Intrinsic Rate Constants Research Articles

Predicting Abiotic TCE Transformation Rate Constants—A Bayesian Hierarchical Approach

AbstractFe(II) minerals can mediate the abiotic reduction of trichloroethylene (TCE), a widespread groundwater contaminant. If reaction rates are sufficiently fast for natural attenuation, the process holds potential for mitigating TCE pollution in groundwater. To assess the variability of abiotic TCE reduction rate constants, we collected pseudo‐first‐order rate constants for natural sediments and rocks from the literature, as well as intrinsic (surface‐area‐normalized) rate constants of individual minerals. Using a Bayesian hierarchical modeling approach, we were able to differentiate the contributions of natural variability and experimental error to the total variance. Applying the model, we also predicted rate constants at new sites, revealing a considerable uncertainty of several orders of magnitude. We investigated whether incorporating additional information about sediment composition could reduce this uncertainty. We tested two sets of predictors: reactive mineral content (measured by X‐ray diffraction) combined with surface areas and intrinsic rate constants, or the extractable Fe(II) content. Knowledge of the mineral composition only marginally reduced the uncertainty of predicted rate constants. We attribute the low information gain to the inability to measure the (reactive) surface areas of individual minerals in sediments or rocks, which are subject to environmental factors like aqueous geochemistry and redox potential. In contrast, knowing the Fe(II) content reduced the uncertainty about the first‐order rate constant by nearly two orders of magnitude, because the relationship between Fe(II) content and rate constants is approximately log–log‐linear. We demonstrate how our approach provides estimates for the range of cleanup times for a simple example of diffusion‐controlled transport in a contaminated aquitard.

Kinetic implications of (hierarchical) zeolite fouling during liquid-phase aromatics upgrading

The kinetics of fouling in large pore zeolites (BEA, MOR), including those with hierarchical pore systems, were probed during the Brønsted acid catalyzed reaction of benzyl alcohol (BA) with 1,3,5-trimethylbenzene (TMB) by varying reactant driving force (i.e., [TMB]0/[BA]0; 11–119). In BEA, initial deactivation rate constants (kD,0) decreased exponentially with [TMB]0/[BA]0, highlighting the significance of oxygenates as deactivation precursors. Further, seeding pores with oxygenates completely suppressed measured rates in parent MOR and BEA, while seeding with TMB had no effect. Comparisons of mass accumulations of different organics (low molecular weight reaction species and coke) as functions of ln([TMB]0/[BA]0) revealed that coke (derived from oxygenate-seeded polyalkylation of TMB) disproportionately controlled deactivation rates by damping apparent (diffusion-controlled) and intrinsic (diffusion-corrected) rate constants through proton losses and/or altered molecule confinement within shrinking pores. These kinetic consequences were delayed upon introduction of mesopores, demonstrating how zeolite porosity impacts coke proliferation and behavior in liquid-phase reactions.

Extraction of the intrinsic rate constant for a photocyclization reaction in capillary microreactors using a simplified reactor model

Extraction of the intrinsic rate constants for a photocyclization reaction from a simplified reactor model.

Exploring chlorination as a removal process for antiretroviral drugs (Nevirapine and Efavirenz) from water: Effect of operational parameters, kinetics, and trihalomethane formation

This study investigates the removal of antiretroviral drugs (ARVs), Nevirapine (NVP) and Efavirenz (EFV), from drinking water through chlorination, focusing on the influence of pH, temperature, chlorine concentration, and ARV initial concentration on the degradation kinetics and trihalomethane (THM) formation. At 25 °C and an initial concentration of 100 μg/L, the degradation trends for both ARVs were examined across varying pH levels. EFV exhibited greater pH sensitivity, with removal increasing from 37 % at pH 5.5 to 68 % at pH 8, while NVP removal increased only from 40 % at pH 5.5 to 48 % at pH 8. Temperature also played an essential role, as the removal of both ARVs increased with rising temperature, indicating an endothermic reaction. Higher chlorine concentrations resulted in increased removal, >90 % at 5 mg/L for both ARVs. Kinetic analysis revealed second-order behaviour, with higher rate constants observed at pH 5.5–7 for NVP and pH 7–8 for EFV. The intrinsic chlorination rate constants for both ARVs were estimated. The calculated half-lives indicated slow removal. The study also assessed contact time for NVP and EFV in a plug-flow reactor (PFR) and continuous stirred tank reactor model (CSTR). Also considering hydraulic residence time (HRT) rather than reaction time to assess the average amount of time that the ARVs spent in the selected reactors. The HRT for a CSTR was lower than the PFR, and the estimated HRT for a CSTR was a feasible value. The minimum HRT came out to 1.56 h (pH 7.5 and a temperature of 25 °C) for NVP and 1.07 h (pH 7 and a temperature of 25 °C) for EFV. Finally, THM formation was observed, with chloroform, dibromochloromethane and bromoform detected. Among the three THMs, the most significant change occurred between 4 and 6 h, with chloroform levels increasing from 31.59 μg/L to 63.49 μg/L. This study underscores the influence of operational parameters on ARV removal via chlorination. Highlighting pH sensitivity, temperature control, precise chlorine dosage, and environmental concerns such as trihalomethane formation ensures effective disinfection while minimizing negative impacts on both water quality and the environment. Emphasizing the importance of a comprehensive approach to ARV removal in water treatment processes is a proactive measure that integrates considerations of environmental impact, public health, regulatory compliance, and long-term sustainability into the design and operation of water treatment facilities.

