Kinetic study on preparation of substoichiometric tungsten oxide WO2.72 via hydrogen reduction process

Controlling the conditions of the oxygen partial pressure and temperature to prepare the WO_2.72 (W₁₈O₄₉) via reduction was possible through thermodynamic consideration. WO_2.72 was synthesized via heating to 1073 K in 5% H₂–95% Ar mixture gas flow from ammonium tungstate which was prepared by hydrothermal process. With the reducing prolonging time, the products were changed from WO_2.72 to WO₂ and then metal W. Thermogravimetric (TG) analysis showed ammonium tungstate decomposed completely to WO₃ at 773 K. Isothermal reductions using TG analysis were carried out at 905 K, 925 K, 945 K and 973 K in 5% H₂–95% Ar mixture gas flow, respectively. The whole reduction from WO₃ to WO_2.72 divided into three parts: initial nucleation and growth stage, final interfacial reaction stage and intermediate stage, was controlled jointly by both mechanisms. Fitting results showed that the initial stage obey the one-dimensional Avrami–Erofeev equation, the apparent activation energy was 132.7 ± 1.1 kJ mol⁻¹ and the pre-exponent factor was 4.82 × 10⁵ min⁻¹; the final stage expressed by 2-dimensional interfacial reaction, the apparent activation energy was 144.0 ± 2.1 kJ mol⁻¹ and the pre-exponent factor was 3.20 × 10⁵ min⁻¹.

     

Seeded growth core-shell (Ag–Au–Pd) ternary nanostructure at room temperature for potential water treatment

Seed-mediated growth is a promising technique for preparation of multi-metallic nanostructures, in which reduction of metal ions takes a place over the surface of another one. Herein, a seed growth mechanism was investigated for synthesis of core-shell Ag–Au–Pd ternary nanostructures through a facile method at room temperature. Ascorbic acid and sodium alginate were used as nano-generator and stabilizing agent, respectively. Spherical shaped monocular Ag nanostructure with size of 13.6 nm grew to 24.4 nm of Ag–Au binary and to 58.8 nm of Ag–Au–Pd ternary core-shell nanostructures. The crystalline shape of nanostructures was approved by X-Ray diffraction analyses. While, FT-IR data approved the redox mechanism for synthesis the as-required nanostructures. The catalytic reactivity of the prepared nanostructures in reductive degradation of methylene blue dye was studied. The results approved the role of Pd in perfection of catalytic degradation of the as-tested dye. The rate constant of dye degradation was considerably enlarged from 62.1 × 10⁻³ m⁻¹ for Ag monocular nanostructures to 403.3 × 10⁻³ m⁻¹ for Ag–Pd binary and to 852.4 × 10⁻³ m⁻¹ for Ag–Au–Pd ternary core-shell nanostructures. The obtained results offer an energy saving method to fabricate core-shell catalytically active ternary nanostructures with promising applicability in water treatment.     

Modelling of mass transfer in facilitated supported liquid membrane transport of copper(II) using MOC-55 TD in Iberfluid

The transport of copper(II) through a supported liquid membrane using MOC-55 TD (oxime derivative), dissolved in Iberfluid, as a carrier has been studied. A physico-chemical model is derived to describe the transport mechanism which consists of: diffusion process through the feed aqueous diffusion layer, fast interfacial chemical reaction and diffusion through the membrane. The experimental data can be explained by mathematical equations describing the rate of transport. The mass transfer coefficient was calculated from the described model as 2.8×10⁻³ cm s⁻¹, the thickness of the aqueous boundary layer as 2.6×10⁻³ cm⁻¹ and the membrane diffusion coefficient of the copper-containing species as 1.2×10⁻⁸ cm² s⁻¹.     

