Temperature dependence of the Fricke dosimeter and spur expansion time in the low-LET high-temperature radiolysis of water up to 350 °C: a Monte-Carlo simulation study

Monte-Carlo simulations of the radiolysis of the ferrous sulfate (Fricke) dosimeter with low-linear energy transfer (LET) radiation (such as (60)Co γ-rays or fast electrons) have been performed as a function of temperature from 25 to 350 °C. The predicted yields of Fe(2+) oxidation are found to increase with increasing temperature up to ～100-150 °C, and then tend to remain essentially constant at higher temperatures, in very good agreement with experiment. By using a simple method based on the direct application of the stoichiometric relationship that exists between the ferric ion yields so obtained G(Fe(3+)) and the sum {3 [g(e(-)(aq) + H˙) + g(HO(2)˙)] + g(˙OH) + 2 g(H(2)O(2))}, where g(e(-)(aq) + H˙), g(HO(2)˙), g(˙OH), and g(H(2)O(2)) are the primary radical and molecular yields of the radiolysis of deaerated 0.4 M H(2)SO(4) aqueous solutions, the lifetime (τ(s)) of the spur and its temperature dependence have been determined. In the spirit of the spur model, τ(s) is an important indicator for overlapping spurs, giving the time required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solution. The calculations show that τ(s) decreases by about an order of magnitude over the 25-350 °C temperature range, going from ～4.2 × 10(-7) s at 25 °C to ～5.7 × 10(-8) s at 350 °C. This decrease in τ(s) with increasing temperature mainly originates from the quicker diffusion of the individual species involved. Moreover, the observed dependence of G(Fe(3+)) on temperature largely reflects the influence of temperature upon the primary free-radical product yields of the radiolysis, especially the yield of H˙ atoms. Above ～200-250 °C, the more and more pronounced intervention of the reaction of H˙ atoms with water also contributes to the variation of G(Fe(3+)), which may decrease or increase slightly, depending on the choice made for the rate constant of this reaction. All calculations reported herein use the radiolysis database of Elliot (Atomic Energy of Canada Limited) and Bartels (University of Notre Dame) that contains all the best currently available information on the rate constants, reaction mechanisms, and g-values in the range 20 to 350 °C.     

Isotopic effect in the radiolysis of water. Diffusion-kinetic modelling up to 300°C

Diffusion-kinetic calculations [1-3] have been analysed to determine the isotopic effect in the radiolysis of water with ionising radiation of linear energy transfer characteristics (LET) from 0.2 to 60 eV/nm and at temperatures up to 300°C. This analysis shows that, for low LET radiation, the spur decay of e^- _aq is slower in D₂O and results in a higher yield of e^- _aq, g(e^- _aq), at 10^-7 -10^-6s after the ionisation event. In low LET radiolysis, g(OD) ≈ g(OH) over the whole range of temperature but in high LET radiolysis g(OD) is clearly lower than g(OH). The isotopic effect on the yields of the radical products is enhanced by increasing LET but diminished by increasing temperature. The yields of the molecular products show the opposite isotopic effect to their radical precursors, namely g(D₂) is 10-20% lower than g(H₂) and g(D₂O₂) > g(H₂O₂). A particularly significant difference between g(D₂O₂) and g(H₂O₂) has been found at LET = 20 eV/nm. The isotopic dependence of the g-values estimated for fast neutron radiolysis is also presented.     

Effect of pressure on the solubility of naphthalene in water at 25°C

Solubility of naphthalene in water was measured at 25°C and pressures up to 200 MPa. The solubility decreased with increasing pressure. From the pressure coefficient of the solubility, the volume change V accompanying the dissolution was estimated as 13.8±0.4 cm ³ -mol ^–1 . Further we estimated the volume change V _CH accompanying hydrophobic hydration as –0.1±0.6 cm ³ -mol ^–1 using the V value, the molar volume of crystalline naphthalene, and the partial molar volume of naphthalene in n-heptane. This V _CH is much larger (i.e., less negative) than that for hydrophobic hydration of alkyl-chain compounds and suggests that the hydration structure of naphthalene differs from that of alkyl-chain compounds.     