Persistent effects of inertia on diffusion-influenced reactions: Theoretical methods and applications.

The Cattaneo-Vernotte model has been widely studied to take momentum relaxation into account in transport equations. Yet, the effect of reactions on the Cattaneo-Vernotte model has not been fully elucidated. At present, it is unclear how the current density associated with reactions can be expressed in the Cattaneo-Vernotte model. Herein, we derive a modified Cattaneo-Vernotte model by applying the projection operator method to the Fokker-Planck-Kramers equation with a reaction sink. The same modified Cattaneo-Vernotte model can be derived by a Grad procedure. We show that the inertial effect influences the reaction rate coefficient differently depending on whether the intrinsic reaction rate constant in the reaction sink term depends on the solute relative velocity or not. The momentum relaxation effect can be expressed by a modified Smoluchowski equation including a memory kernel using the Cattaneo-Vernotte model. When the intrinsic reaction rate constant is independent of the reactant velocity and is localized, the modified Smoluchowski equation should be generalized to include a reaction term without a memory kernel. When the intrinsic reaction rate constant depends on the relative velocity of reactants, an additional reaction term with a memory kernel is required because of competition between the current density associated with the reaction and the diffusive flux during momentum relaxation. The competition effect influences even the long-time reaction rate coefficient.

Solar photocatalysis application in UWWTP outlets - simulations based on predictive models in flat-plate reactors and pollutant degradation studies with in silico toxicity assessment

The application of the solar photocatalysis for the degradation of residual pollutants found in surface water was demonstrated. Semi-pilot scale flat-plate cascade reactor (FPCR) was used to study the degradation of model organic pollutants: enrofloxacin (ENRO), 17β-estradiol (E2) and 1H-benzotriazole (1H-BT) over TiO2 thin-film supported on glass fibers. A modular panel with full-spectra solar lamps with appropriate UVB and UVA irradiation levels was used as a simulation of sunlight. Pollutant degradation in FPCR was estimated using predictive models; intrinsic reaction rate constants (ki) for ENRO, E2 and 1H-BT independent of the reactor size, flow rate and irradiation conditions were determined: 9.60, 3.35 and 0.37 109 s−1 W−0.5 m1.5, respectively. Main degradation products (DPs), formed upon hydroxylation, ring opening and oxidation, were identified using LC-QTOF-MS. The ecotoxicological impact was assessed via T.E.S.T. and ECOSAR open-source tools showing the formation of less harmful DPs after sufficient reaction time. Pollutant degradation was simulated at four locations of interest, i.e. exhausts from urban wastewater treatment plants (UWWTPs) in Zagreb, Croatia (45°N), Krakow, Poland (50°N), Sevilla, Spain (37°N) and Ioannina, Greece (39.6°N). Results have proved that a simple flat-plate system with supported photocatalysts can be easily scaled up and incorporated at the outlet of UWWTP for the reduction of pollutant load and related toxicity. The exhaust canal in Zagreb with the estimated length of a photocatalytic layer of 122 m for the > 90% degradation of all target pollutants was discussed as the best installation site among studied locations. Environmental ImplicationA multi-disciplinary approach to the tentative application of TiO2 solar photocatalysis outdoors to reduce pollutant loads and toxicity in surface waters was demonstrated. Possible application at four selected locations in Europe, as an additional step in water treatment after urban wastewater treatment plants (UWWTPs) was discussed. Target pollutants were studied under environmentally relevant conditions (sunlight levels, water matrix, simulation of process on a real scale at selected geographical location), at both higher and low concentrations.