Dissipation of chlortetracycline in the aquatic environment: Characterization in terms of a generalized multiphase pseudo–zero-order rate law

The kinetics of the dissipation of chlortetracycline in the aquatic environment was studied over a period of 90 days using microcosm experiments and distilled water controls. The distilled water control experiments, carried out under dark conditions as well as exposed to natural sunlight, exhibited biphasic linear rates of dissipation. The microcosm experiments exhibited triphasic linear rates of degradation both in the water phase (2.7 × 10⁻², 7 × 10⁻³, 1.3 × 10⁻³ μg g⁻¹ day^–1) and the sediment phase (3.4 × 10⁻², 6 × 10⁻³, 1 × 10⁻³ μg g⁻¹ day^–1). The initial slow rate of dissipation in the dark control (3 × 10⁻³ μg g⁻¹ day^–1) was attributed to a combination of evaporation and hydrolysis, whereas the subsequent fast rate (1.8 × 10⁻³ μg g⁻¹ day^–1) was attributed to a combination of evaporation, hydrolysis, and microbial degradation. For the sunlight-exposed control, the initial slow rate of dissipation (1.5 × 10⁻³ μg g⁻¹ day^–1) was attributed to a combination of evaporation, hydrolysis, and photolysis, whereas the subsequent fast rate was attributed to a combination of evaporation, hydrolysis, photolysis, and microbial degradation (5.1 × 10⁻³ μg g⁻¹ day^–1). The initial fast rate of dissipation in the water phase of the microcosm experiment is attributed to a combination of evaporation, hydrolysis, photolysis, and microbial degradation, whereas all subsequent slow rates in the water phase and all rates of degradation in the sediment phase are attributed to microbial degradation of the colloidal and sediment particle adsorbed antibiotic. A multiphase zero-order kinetic model is presented that takes into account (a) dissipation of the antibiotic via evaporation, hydrolysis, photolysis, microbial degradation, and adsorption by colloidal and sediment particles and (b) the dependence of the dissipation rate on the concentration of the antibiotic, type and count of microorganisms, and type and concentration of colloidal particles and sediment particle adsorption sites within a given aquatic environment.     

Study on degrees of mesomorphic zone of polymer.Ⅰ.Determination of degrees of crystallinity of Eucommia ulmoides gum and natural rubber by dynamic mechanical thermal analysis

According to the two-phase model theories of polymer crystallization, the degree of crystallinity of natural rubber (NR) induced to crystallize at −25 °C can be directly determined by dynamic mechanical thermal analysis (DMTA) by testing its tanδ peak areas of crystalline state and 100% amorphous state respectively. The degree of crystallinity is 86% when the induction time increases to 12 h. However, due to the tanδ peak area of 100% amorphous state cannot be measured by DMTA directly, the degree of crystallinity of Eucommia Ulmoides (EU) gum can only be determined indirectly with DMTA by using the tanδ peak areas of 100% amorphous state extrapolated from the linear equation (y = ax + b) which is derived from the relationship between tanδ peak areas and crosslink densities. The degree of crystallinity is 87%, which is very close to NR. The crystallization of NR can be prevented effectively by increasing slightly the crosslink density to the critical crystallization point, which is 6.20 × 10⁻⁵mol/cm⁻³ for vulcanized NR induced to crystallize at −25 °C for 4 h. However, it is very hard for EU gum to prevent the crystallization by crosslinking. Because, with increasing of the crosslink density, the EU gum will firstly show a percolation threshold, which is 53.34 × 10⁻⁵mol/cm⁻³, and the degree of crystallinity is 71.04%. Then, like NR, the degree of crystallinity of EU gum will decrease rapidly to zero by increasing the crosslink density to the critical crystallization point, which is 68.89 × 10⁻⁵mol/cm⁻³. In the end of the paper, the linear relationship of the equation is verified and the influence factors on slope a and intercept b of the linear equation are discussed as well.     

Crystallization kinetics study of melt-spun Zr66.7Ni33.3 amorphous alloy by electrical resistivity measurements

In this paper, the electronic transport properties of as-spun Zr_66.7Ni_33.3 alloys were studied in detail by a combination of electrical resistivity and absolute thermoelectric power measurements over a temperature range from 25 up to 400 °C. Moreover, the isochronal and isothermal crystallization kinetics of Zr_66.7Ni_33.3 glassy alloy has been investigated based on the electrical resistivity measurements. The comparative study of the crystallization kinetics of these binary amorphous alloys was carried out, for the first time to our knowledge, using an accurate method for electrical resistivity measurements. In the isochronal heating process, the apparent activation energy for crystallization was determined to be, respectively, 371.4 kJ mol⁻¹ and 382.2 kJ mol⁻¹, by means of Kissinger and Ozawa methods. The Johnson–Mehl–Avrami model was used to describe the isothermal transformation kinetics, and the local Avrami exponent has been determined in the range from 2.97 to 3.23 with an average value of 3.1, implying a mainly diffusion-controlled three-dimensional growth with an increasing nucleation rate. Based on an Arrhenius relationship, the local activation energy was analyzed, which yields an average value E_x = 376.2 kJ mol⁻¹.

