized by single‐crystal X‐ray diffraction. The thermal behavior of (NH₄)₂DNMT was studied with DSC and TG‐DTG methods. The kinetic equation of the thermal decomposition reaction is: dα/dT=10^13.17/3β(1−α)⁻² exp(−1.388×10⁵/RT). The critical temperature of thermal explosion is 182.7 °C. The specific heat capacity of (NH₄)₂DNMT was determined and the molar heat capacity is 301 J mol⁻¹ K⁻¹ at 298.15 K. The adiabatic time‐to‐explosion of (NH₄)₂DNMT was calculated to be 277 s. The detonation velocity and detonation pressure were also estimated. All results showed that (NH₄)₂DNMT presents good performance.     

Kinetic and thermodynamic studies on oxidative desulphurisation of organic sulphur from Indian coal at 50–150°C

This paper reports a relatively simple low-temperature non-isothermal oxidative desulphurisation of coal organic sulphur by weakening the CS bond using HgCl₂ solution to an inorganic sulphur-free high-sulphur Indian coal. When oxidised from 50°C to 150°C in air under normal atmospheric pressure, there is continuous decrease of organic sulphur content in the samples of the feed and Hg-treated coals. Desulphurisation is more in the Hg-treated coal (4.97–14.53 wt.%) than in the feed coal (3.72–10.93 wt.%). Kinetic study reveals that the oxidative desulphurisation process follows pseudo-first order kinetics and the rate constants have been found to be in the range (3.09–5.06)×10⁻⁵ s⁻¹ for feed coal and (4.19–6.80)×10⁻⁵ s⁻¹ for Hg-treated coal. The activation energies for the sulphur loss reaction in the oxidative desulphurisation process by using the pseudo-first order kinetic (feed coal: 2.21×10² J mol⁻¹; Hg-treated coal: 1.53×10² J mol⁻¹) have been found to be almost similar to those calculated by applying the Coats and Redfern's equation (feed coal: 2.19×10² J mol⁻¹; Hg-treated coal: 1.53×10² J mol⁻¹). However, the value is higher (feed coal: 3.50×10² J mol⁻¹; Hg-treated coal: 2.70×10² J mol⁻¹) when Horowitz and Metzger's equation is applied. The frequency factors computed by the pseudo-first order kinetics are very low and have been found to be 2.66×10⁻⁵ s⁻¹ for feed coal and 3.96×10⁻⁵ s⁻¹ for Hg-treated coal, suggesting very low rate of successful collisions for the formation of the activated complex. Evaluation of thermodynamic parameters viz., ΔH, ΔU, ΔS and ΔG, reveals that this oxidative desulphurisation process is non-spontaneous in nature and the degree of non-spontaneity of such a process in the feed coal is more relative to that of the Hg-treated coal.     

The role of mass transfer in the electrolytic reduction of hexavalent chromium at gas evolving rotating cylinder electrodes

The rate of electrolytic reduction of hexavalent chromium from acidic solution at a hydrogen-evolving rotating cylinder lead cathode was studied under conditions of different current densities, Cr⁶⁺ concentrations and rotation speeds. The rate of the reaction was found to follow a first order rate equation. The specific reaction rate constant was found to increase with increasing rotation speed until a limiting value was reached with further increase in rotation speed. Mechanistic study of the reaction has shown that at relatively low rotation speeds the reduction of Cr⁶⁺ is partially diffusion controlled, at higher speeds the reaction becomes chemically controlled. The limiting specific reaction rate constant was related to the operating current density by the equationK=0.044i ^1.385. The current efficiency of Cr⁶⁺-reduction was measured as a function of current density, initial Cr⁶⁺ concentration and rotation speed. Possible practical applications are discussed.Nomenclature A electrode area (cm²) - a, b constants in Equations 5 and 13, respectively - C bulk concentration of Cr⁶⁺ at timet(M) - C _o initial concentration of Cr⁶⁺ (M) - C _i interfacial concentration of Cr⁶⁺ (M) - d cylinder diameter (cm) - D diffusivity of Cr⁶⁺ (cm² s^–1) - e _o standard electrode potential (V) - F Faraday's constant (96 487 C) - current consumed in hydrogen discharge (A) - i current density (A cm^–2) - I cell current (A) - K _l mass transfer coefficient (cm s^–1) - K _r mass transfer coefficient due to cylinder rotation (cm s^–1) - K _o natural convection mass transfer coefficient (cm s^–1) - K _g mass transfer coefficient due to hydrogen stirring (cm s^–1) - K ₂ specific reaction rate constant (cm s^–1) - K overall rate constant (cm s^–1) - m theoretical amount of Cr⁶⁺ reduced during electrolysis (g) - P gas pressure (atm) - R gas constant (atm cm³ mol^–1 K^–1) - T temperature (K) - t time (s) - V linear speed of the rotating cylinder (cm s^–1) - hydrogen discharge rate (cm³ cm^–2 s^–1) - V _s solution volume (cm³) - z electrochemical equivalent (g C^–1) - Z number of electrons involved in the reaction - Re Reynolds number (Vd/v) - Sh Sherwood number (K _r d/D) - Sc Schmidt number (v/D) - rotation speed (r.p.m.) - kinematic viscosity (cm² s^–1)     