Overlooked Transformation of Nitrated Polycyclic Aromatic Hydrocarbons in Natural Waters: Role of Self-Photosensitization.

Photochemical transformation is an important process that involves trace organic contaminants (TrOCs) in sunlit surface waters. However, the environmental implications of their self-photosensitization pathway have been largely overlooked. Here, we selected 1-nitronaphthalene (1NN), a representative nitrated polycyclic aromatic hydrocarbon, to study the self-photosensitization process. We investigated the excited-state properties and relaxation kinetics of 1NN after sunlight absorption. The intrinsic decay rate constants of triplet (31NN*) and singlet (11NN*) excited states were estimated to be 1.5 × 106 and 2.5 × 108 s-1, respectively. Our results provided quantitative evidence for the environmental relevance of 31NN* in waters. Possible reactions of 31NN* with various water components were evaluated. With the reduction and oxidation potentials of -0.37 and 1.95 V, 31NN* can be either oxidized or reduced by dissolved organic matter isolates and surrogates. We also showed that hydroxyl (•OH) and sulfate (SO4•-) radicals can be generated via the 31NN*-induced oxidation of inorganic ions (OH- and SO42-, respectively). We further investigated the reaction kinetics of 31NN* and OH- forming •OH, an important photoinduced reactive intermediate, through complementary experimental and theoretical approaches. The rate constants for the reactions of 31NN* with OH- and 1NN with •OH were determined to be 4.22 × 107 and 3.95 ± 0.01 × 109 M-1 s-1, respectively. These findings yield new insights into self-photosensitization as a pathway for TrOC attenuation and provide more mechanistic details into their environmental fate.

Kinetics of CaCO3 decomposition at low CO2 partial pressure in a vacuum fixed bed

Calcium looping often suffers from sintering during CaCO3 decomposition process, and CaCO3 decomposition in vacuum is a potential solution. Kinetic analysis of CaCO3 decomposition in vacuum is seldom, and most of the kinetics in the literature is obtained from thermogravimetric analyzer in atmospheric pressure. In addition, the reported kinetic data seems scattered, and varies depending on the analyzer employed. This study is focused on the intrinsic kinetics of isothermal CaCO3 decomposition in a vacuum fixed bed from 700 °C to 850 °C. Besides conventional internal/external mass transfer elimination via reducing size/increasing gas velocity, the effect of CO2 partial pressure is specially discussed under vacuum condition. It is found that, this influence could be eliminated only when the relative difference between equilibrium pressure and maximum CO2 partial pressure (Peq -P*CO2)/Peq was higher than 0.98. Tests at atmospheric pressure were also conducted with the same particle size and gas volumetric feeding rate for comparison. The intrinsic reaction rate constants of CaCO3 decomposition in vacuum are 1.05 × 10-3 s−1– 4.76 × 10-3 s−1 between 700 °C and 850 °C, and the activation energy of this reaction is 92.58 kJ/mol.

Synthesis and Characterization of Dense Carbon Films as Model Surfaces to Estimate Electron Transfer Kinetics on Redox Flow Battery Electrodes.

Redox flow batteries (RFBs) are a promising electrochemical technology for the efficient and reliable delivery of electricity, providing opportunities to integrate intermittent renewable resources and to support unreliable and/or aging grid infrastructure. Within the RFB, porous carbonaceous electrodes facilitate the electrochemical reactions, distribute the flowing electrolyte, and conduct electrons. Understanding electrode reaction kinetics is crucial for improving RFB performance and lowering costs. However, assessing reaction kinetics on porous electrodes is challenging as their complex structure frustrates canonical electroanalytical techniques used to quantify performance descriptors. Here, we outline a strategy to estimate electron transfer kinetics on planar electrode materials of similar surface chemistry to those used in RFBs. First, we describe a bottom-up synthetic process to produce flat, dense carbon films to enable the evaluation of electron transfer kinetics using traditional electrochemical approaches. Next, we characterize the physicochemical properties of the films using a suite of spectroscopic methods, confirming that their surface characteristics align with those of widely used porous electrodes. Last, we study the electrochemical performance of the films in a custom-designed cell architecture, extracting intrinsic heterogeneous kinetic rate constants for two iron-based redox couples in aqueous electrolytes using standard electrochemical methods (i.e., cyclic voltammetry, electrochemical impedance, and spectroscopy). We anticipate that the synthetic methods and experimental protocols described here are applicable to a range of electrocatalysts and redox couples.