A new determination of the structure of water at 25°C

The results of a new neutron diffraction experiment to measure the structure of water are presented. The data, measured at the McMaster Nuclear Reactor, are of a high quality and are analysed to yield the hydrogen-hydrogen pair correlation function using a subtraction procedure which has been used in previous experiments of this kind. This procedure circumvents the necessity of applying inelasticity corrections. The results are in good agreement with earlier work and serve to establish the general correctness of the subtraction procedure when used to determine hydrogen correlations. The data are further analysed to yield separate oxygen-hydrogen and oxygen-oxygen partial structure factors for liquid water. For the second part of the analysis an effective mass model of the dynamic scattering law is used, with the model parameter, the effective mass of the scattering particle, chosen by a least-squares fit to the measured differential cross sections. The final pair correlation functions are obtained using a maximum entropy analysis of the structure functions.     

Calculating the permittivity and polarizability of methanol–water mixtures at 20°C

The static permittivity and polarizability of methanol–water mixtures as functions of composition at 20°C are calculated on the basis of models proposed earlier.     

Measurement of the hall mobility of injected electrons in liquid tetramethylsilane between 22 and 164 °C

We have measured the Hall mobility of excess electrons in liquid tetramethylsilane (TMS) between room temperature and 164°C. The novel result is that, within an experimental error of ≈10%, the Hall mobility μ_h is equal to the drift mobility μ_d; μ_h = 101 ± 7 cm²/V s at 22°C and 53 ± 5 cm²/Vs at 164°C. This indicates that, contrary to what is observed in neopentane, traps are unimportant and, in particular, are not responsible for the mobility drop observed close to the critical point in TMS.     

The enthalpies of interaction of glycine with some amides and ureas in water at 25°C

Measurement of solution enthalpyiesof glycine in aqueous solutions of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), monomethylurea (MMU), 1,3-diethylurea (DEU) and tetramethylurea (TMU) at 25°C have been undertaken. On the basis of the results, enthalpic coefficients of heterotactic interactions between a glycine zwitterion and a molecule of organic substance in aqueous solutions have been calculated. Using the additivity of groups concept by Savage and Wood (SWAG), contributions of each functional group of studied amides and ureas have been estimated. In this model a zwitterion of glycine has been considered as an individual equal to a single functional group.     

Solubility in the magnesium chlorate-carbamide-water system at 50°C

The solubility of phases in the magnesium chlorate-carbamide-water system was studied by the isothermal solubility method at 50°C. The crystallization branches of carbamide, magnesium chlorate hexahydrate, Mg(ClO₃)₂ · 6CO(NH₂)₂, Mg(ClO₃)₂ · 4CO(NH₂)₂ · 2H₂O, and Mg(ClO₃)₂ · 2CO(NH₂)₂ · 4H₂O were revealed in the phase diagram.     

The influence of water on the isomerization of α-pinene in a supercritical aqueous-alcoholic solvent

The influence of water as a cosolvent and catalyst of the isomerization of α-pinene in a supercritical aqueous-alcoholic (ethanol) solvent was studied experimentally. At T = 657 K and p = 230 atm, an increase in the concentration of water in the reaction mixture was found to increase the rate of the reaction and its selectivity with respect to the desired product, limonene. Water exhibited the properties of an acid catalyst because of its ionization. Mathematical experimental data processing was performed to evaluate and separate the contributions of the radical and ionic paths to the total rate of the reactions that occurred during the thermal isomerization of α-pinene.     