     

A Photoprotective Effect by Cation-π-Interaction? Quenching of Singlet Oxygen by an Indole Cation-π Model System

We investigated the effect of the cation-π interaction on the susceptibility of a tryptophan model system toward interaction with singlet oxygen, that is, type II photooxidation. The model system consists of two indole units linked to a lariat crown ether to measure the total rate of removal of singlet oxygen by the indole units in the presence of sodium cations (i.e. indole units subject to a cation-π interaction) and in the absence of this interaction. We found that the cation-π interaction significantly decreases the total rate of removal of singlet oxygen (k_T) for the model system, that is, (k_T = 2.4 ± 0.2) × 10⁸ m ⁻¹ s⁻¹ without sodium cation vs (k_T = 6.9 ± 0.9) × 10⁷ m ⁻¹ s⁻¹ upon complexation of sodium cation to the crown ether. Furthermore, we found that the indole moieties undergo type I photooxidation processes with triplet excited methylene blue; this effect is also inhibited by the cation-π interaction. The chemical rate of reaction of the indole groups with singlet oxygen is also slower upon complexation of sodium cation in our model system, although we were unable to obtain an exact ratio due to differences of the chemical reaction rates of the two indole moieties.     

Structural and electrical characteristics of ALD-HfO2/n-Si gate stack with SiON interfacial layer for advanced CMOS technology

We report the fabrication of an ultra-thin silicon oxynitride (SiON) as an interfacial layer (IL) for n-Si/ALD-HfO₂ gate stack with reduced leakage current. The XRD, AFM, FTIR, FESEM and EDAX characterizations have been performed for structural and morphological studies. Electrical parameters such as dielectric constant (K), interface trap density (D_it), leakage current density (J), effective oxide charge (Q_eff), barrier height (Φ_bo), ideality factor (ƞ), breakdown-voltage (V_br) and series resistance (R_s) were extracted through C-V, G-V and I-V measurements. The determined values of K, D_it, J, Q_eff, Φ_bo, ƞ, V_br and R_s are 14.4, 0.5 × 10 ¹¹ eV⁻¹ cm⁻², 2.2 × 10⁻⁹ A/cm², 0.3 × 10¹³ cm⁻², 0.42, 2.1, −0.33 and 14.5 MΩ respectively. SiON growth prior to HfO₂ deposition has curtailed the problem of high leakage current density and interfacial traps due to sufficient amount of N₂ incorporated at the interface.     

Discovering Intrinsic Magnetism of Silk Fibroins

Silk fibroin (SF) is an interesting biomaterials with intriguing mechanical, optical, piezoelectric, and biocompatible properties. The present study reports the first magnetic experiments to show that organic radicals in the building blocks of SF induce permanent magnetization in these materials. A saturation magnetization of 2.8 × 10⁻³ emu g⁻¹ and non-zero remnant magnetization of 2.2 × 10⁻⁴ emu g⁻¹ with a coercive field of 65 Oe is observed in multiple SFs. Tightly bound β-sheets of polyglycine-alanine motifs provides sufficient steric hinderance to organic radicals and protects them from attack of reactive species. Moreover, it also shows that silk's ferromagnetic character can be enhanced by simple mechanical stretching and heating. This work demonstrates a new paradigm of magnetic proteins and opens a route toward the biomimetic discovery of iron-free magnetic proteins.     

Kinetics and mechanism of formation of gehlenite,Al–Si spinel and anorthite from the mixture of kaolinite and calcite

The kinetics and mechanism of formation of gehlenite, Al–Si spinel phase, wollastonite and anorthite from the mixture of kaolinite and calcite was investigated by differential thermal analysis under the heating rate from 283 to 293 K min⁻¹ using Kissinger equation. The changes in the phase composition of the sample during the thermal treatment were investigated via simultaneous TG-DTA, in situ high-temperature x-ray diffraction analysis and high-temperature heating-microscopy. The crystallizations of gehlenite and Al–Si spinel phase show apparent activation energy of (411 ± 5) kJ mol⁻¹ and (550 ± 9) kJ mol⁻¹, respectively. The value of kinetic exponent corresponds to the process limited by the decreasing nucleation rate for gehlenite while constant nucleation rate is determined for Al–Si spinel phase. Anorthite crystallizes from the eutectic melt and the process shows the apparent activation energy of (1140 ± 25) kJ mol⁻¹. The process is limited by the constant nucleation rate of a new phase.     

