Temperature dependence of121Sb nuclear quadrupole resonance frequencies in the spectra of M2Sb2SO4F6 (M = K,Rb, NH4)

The temperature dependence of the frequencies of¹²¹Sb nuclear quadrupole resonance in the spectra of M₂Sb₂SO₄F₆ (M = K, Rb, NH₄) was studied in the temperature range from 77 to 340 K. A secondary phase transition was found above 320 K for (NH₄)₂SO₄F₆.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1831–1833, October, 1994.This study was sponsored by the Russian Foundation for Basic Research (Grant No. 93-03-4394).     

The effect of particle size on the thermal decomposition kinetics of potassium bromate

The thermal decomposition of potassium bromate (KBrO₃) has been studied as a function of particle size, in the range 53?C150???m, by isothermal thermogravimetry at different temperatures, viz. 668, 673, 678, and 683?K in static air atmosphere. The theoretical and experimental mass loss data are in good agreement for the thermal decomposition of all samples of KBrO₃ at all temperatures studied. The isothermal decomposition of all samples of KBrO₃ was subjected to both model fitting and model-free (isoconversional) kinetic methods of analysis. Isothermal model fitting analysis shows that the thermal decomposition kinetics of all the samples of KBrO₃ studied can be best described by the contracting square equation. Contrary to the expected increase in rate followed by a decrease with decrease in particle size, KBrO₃ shows a regular increase in rate with reduction in particle size, which, we suggest, is an impact of melting of this solid during decomposition.     

Electrostatic rigidity of polyelectrolytes from reparametrization invariance

The persistence length l_p of a polyelectrolyte chain can be represented as l_p = l_O + l_e where l_O is the bare persistence and I_e is the electrostatic contribution coming from the effects of electrostatic chain self-interactions. Using a reparametrization-invariant path integral model of semiflexible polymers we find that I_e depends on the ionic strength I as I_e ∼ I^−1/2. This result accords with experimental observations and recent Monte Carlo simulations. Reparametrization-invariance is apparently an essential constrainet in selecting acceptable models of semiflexible polymers.     

Surface tension measurements of imidazolium-based ionic liquids at liquid–vapor equilibrium

A series of high quality 1-alkyl-3-methylimidazolium-based ionic liquids are synthesized and used for studying their surface tension. The capillary rise method is used for measuring the surface tension of I⁻, Cl⁻, PF₆⁻, and BF₄⁻ salts in the temperature range 298–393 K. The capillary apparatus is evacuated and sealed under vacuum. The experimental results show that surface tension of these compounds depend systematically on temperature.     

Aqueous Polymerization of Methyl Methacrylate

Aqueous polymerization of methyl methacrylate (MMA), initiated by the potassium bromate-thioglycollic acid (TGA) redox system, has been studied at 30 ± 0.2° C under positive pressure of nitrogen. The rate is given by K[MMA] [TGA] ⁰[KBrO₃]^x where × = 1 for lower KBrO₃ concentrations and 0.5 for higher KBrO₃ concentrations. The reaction has been studied over the 20–45°C range. The activation energy was found to be 65.72 kJ/mol (15.71 kcal/mol) in the investigated range of temperature. Inorganic electrolytes except MnSO₄·4H₂O and Na₂C₂O₄ depress both the rate of polymerization and the maximum conversion. All the alcohols (viz., MeOH, EtOH, iso-PrOH, tert-BuOH) and acetone depress the rate of polymerization as well as the maximum conversion.     

Enthalpies of dilution of aqueous electrolytes: The NaCl/H2O system

Analysis of enthalpy of dilution data for the system NaCl/H₂O at 298.15°K with correlating equations presently in widespread use shows that these do not satisfactorily fit the data at the lowest available concentrations. A new approach based on the extended Debye-Hückel theory is suggested. It is shown that a plot of the ratio of the apparent molar enthalpy of dilution to the change in ionic strength, L/I, versus (I _i ^1/2 –I _f ^1/2 )/I, should give a straight line at low enough concentration with a slope 2S _H/3. The intercept is related to the Debye distance parameterA and the coefficient of the first virial correction term. The quantityS _H is the limiting Debye slope as calculated from the properties of the pure solvent. These expectations are substantiated by the fits. Values of the parameters are compared with older estimates, and it is concluded that the choiceA=0 is reasonable.     