Comment on the possible role of the reaction H+H2O→H2+OH in the radiolysis of water at high temperatures

In a recent paper (Radiation Physics and Chemistry, 2005, vol. 74, pp. 210) it was suggested that the anomalous increase of molecular hydrogen radiolysis yields observed in high-temperature water is explained by a high activation energy for the reaction H+H₂O→H₂+OH. In this comment we present thermodynamic arguments to demonstrate that this reaction cannot be as fast as suggested. A best estimate for the rate constant is 2.2×10³ M⁻¹ s⁻¹ at 300 °C. Central to this argument is an estimate of the OH radical hydration free energy vs. temperature, ΔG_hyd(OH)=0.0278t−18.4 kJ/mole (t in °C, equidensity standard states), which is based on analogy with the hydration free energy of water and of hydrogen peroxide.     

Interaction in the manganese sulfate-carbamide-sulfuric acid-water system at 25°C

Heterogeneous equilibria in the manganese-carbamide-sulfuric acid-water quaternary system at 25°C are studied using the solubility method. The concentration boundaries are determined for crystallization of the initial solid components, eutectic compositions of the ternary systems, and binary compounds formed in the carbamide-sulfuric acid-water and carbamide-manganese sulfate-water systems, as well as for new compounds simultaneously containing carbamide, manganese sulfate, and sulfuric acid at a 1: 4: 1 and 1: 2: 1 ratios.     

Diffusion coefficients of the system α-cyclodextrin-n-butylurea-water at 25°C

Diffusion coefficients were measured for the ternary system -cyclodextrin(I)-n-butylurea(2)-water at three average concentrations. The cross coefficient D₁₂ was found to be almost zero and D₂₁ large and negative. These results are in agreement with the presence of an inclusion complex whose mobility is close to that of the host cyclodextrin molecules. The values of the four experimental diffusion coefficients are used to compute a value of the inclusion constant which is in reasonably good agreement with the calorimetric value.     

Solution behavior of metoclopramide in aqueous-alcoholic solutions at 30°C

Densities (ρ) and refractive indices (n_D) of solutions of antiemetic drug metoclopramide (4-amino-5-chloro-N-(2-(diethylamino)ethyl)-2-methoxybenzamide hydrochloride hydrate) in methanolwater and ethanol-water mixtures of different compositions were measured at 30°C. Apparent molar volume (φ_v) of the drug was calculated from density data and partial molar volumes (φ_v⁰) were determined from Massons relation. Concentration dependence of nD has been studied to determine refractive indices of solution at infinite dilution (n_D⁰). Results have been interpreted in terms of solute-solvent interactions.     

Volumetric properties of cyclic hydrocarbons in tetrachloromethane at 25°C

By means of a vibrating-tube densimeter, the densities at 25°C have been determined for binary mixtures of tetrachloromethane with a liquid (cyclodecane, cis-decahydronaphthalene, trans-decahydronaphthalene, bicyclohexyl, pentane) or a solid hydrocarbon (cyclododecane, cyclopentadecane, norbornane, adamantane, octahydro-4,7-methano-1H-indene). Excess molar volumes have been obtained in the whole mole fraction range for mixtures containing a liquid hydrocarbon. For solid cycloalkanes, apparent molar volumes have been evaluated in the whole range of miscibility. The partial molar volumes at infinite dilution have been calculated for all examined cycloalkanes and compared with those of n-alkanes. The dependence of upon the size and shape of the ring or cage structure of the solute is discussed. The capability of the Flory theory to reproduce V^E for these mixtures is also tested.     

The Solubility of Hydrogen Sulfide in Acetonitrile at 25°C

Abstract

The solubility of hydrogen sulfide in acetonitrile at 25°C has been determined both gravimetrically and titrimetrically to be 0. 528 M. The effects of the presence of water and an electrolyte (LiClO₄) on this parameter are reported.     