Electroosmotic flow in fused deposition modeling (FDM) 3D-printed microchannels

Electroosmotic flow (EOF) was determined in tridimensional (3D)-printed microchannels with dimensions smaller than 100 µm. Fused deposition modeling 3D printing using thermoplastic filaments of PETG (polyethylene terephthalate glycol), PLA (polylactic acid), and ABS (acrylonitrile butadiene styrene) were used to fabricate the microchannels. The current monitoring method and sodium phosphate solutions at different pH values (3–10) were used for the EOF mobility (µ_EOF) measurements, which ranged from 2.00 × 10⁻⁴ to 12.52 × 10⁻⁴ cm² V⁻¹ s⁻¹. The highest and the smallest µ_EOF were obtained for the PLA and PETG microchannels, respectively. Adding the cationic surfactant cetyltrimethylammonium bromide to the sodium phosphate solution caused EOF direction reversion in all the studied microchannels. The obtained results can be interesting for developing 3D-printed microfluidic devices, in which EOF is relevant.     

Successful spectrofluorometric and chemiluminescence methods for the estimation of azathioprine as an immunosuppressive drug in pharmaceutical preparation

The determination of azathioprine (AZA), a mercaptopurine derivative, has attracted a particular attention in most researches. Herein, spectrofluorometric (I) and chemiluminescence (II) methods were successfully confirmed to estimate azathioprine (AZA) in bulk powder form and pharmaceutical preparation. The optimum conditions to improve the fluorescence and chemiluminescence intensity of AZA were investigated. The method (I) measured the native fluorescence intensity of AZA in methanol as solvent, without any addition. The micelle-enhanced fluorescence of AZA was affected by the surfactant addition, type of solvent, pH value, and the volume of sodium dodecyl sulfate (SDS). The method (II) consisted in the improvement of the weak signal of calcein–KMnO₄ chemiluminescence system by introducing synthesized silver nanoparticles (AgNPs) in the presence of AZA. The analytical curves were linear in the concentrations range 5.0 × 10⁻⁸ − 1.0 × 10⁻⁴ M and 5.0 × 10⁻⁹ − 2.0 × 10⁻³ M with a detection limit equal to 1.5 × 10⁻⁹ M and 2.6 × 10 ⁻¹⁰ M for techniques (I) and (II), respectively. The statistical analysis indicated no significant difference between the proposed and reported methods. The suggested techniques were successfully used to determine azathioprine in its tablet form and were validated according to ICH guidelines.     

Study of the reactions of OH with HCl,HBr, and HI between 298 K and 460 K

The reactions between OH radicals and hydrogen halides (HCl, HBr, HI) have been studied between 298 and 460 K by using a discharge flow-electron paramagnetic resonance technique. The rate constants were found to be k_HCl(298 K) = (7.9 ± 1.3) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ with a weak positive temperature dependence, k_HBr (298-460 K) = (1.04 ± 0.2) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, and k_HI(298 K) = (3.0 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, respectively. The homogeneous nature of these reactions has been experimentally tested.     

Kinetics of the diazotization and azo coupling reactions of procaine in the micellar media