Aqueous polymerization of methacrylamide

The aqueous polymerization of methacrylamide (I) initiated by KBrO₃–thioglycolic acid (TGA) has been studied at 30 ± 0.2°C in nitrogen. The rate is given by K[M]^1.19 [thioglycolic acid]¹ [KBrO₃]^0.53 for 10–15% conversion. Activation energy was found to be 53.96 kJ/mole (12.92 kcal/mole) in the investigated range of temperature 30–45°C. The role of addition of a series of aliphatic alcohols and some salts was also determined. The kinetics of polymerization was followed iodometrically.     

Thermal and dielectric studies of 2,2’-dihydroxybenzophenone

Thermal and dielectric properties of 2,2’-dihydroxybenzophenone were studied in relation with the potential progress of crystal nucleation and growth below the ordinary glass transition temperature, T_gα. Differential scanning calorimetry was carried out in a range 100-350 K. The α glass transition was found to occur at T_gα=239 K. Crystallization and fusion were observed to take place when the sample was cooled down to 103 K, but not observed when cooled to 203 K. Crystal nucleation was interpreted as having happened during annealing for a short time at 103 K which is much below the T_gα. Heat capacities were measured in a range 7-350 K by an intermittent heating method with an adiabatic calorimeter. The temperature, enthalpy and entropy of fusion were determined to be 334.46 K, 20.07 kJ mol^-1 and 60.01 J K^-1mol^-1, respectively. Crystal growth was found to proceed even at 220 K below the T_gα, but no glass transition was detected below 220 K. Dielectric losses were measured in a temperature range of 100-250 K and a frequency range of 30Hz-10 kHz. β-Relaxation process was found dielectrically with the activation energy of 22.6 kJ mol^-1, and the corresponding glass transition was expected to occur at 76.9 K. It is discussed, based on the “structurally ordered clusters aggregation” model for supercooled liquids and glasses, that the β process is potentially attributed to the crystal nucleation progressing at 103 K.     

Effect of Divalent ion Dopants on the Kinetics of Thermal Decomposition of Potassium Bromate

Isothermal decomposition of KBrO₃ has been studied as a function of concentration of the dopants, SO₄ ^2- and Ba²⁺ by isothermal thermogravimetry in the temperature range 668 - 683 K. The rate law and the activation energies remained unaltered by doping. The results suggest a diffusion-controlled mechanism, the diffusing species being both K⁺ and BrO₃ ^-.     

Estimation of Physico-chemical Properties of Ionic Liquid EPReO_4 Using Surface Tension and Density

An ionic liquid (IL) EPReO₄ (N‐ethylpyridinium rheniumate) was prepared. The density and surface tension values of the IL were determined in the temperature range of 293.15–343.15 K. The ionic volume and surface entropy of the IL were estimated by extrapolation, respectively. In terms of Glasser's theory, the standard molar entropy and lattice energy of the IL were estimated, respectively. Using Kabo's and Rebelo's methods, the molar enthalpy values of vaporization of the IL, Δ^g_lH⁰_m (298 K), at 298 K and, Δ^g_lH⁰_m (T_b), at hypothetical normal boiling point were estimated, respectively. According to the interstice model, the thermal expansion coefficient of IL EPReO₄ (α) was calculated and compared with experimental value, finding their magnitude order is in good agreement by 8.98%.     

A study of time dependence of ortho-positronium annihilation in a poly(butadiene) at different temperatures: a meaning of I3 parameter

Positron annihilation measurements as a function of temperature and time have been carried out on a poly(butadiene). The measurements were performed at several temperature points from 14 to 225 K. The measurement time was several hours to four days. The analysis of data shows the following features:

(i) the value of τ₃ does not depend on the rate of cooling or time,

(ii) the value of I₃ depends on the rate of cooling and the history of thermal treatment,

(iii) the dependence of I₃ on time can be described by Debye function. But the rise in I₃ is observed at very low temperatures,

(iv) the I₃ decays to value of I₃ observed during very slow cooling.