Refractivity and polarizability of mixtures of L-histidine–metformin hydrochloride–water at 30°C

The molar refractivity and polarizability of mixtures of L-histidine (0.01–0.11 mol L^–1)–metformin hydrochloride (0.03, 0.05, 0.07 mol L^–1)–water were calculated from density and refractive index data at 30°C. Enhancement in the polarizability has been observed with increase in L-histidine concentration as well as metformin hydrochloride content in the solution. The molar refractivity and polarizability of solutions increased appreciably after 0.09 mol L^–1 L-histidine in each aqueous solution.     

Kinetics of the Micellar Effects on the Alkaline Hydrolysis of 2-Chloro-quinoxaline in Acetonitrile-Water Mixtures at 35°C

Pseudo-first-order reaction rate for alkaline hydrolysis of 2-chloroquinoxaline (2-CQX) is carried out in acetonitrile (AN)-water (H₂O) mixtures at 35°C. Cationic surfactants as dodecyltrimethylammonium bromide (DOTAB) and an anionic surfactant as sodium dodecylsulphate (SDS) are used above their critical micelle concentration (cmc) to study the effect of micelles on reaction rate. When increasing the percentage of volume of AN, the rate profiles with DOTAB are shown to slightly increase with increasing surfactant concentration, while that with SDS are found to smoothly decrease. The micellar effect is explained in terms of a modified pseudo-phase ion exchange model. The binding constant (K_S) between 2-CQX and DOTAB as micelle showed a decrease by increasing percentage of volume of AN, while that with SDS increased. The counterion micellar coverage degrees (β) are found to be 0.55 and 0.85 with DOTAB and SDS systems, respectively, at all range of volume percentage of AN. Finally, the calculated ratio between rate constants in water to that in the micelle region k_w/k_M at different volume percentage of AN indicated that DOTAB enhances the reaction rate while SDS inhibits it.      

Potentiometric study of the acid-base equilibrium of loprazolam at 25 °C in 0.1M NaCl medium

The acid-base behaviour of Loprazolam has been studied using an automated potentiometric titration system. The temperature was kept constant at 25 °C and the ionic medium was 0.1 mol dm^–3 NaCl in different ethanol-water mixtures (5–20%). The values of the constants obtained at the different ethanol compositions have been extrapolated to give the corresponding stoichiometric constant in water, log = 6.39 ± 0.15     

Study of the behavior of chromium(III) in potassium nitrate solutions at 25°C

Solubility in the KNO₃-Cr(NO₃)₃-H₂O system at 25°C was studied by an isothermal method. The existence of solid phases of potassium nitrate and chromium(III) nitrate nonahydrate was confirmed by constructing a phase diagram, chemical analysis, and IR spectroscopy. Crystallization of potassium nitrate from aqueous solutions was studied. Potassium nitrate does not interact with chromium(III) nitrate within the range of micro and macro concentrations. The capture of chromium(III) by KNO₃ crystals is due to adsorption and occlusion of a mother solution.     

Reply to comment on the possible role of the reaction H+H2O→H2+OH in the radiolysis of water at high temperatures

In reply to “Comment on the possible role of reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures” (Bartels, 2009 Comment on the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 78, 191–194) we present an alternative thermodynamic estimation of the reaction rate constant k. Based on the non-symmetric standard state convention we have calculated that the Gibbs energy of reaction Δ_rG=57.26 kJ mol^?1 and the reaction rate constant k=7.23×10^?5 M^?1 s^?1 at ambient temperature. Re-analysis of the thermodynamic estimation (Bartels, 2009 Comment on the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 78, 191–194) showed that the upper limit for the rate constant at 573 K is k=1.75×10⁴ M^?1 s^?1 compared to the value predicted by the diffusion-kinetic modelling (3.18±1.25)×10⁴ M^?1 s^?1 (Swiatla-Wojcik, D., Buxton, G.V., 2005. On the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 74(3–4), 210–219). The presented thermodynamic evaluation of k(573) is based on the assumption that k can be calculated from Δ_rG and the rate constant of the reverse reaction which, as discussed, are both uncertain at high temperatures.