Syntheses and gas-transport properties of alkylsilane-modified SBS membranes

Alkylsilane-modified styrene-butadiene-styrene (SBS) polymers were prepared via two steps. In the first step, dimethylchlorosilane HSi(CH₃)₂Cl and SBS (Korea Kumho Petrochem. Co.; 35% vinyl content; M_o = 48,000, M_o/M_n = 1.05) were reacted in toluene using H₂PtCl₆ catalyst to produce a hydrosilylated polymer. In the second step, chloride in the silyl group was replaced with alkyl by reaction with RLi (R = Me, n-Bu). Under severe conditions, silylation occurs on all the double bonds of the polymer so that their elastomeric nature disappears. However, under mild conditions, selective hydrosilylation of the pendant vinyl groups of SBS could be accomplished so that the polymer retained its elastomeric properties with two low T_g values. These modified SBS polymers were easily dissolved in THF, and thus could be formed into film-type membranes by solvent casting on teflon plates. From the measurements of their gas permeability for O₂ and N₂, they showed higher selectivity for O₂ compared with the unmodified SBS. The permeability for oxygen was dependent on the size of the alkylsilyl group. Methylsilane and butylsilane-modified SBS membranes showed the higher permeability values of 31.2 and 37.5 barrer (1 barrer = 1 × 10⁻¹⁰ cm³ (STP) cm cm⁻² s⁻¹ cmHg⁻¹) at 30°C, respectively, compared with 2.39 barrer of the unmodified SBS. Furthermore, their selectivities for O₂ over N₂ were increased to 3.0 and 3.2, respectively, from 2.5 of the unmodified SBS case. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1117–1122, 1997     

Polymerization of methyl methacrylate by imidazole–carbon tetrachloride charge-transfer system

The kinetics of charge-transfer (CT) polymerization of methyl methacrylate (MMA) in the presence of imidazole (Imy) and CCl₄ was studied in dimethyl sulfoxide (DMSO) at 60°C. The rate of polymerization (R_p) is sensitive to the [CCl₄] up to a concentration of 0.60 mol L⁻¹, but at a higher concentration, it is practically independent of the [CCl₄]. When [CCl₄] > [Imy], R_p is proportional, to [MMA]^1.45±0.15 and [Imy]^0.53±0.04 and the average rate constant for the polymerization of MMA is 3.25±0.41 × 10⁻⁶ L mol⁻¹ s⁻¹. This article also reports the polymerization of MMA initiated by Imy and CCl₄ and accelerated by hexakis (dimethyl sulfoxide) iron (III) perchlorate, [Fe(DMSO)₆] (CIO₄)₃ (A), at 60°C. The presence of Fe(Imy)³⁺₃ in the polymerization system produced well-defined induction periods. The rate constant at 60°C for the interaction of the poly(MMA) radical toward Fe(Imy)³⁺₃ is 7.19 × 10⁴ L mol⁻¹ s⁻¹. A probable reaction mechanism for the polymerization system has been postulated to explain the observed results. © 1996 John Wiley & Sons, Inc.     

Radical polymerization in disperse systems,by J. Bartoň, I. Capek. Ellis Horwood Ltd,Chichester/VEDA,Bratislava, 1994. pp. 352, price £68. ISBN 0-13-752353-X

Radical copolymerization of zinc acrylate (ZnA₂) with acrylonitrile (AN), initiated by As₂S₃–styrene complex(I), in dimethyl Sulphoxide (DMSO) at 90 ± 0.1°C for 1.0h under inert atmosphere, yields non-alternating copolymers. The kinetic expression is R_p ∝ [I]^0.33 [ZnA₂]^0.25 [AN]^0.44, i.e. the system follows non-ideal kinetics, which is due to primary radical termination as well as degradative chain transfer reactions. The values for activation energy (E) and k²_p/k_t are 128kJ mol⁻¹ and 8.57 × 10⁻⁷ litre mol⁻¹ s⁻¹, respectively. Thermal stability, solubility in different solvents, and IR and NMR spectra have been evaluated.     