The kinetics of the diazotization reaction of procaine in the presence of anionic micelles of sodium dodecyl sulfate (SDS) and cationic micelles of cetyltrimethyl ammonium bromide (CTAB), dodecyltrimethyl ammonium bromide (DDTAB) and tetradecyltrimethyl ammonium bromide (TDTAB) were carried out spectrophotometrically at λ_max = 289 nm. The values of the pseudo first order rate constant were found to be linearly dependent upon the [NaNO₂] in the concentration range of 1.0 × 10⁻³ mol dm⁻³ to 12.0 × 10⁻³ mol dm⁻³ in the presence of 2.0 × 10⁻² mol dm⁻³ acetic acid. The concentration of procaine was kept constant at 6.50 × 10⁻⁵ mol dm⁻³. The addition of the cationic surfactants increased the reaction rate and gave plateau like curve. The addition of SDS micelles to the reactants initially increased the rate of reaction and gave maximum like curve. The maximum value of the rate constant was found to be 9.44 × 10⁻³ s⁻¹ at 2.00 × 10⁻³ mol dm⁻³ SDS concentration. The azo coupling of diazonium ion with β-naphthol (at λ_max = 488) nm was found to linearly dependent upon [ProcN₂⁺] in the presence of both the cationic micelles (CTAB, DDTAB and TDTAB) and anionic micelles (SDS). Both the cationic and anionic micelles inhibited the rate of reactions. The kinetic results in the presence of micelles are explained using the Berezin pseudophase model. This model was also used to determine the kinetic parameters e.g. k_m, K_s from the observed results of the variation of rate constant at different [surfactants].     

Rate coefficients for reactions of OH radicals with CH3D,CH2D2, CHD3, and CD4

Fourier transform infrared (FTIR) smog chamber techniques were used to investigate the atmospheric chemistry of the isotopologues of methane. Relative rate measurements were performed to determine the kinetics of the reaction of the isotopologues of methane with OH radicals in cm³ molecule⁻¹ s⁻¹ units: k(CH₃D + OH) = (5.19 ± 0.90) × 10⁻¹⁵, k(CH₂D₂ + OH) = (4.11 ± 0.74) × 10⁻¹⁵, k(CHD₃ + OH) = (2.14 ± 0.43) × 10⁻¹⁵, and k(CD₄ + OH) = (1.17 ± 0.19) × 10⁻¹⁵ in 700 Torr of air diluent at 296 ± 2 K. Using the determined OH rate coefficients, the atmospheric lifetimes for CH_4–xD_x (x = 1–4) were estimated to be 6.1, 7.7, 14.8, and 27.0 years, respectively. The results are discussed in relation to previous measurements of these rate coefficients.     

Pulse radiolysis study of the formation and the reactivity of baicalin radical anion,and in comparison with rutin,quercetin and acyrlate ester radical anions in ethanol

The reaction of solvated electrons with baicalin in N₂-saturated ethanol has been studied by pulse radiolysis. The results show that a solvated electron can add to baicalin and generate a baicalin radical anion with a maximum UV absorbance peak at 360 nm. Its molar extinction coefficient at this wavelength is 1.3×10⁴ M⁻¹ cm⁻¹. The rate constant for the build-up of the baicalin radical anion is 1.3(±0.4)×10¹⁰ M⁻¹ s⁻¹. Decay of the radical anion is induced by a proton transfer reaction and a recombination reaction, which involves a pseudo-first-order reaction with rate constant 2.6(±0.4)×10³ s⁻¹ and a second-order reaction with rate constant 1.3(±0.2)×10⁹ M⁻¹ s⁻¹. The effect of acetaldehyde on the decay of the baicalin radical anion was also investigated. Electron transfer between the baicalin radical anion and acetaldehyde was not observed, probably due to the low rate of electron transfer between the baicalin radical anion and acetaldehyde. Reactivity of the rutin, quercetin, baicalin and ethyl acrylate radical anions are also compared.     

Effect of cooling rate on the crystal/mesophase polymorphism of polyamide 6

The solidification of the quiescent polyamide 6 (PA 6) melt has been analyzed as a function of the cooling rate in a wide range between 1.67 × 10⁻² and close to 2 × 10² K s⁻¹, by means of differential scanning calorimetry at a low cooling rate of up to about 1 K s⁻¹, and by the recording of continuous cooling curves and time-resolved X-ray diffraction on cooling at a higher rate. The performed experiments allowed for the first time to establish the relationship between the cooling rate, the crystallization temperature, and the X-ray structure of PA 6. The exclusive formation of monoclinic α-crystals is only detected if the crystallization temperature is higher than about 430 K or if the cooling rate is slower than about 5 K s⁻¹, respectively. The formation of α-crystals is increasingly replaced by the development of mesophase with increasing cooling rate, accompanied with a decrease of the temperature of crystallization/ordering. Finally, completely amorphous samples were obtained on cooling faster than about 10² K s⁻¹. The continuous decrease of the temperature of crystallization with increasing cooling rate, regardless of the specific structure formed, precludes a primary effect of the nucleation mechanism on the α-crystal/mesophase polymorphism of PA 6. A preliminary discussion of the effect of molar mass of PA 6 on the cooling rate-dependent polymorphism is also included.     