Mechanistic study of osmium(VIII) promoted oxidation of crotonic acid by aqueous alkaline solution of potassium bromate

Kinetics and mechanism of the Os(VIII) catalysed oxidation of crotonic acid (CA) by KBrO₃ in alkaline medium have been investigated. Zero order dependence in [KBrO₃] was observed, while first order with respect to CA in its lower concentration range tends to zero order at its higher concentration range. The order in [Os(VIII)] was found to be unity and a positive effect of [OH⁻] was observed. Variation of the ionic strength (μ) and dielectric constant of the medium and addition of Hg(OAc)₂ (used as Br⁻ scavenger) had an insignificant effect on the rate of reaction. Thermodynamic parameters have also been calculated and reported. A suitable mechanism consistent with the observed kinetic results has been suggested and the related rate law deduced.     

Crystal Structures and Ionic Conductivities of Ternary Derivatives of the Silver and Copper Monohalides: I. Superionic Phases of Stoichiometry MA4I5:RbAg4I5, KAg4I5, and KCu4I5

The superionic properties of the compounds RbAg₄I₅, KAg₄I₅ and KCu₄I₅ have been investigated by powder neutron diffraction and complex impedance spectroscopy. RbAg₄I₅ and KAg₄I₅ have room-temperature ionic conductivities of σ=0.21(6) and 0.08(5) Ω⁻¹ cm⁻¹, respectively, which increase gradually on increasing temperature. KCu₄I₅ is only stable in the temperature range between 515(5) K and its melting point of 605 K, and its ionic conductivity is σ=0.61(8) Ω⁻¹ cm⁻¹, at T=540 K. At lower temperatures, KCu₄I₅ disproportionates into KI+4CuI and the ionic conductivity falls by over three orders of magnitude. Least-squares refinements of the powder neutron diffraction data for RbAg₄I₅ at ambient temperature confirm the reported structure (space group P4₁32, Z=4, a=11.23934(3) Å), though with some differences in the preferred locations of the mobile Ag⁺. KAg₄I₅ and KCu₄I₅ are found to adopt the same basic structure as RbAg₄I₅, with the I− forming a β-Mn-type sublattice, with the K⁺ located in a distorted octahedral environment and the Ag⁺(Cu⁺) predominantly distributed over two sites which are tetrahedrally co-ordinated to I⁻. The implications for the conduction mechanism within these compounds are discussed, using a novel maximum entropy difference Fourier technique to map the distribution of the Ag⁺(Cu⁺) within the unit cell.     

Reaction of excited iodine atoms with methyl iodide: Rate constant determinations

The rate constant for the reaction I(²P_1/2) + CH₃I → I₂ + CH₃ has been reevaluated taking into account both collisional deactivation of excited iodine atoms and loss of I₂ by I₂ + CH₃ → I + CH₃I. The reevaluation is based upon data obtained (R. T. Meyer), J. Chem. Phys., 46 , 4146 (1967) from the flash photolysis of CH₃I using time-resolved mass spectrometry to measure the rate of I₂ formation. Computer simulations of the complete kinetic system and a closed-form solution of a simplified set of the differential equations yielded a value of 6(± 4) × 10⁶ 1./mole-sec for the excited iodine atom reaction in the temperature region of 316 to 447 K. A slight temperature dependence was observed, but an activation energy could not be evaluated quantitatively due to the small temperature range studied. An upper limit for the collisional deactivation of I(²P_1/2) with CH₃I was also determined (2.4 × 10⁷ 1./mole-sec).     

Thermochemical properties of the Fe-analogue of cryolite, Na3FeF6

Relative enthalpies for low-and high-temperature modifications of Na₃FeF₆ and for the Na₃FeF₆ melt have been measured by drop calorimetry in the temperature range 723–1318 K. Enthalpy of modification transition at 920 K, δ_trans H(Na₃FeF₆, 920 K) = (19 ± 3) kJ mol⁻¹ and enthalpy of fusion at the temperature of fusion 1255 K, δ_fusH(Na₃FeF₆, 1255 K) = (89 ± 3) kJ mol⁻¹ have been determined from the experimental data. Following heat capacities were obtained for the crystalline phases and for the melt, respectively: C _p(Na₃FeF₆, cr, α) = (294 ± 14) J (mol K)⁻¹, for 723 = T/K ≤ 920, C _p(Na₃FeF₆, cr, β) = (300 ± 11) J (mol K)⁻¹ for 920 ≤ T/K = 1233 and C _p(Na₃FeF₆, melt) = (275 ± 22) J (mol K)⁻¹ for 1258 ≤ T/K ≤ 1318. The obtained enthalpies indicate that melting of Na3FeF6 proceeds through a continuous series of temperature dependent equilibrium states, likely associated with the production of a solid solution.      