Tetrakis(nitratoxycarbon)methane (Née CLL‐1) as a Potential Explosive Ingredient: a Theoretical Study

Ab initio electronic structure calculations at the MP2/cc‐pVTZ level predict the vibrational stability of the theoretical molecule tetrakis(nitratoxycarbon)methane, designated CLL‐1. The gas phase enthalpy of formation, predicted to be +1029.3 kJ mol⁻¹ using the G3(MP2) method, and the estimated density of 1.87 g cm⁻³ are used to predict the explosive performance properties using the equilibrium thermochemical code CHEETAH. The predicted detonation velocity (8.61 km s⁻¹) and pressure (33.1 GPa) are similar to those of RDX, but with a significantly higher detonation temperature (6740 K). Finally, the stability of this theoretical molecule is investigated by calculating the lowest energy unimolecular decomposition pathways of the HCO₃N model compound as well as barriers to rearrangement upon interaction of two HCO₃N molecules.     

Thermochemistry of Evaporation and Sublimation of 2,4,6‐Triazido‐1,3,5‐triazine

Saturated vapor pressure over cyanuric triazide melt was measured in the temperature range 393.15–453.15 K using glass membrane Bourdon pressure gauges. Measured evaporation heat and evaporation entropy are equal to 61.1±3.3 kJ mol^–1 and 111.3±6.1 J mol^–1 K^–1, respectively. According to DSC data, melting heat and melting entropy of cyanuric triazide are 22.2±1.3 kJ mol^–1 and 60.5±3.5 J mol^–1 K^–1, respectively. Based on the results obtained, the following dependence was found for the saturated vapor pressure over solid cyanuric triazide: P _s [Pa]=10^14.0 ⋅ exp[(–83300±3300)/RT ]. Concequently, sublimation heat and sublimation entropy of cyanuric triazide are equal to 83.3±3.3 kJ mol^–1 and 171.8±9.6 J mol^–1 K^–1, respectively. Saturated vapor pressure over solid cyanuric triazide at room temperature is equal to 0.25 Pa (1.9×10⁻³ Torr). It is concluded that high volatility of cyanuric triazide is caused by its low sublimation heat.     

Homogeneous liquid–liquid extraction using perfluorooctanesulfonic acid and calcium and its application to the separation and recovery of vitamin B12

A new phase separation phenomenon was observed in which the perfluorooctanesulfonate ion (PFOS⁻) and calcium ion form an ion‐pair associator and the sedimented liquid phase occurs from the homogeneous aqueous solution. This phenomenon was observed in the neutral pH region at room temperature (25 °C). The optimum concentration conditions for the reagents were [PFOS⁻]_T = 7 × 10⁻³ mol dm^‐3 and [Ca²⁺]_T = 1.1 mol dm^‐3. When these findings were applied to the homogeneous liquid–liquid extraction of vitamin B₁₂, the extraction percentage (E) was 83% and the concentration ratio (ie V_a/V_s, where V_a is the volume of the aqueous phase and V_s is the volume of the sedimented liquid phase) was a maximum of 149. The recovery of vitamin B₁₂ was achieved by adding the propanol–acetone (20 : 80 v/v%) mixed solvent to the sedimented liquid phase; the vitamin B₁₂ precipitated and was filtered. Both the PFOS⁻ and Ca²⁺ were removed by dissolution in the mixed solvent. The recovery percentage of vitamin B₁₂ was 78%. © 1999 Society of Chemical Industry     

The Dipole Correction Method for Extracting Excited State Potentials and Electronic Transition Dipoles from Fluorescence Data

We have developed a new inversion scheme for the accurate extraction of excited state potentials from fluorescence line positions and line strengths which does not make use of the Franck Condon Approximation (FCA). Our “dipole correction” method also enables the extraction of the coordinate dependence of the electronic transition dipoles. The accuracy of the potential energy surfaces (PES) thus extracted is much higher than that of the FCA- derived PES. The procedure, illustrated for the Na₂ A(¹Σ⁺_u) → X(¹Σ⁺_g) P-branch emission, results in global errors of 0.1 cm⁻¹, and average errors near the PES minimum of 0.03 cm⁻¹, with A → X electronic transition dipole function accuracies better than 1×10⁻³ Debye. We also show that it is possible to use emission data from a few select states: Global errors as small as 0.08 cm⁻¹ for the Na₂ B(¹Π_u) PES, using emissions data from only the s=0–5 low-lying levels or the s=20–23 states, are demonstrated.     