Development of hexa- and mono-celsian in the crystallization of banalsite BaNa2Al4Si4O16 (LiF-CaF2-B2O3) glasses

On the basis of the mineral banalsit (BaNa₂Al₄Si₄O₁₆) and the addition of small B₂O₃ concentrations, transparent glasses were prepared. Furthermore, in order to achieve nucleation, LiF and CaF₂ were added. Hexacelsian was formed in bulk crystallized glass samples whereas, monocelsian, as well as small quantities of nepheline and banalsite were crystallized from sintered glass powder. The scanning electron micrographs of the sintered samples show high crystallinity and crystals with sizes from nano to micrometers. The SEM micrographs and the EDX microanalyses show that nano size rods of monocelsian surrounded by micrometer-sized hexagonal nepheline, banalsite or residual glassy phase occur. The coefficient of thermal expansion of the samples sintered at 1000 °C was higher (12.93–9.52 × 10⁻⁶ K⁻¹) in hexacelsian containing samples than in monocelsian (2.24–7.35 × 10⁻⁶ K⁻¹) containing ones. The samples also showed notably different densities of 2.6424 and 2.4718 g/cm³, respectively.     

Experimental determination of the rate constants of the reactions of HO2 + DO2 and DO2 + DO2

The rate constants of the reactions of DO₂ + HO₂ (R1) and DO₂ + DO₂ (R2) have been determined by the simultaneous, selective, and quantitative measurement of HO₂ and DO₂ by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near infrared, coupled to a radical generation by laser photolysis. HO₂ was generated by photolyzing Cl₂ in the presence of CH₃OH and O₂. Low concentrations of DO₂ were generated simultaneously by adding low concentrations of D₂O to the reaction mixture, leading through isotopic exchange on tubing and reactor walls to formation of low concentrations of CH₃OD and thus formation of DO₂. Excess DO₂ was generated by photolyzing Cl₂ in the presence of CD₃OD and O₂, small concentrations of HO₂ were always generated simultaneously by isotopic exchange between CD₃OD and residual H₂O. The rate constant k₁ at 295 K was found to be pressure independent in the range 25–200 Torr helium, but increased with increasing D₂O concentration k₁ = (1.67 ± 0.03) × 10⁻¹² × (1 + (8.2 ± 1.6) × 10⁻¹⁸ cm³ × [D₂O] cm⁻³) cm³ s⁻¹. The rate constant for the DO₂ self-reaction k₂ has been measured under excess DO₂ concentration, and the DO₂ concentration has been determined by fitting the HO₂ decays, now governed by their reaction with DO₂, to the rate constant k₁. A rate constant with insignificant pressure dependence was found: k₂ = (4.1 ± 0.6) × 10⁻¹³ (1 + (2 ± 2) × 10⁻²⁰ cm³ × [He] cm⁻³) cm³ s⁻¹ as well as an increase of k₂ with increasing D₂O concentration was observed: k₂ = (4.14 ± 0.02) × 10⁻¹³ × (1 + (6.5 ± 1.3) × 10⁻¹⁸ cm³ × [D₂O] cm⁻³) cm³ s⁻¹. The result for k₂ is in excellent agreement with literature values, whereas this is the first determination of k₁.     

Abnormally slow reaction of oppositely charged ions: The kinetics of dopamine hydrochloride oxidation by ammonium peroxydisulfate

The kinetic curves for oxidation of dopamine hydrochloride in aqueous solution in the presence of ammonium peroxydisulfate were obtained by UV–vis spectroscopy and potentiometry. It was shown that the reaction follows the first-order kinetic equation and proceeds at a low rate. The values for the activation energy and the preexponential factor were determined as 75 kJ × mol⁻¹ and 4 × 10⁸ s⁻¹, respectively. The activation entropy was found having a negative value of −89 J × mol⁻¹ × K⁻¹. The first reaction order, the low preexponential factor and the negative activation entropy value for the reaction between the 2-(3,4-dihydroxyphenyl)ethanammonium cation and the peroxydisulfate anion were explained by the formation of ionic associates, which slowly enter into the internal redox reaction.