Quantitative interpretation to the chain mechanism of free radical reactions in cyclohexane pyrolysis

Pyrolysis of cyclohexane was conducted with a plug flow tube reactor in the temperature range of 873-973 K. Based on the experimental data, the mechanism and kinetic model of cyclohexane pyrolysis reaction were proposed. The kinetic analysis shows that overall conversion of cyclohexane is a first order reaction, of which the rate constant increased from 0.0086 to 0.0225 to 0.0623 s-1 with the increase of temperature from 873 to 923 to 973 K, and the apparent activation energy was determined to be 155.0±1.0 kJ mol-1. The mechanism suggests that the cyclohexane is consumed by four processes:the homolysis of C-C bond (Path I), the homolysis of C-H bond (Path II) in reaction chain initia- tion, the H-abstraction of various radicals from the feed molecules in reaction chain propagation (Path III), and the process associated with coke formation (Path IV). The reaction path probability (RPP) ratio of XPath I:XPath II : XPath III : XPath IV was 0.5420:0.0045:0.3897:0.0638 at 873 K, and 0.4336 : 0.0061 : 0.4885 : 0.0718 at 973 K, respectively.     

Nuclear magnetic resonance study of Li and H diffusion in the high-temperature solid phase of LiBH4

In order to study the atomic jump motions in the high-temperature solid phase of LiBH₄, we have measured the ¹H and ¹¹B nuclear magnetic resonance (NMR) spectra and the ¹H, ⁷Li and ¹¹B spin–lattice relaxation rates in this compound over the resonance frequency range of 14–34.4 MHz. In the temperature range 384–500 K, all the spin–lattice relaxation data are satisfactorily described in terms of a thermally activated jump motion of Li ions with the pre-exponential factor τ₀=1.1×10⁻¹⁵ s and the activation energy E_a=0.56 eV. The observed frequency dependences of the spin–lattice relaxation rates in this temperature range exclude a presence of any distributions of the Li jump rate or any other jump processes on the frequency scale of 10⁷–10¹⁰ s⁻¹. The strong narrowing of the ¹H and ¹¹B NMR lines above 440 K is consistent with the onset of diffusive motion of the BH₄ tetrahedra.     

A New and Sensitive Catalytic Resonance Scattering Spectral Assay for the Detection of Laccase Activity Using H2O2‐I−‐TDMAC System

In pH 3.8 acetic acid‐sodium acetate (HAC‐NaAC) buffer solution, laccase exhibited a strong catalytic effect on the H₂O₂ oxidation of I ^? to form I₂, and I₂ combined with excess I ^? to form I₃^? that reacted with cationic surfactants of tetradecyl dimethylbenzyl ammonium chloride (TDMAC) to produce the (TDMAC‐I₃)_n association complex particles, which exhibited a strong resonance scattering (RS) peak at 468 nm. Under the chosen conditions, as the concentration of laccase activity increased, the RS intensity at 468 nm (I_{468 nm}) increased linearly. The increased RS intensity ΔI_{468 nm} was linear to laccase activity in the range of 0.08–0.96 U/mL, with a regression equation of ΔI_{468 nm}?88.8U?1.9, and a detection limit of 0.02 U/mL laccase. This proposed method was applied to detect laccase activity in waste water, with satisfactory results.     

Heat Capacities of Hydroxy and Aminoderivatives of Benzene

Heat capacities in the liquid phase C ^l of methylbenzeneamines and heat capacities in the solid phase C ^s of benzenediols and of 4-methylbenzeneamine were measured by commercial Setaram heat conduction and power compensated calorimeters. Results obtained cover the following temperature range (depending on the compound and state of aggregation): 2-methylbenzeneamine 313 to 371 K, 3-methylbenzeneamine 263 to 371 K, 4-methylbenzeneamine 133 to 353 K, 1,2-benzenediol 153 to 353 K, 1,3-benzenediol 173 to 353 K, 1,4-benzenediol 133 to 403 K. The heat capacity data obtained in this work were merged with experimental data from literature, critically assessed and sets of recommended data were developed by correlating selected data as a function of temperature. This revised version was published online in August 2006 with corrections to the Cover Date.