Effect of processing parameters on microstructure and ablation behavior of SiC/ZrB2 coating for graphite

In this research, a SiC/ZrB₂ coating was produced on graphite by reactive melt infiltration and plasma spraying method. The coating characterization was performed using XRD analysis, electron microscopy equipped with energy dispersive spectrometer (EDS), and supersonic flame ablation test at 2073 K. The results indicated that the dense C/SiC coating with good ablation resistance can be obtained at 1873 K. The coating thickness decreased with increasing infiltration temperature. The results of ablation test showed that by increasing the infiltration temperature and holding time, weight loss and mass ablation rate decreased from 22.63% to 9.83% and 3.63 × 10⁻³ g cm⁻² s⁻¹ to 1.34 × 10⁻³ g cm⁻² s⁻¹, respectively. The results showed that by using the ZrB₂ as outer coating the ablation resistance improved remarkably. The weight loss and mass ablation rates for the SiC/ZrB₂ coating were 12.79% and 1.857 × 10⁻³ g cm⁻² s⁻¹, respectively.     

Poly(L‐lactide): v. effects of storage in swelling solvents on physical properties and structure of poly(L‐lactide)

The effects of storage at 25°C in swelling solvents having different solubility parameter (δ_s) values of 16.8–26.0 J^0.5 cm^−1.5 on the physical properties and structure of as‐cast poly(L ‐lactide) (PLLA) films was investigated by the degree of swelling (DS), differential scanning calorimetry (DSC), and tensile tests. It was found that PLLA film shows durabity to swelling solvents having δ_s values much lower or higher than the value range of 19–20.5 J^0.5 cm^−1.5 and that the polymer solubility parameter (δ_p) for PLLA is in the value range of 19–20.5 J^0.5 cm^−1.5. The decrease in the glass transition temperature (T_g) and tensile properties and the increase in melting temperature (T_m) and crystallinity (x_c) were larger for PLLA films swollen in solvents having a high DS at 7 days (DS_7days). The slight increase in T_m and x_c for PLLA films after swelling in solvents with high DS_7days values was due to the crystallization of PLLA that occurred during swelling, while the small increase in T_g and elongation at break (ε_B) for PLLA films after immersion in the solvents having low DS_7days values was ascribed to stabilized chain packing in the amorphous region. The T_g, ε_B, and Young's modulus of the PLLA films after swelling in the solvents varied in the ranges of 47–57°C, 4–8%, and 55–77 kg/mm², depending on their DS_7days or δ_s values. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1582–1589, 2001     

Cytarabine trapping in poly(2-hydroxyethyl methacrylate-co-acrylamide) hydrogels: drug delivery studies

Copolymeric hydrogels of poly(2-hydroxyethyl methacrylate-co-acrylamide) [p(HEMA-co-A)] crosslinked with ethylene glycol dimethacrylate, with a high equilibrium degree of swelling (37–65 wt%) in saline solution (NaCl 0.9 wt%) were synthesized as devices for controlled release of cytarabine (ara-C). Two compositions of the copolymer, each with a different degree of crosslinking have been studied, HEMA80/A20 and HEMA60/A40. The antineoplasic drug was included in the feed mixture of polymerization, and discs 3.7 ± 0.4 mm thick and 11.8 ± 0.2 mm in diameter with 5–40 mg (1.0–8.3 wt%) of ara-C were obtained. The diffusion studies followed Fick's second law. The diffusion coefficients for swelling of the gels were between 3.60 × 10⁻¹¹ and 15.8 × 10⁻¹¹ m² s⁻¹; those for release of ara-C were between 0.31 × 10⁻¹¹ and 7.18 × 10⁻¹¹ m² s⁻¹. The activation energies for swelling were in the range 23.4–31.9 kJ mol⁻¹ and those for ara-C release were 42.2–61.6 kJ mol⁻¹; their values indicate that the drug release process depends on drug–matrix and drug–water interactions that are influenced by the aqueous solution content and the network size of the gels. Total release of the drug takes place between 17 h from H60/A40/E2 at 310 K and 6 days from H80/A20/E10 at 288 K. Ara-C degradation was not observed either during loading of the gels or during drug release. © 1999 Society of Chemical Industry