Formulation of Nifedipine–Polyvinyl Pyrrolidone (PVP) Solid Dispersion System and Intrinsic Dissolution Rate Evaluation

Nifedipine is a drug that acts as an antihypertensive and anti-angina. Nifedipine is known as a drug with poor water solubility. This characteristic will affect the intrinsic dissolution rate so that it can affect the absorption process and reduce the amount of drug that reaches systemic circulation. One of the strategies to increase the intrinsic dissolution rate is developing nifedipine to solid dispersions form. This study aims to observe the intrinsic dissolution rate of nifedipine after it has been made into a solid dispersion. Four samples were prepared, including three solid dispersions of nifedipine-PVP K-30 and one sample of pure nifedipine. The results of the intrinsic dissolution tests are then interpreted through the intrinsic dissolution rate constant (G). The solid dispersions with concentration of nifedipine-PVP K-30 90%:10%; 75%:25%; 60%:40% (w/w), and pure nifedipine produced G values of 3.63; 9.33; 12.63; and 2.08 µg/mm2. min1, consecutively. It shows that the formulation of nifedipine-PVP K-30 solid dispersions has higher G values than pure nifedipine. In addition, increasing PVP K-30 concentration up to 40 % (w/w) can increase the intrinsic dissolution rate of the nifedipine-PVP K-30 solid dispersion system.

Hemodialysis as emergency treatment of a severe baclofen intoxication in a 3 kg dog

To describe the clinical efficacy and drug removal kinetics of hemodialysis (HD) as emergency treatment in a small size dog with severe baclofen intoxication. A 2-year-old dog was presented in stupor to the emergency service a few hours after ingestion of up to 25 mg of baclofen. Medical stabilisation was attempted but was unsuccessful in improving the neurological condition and the patient rapidly progressed to coma. A 4-h session of HD was performed in emergency with near complete resolution of neurological signs and only mild disorientation by the end of the treatment. No adverse side effects occurred during HD. Baclofen concentration was measured serially during the session. Drug extraction ratio, clearance and mass removal by the dialyser were calculated. Dialytic elimination rate constant (Kd ) was seven times higher than the intrinsic elimination rate constant (Kintr ) and the half-life (t½) during HD was nearly nine times shorter than the endogenous one. This is the first case report providing pharmacokinetic data associated with HD treatment of severe baclofen intoxication in a dog.

Study of Photocatalytic Oxidation of Micropollutants in Water and Intensification Case Study

During the last decades, heterogenous photocatalysis has shown as the most promising advanced oxidation process for the removal of micropollutants due to degradation rate, sustainability, non-toxicity, and low-cost. Synergistic interaction of light irradiation, photocatalysts, and highly reactive species are used to break down pollutants toward inert products. Even though titanium dioxide (TiO2) is the most researched photocatalyst, to overcome shortcomings, various modifications have been made to intensify photocatalytic activity in visible spectra range among which is modification with multiwalled carbon nanotubes (MWCNTs). Therefore, photocatalytic oxidation and its intensification by photocatalyst’s modification was studied on the example of four micropollutants (diclofenac, DF; imidacloprid, IMI; 1-H benzotriazole, BT; methylene blue, MB) degradation. Compound parabolic collector (CPC) reactor was used as, nowadays, it has been considered the state-of-the-art system due to its usage of both direct and diffuse solar radiation and quantum efficiency. A commercially available TiO2 P25 and nanocomposite of TiO2 and MWCNT were immobilized on a glass fiber mesh by sol-gel method. Full-spectra solar lamps with appropriate UVB and UVA irradiation levels were used in all experiments. Photocatalytic degradation of DF, IMI, BT, and MB by immobilized TiO2 and TiO2/CNT photocatalysts was achieved. Mathematical modelling which included mass transfer and photon absorption was applied and intrinsic reaction rate constants were estimated: kDF=3.56 × 10−10s−1W−0.5m1.5, kIMI=8.90 × 10−11s−1W−0.5m1.5, kBT=1.20 × 10−9s−1W−0.5m1.5, kMB=1.62 × 10−10s−1W−0.5m1.5. Intensification of photocatalysis by TiO2/CNT was observed for DF, IMI, and MB, while that was not the case for BT. The developed model can be effectively applied for different irradiation conditions which makes it extremely versatile and adaptable when predicting the degradation extents throughout the year using sunlight as the energy source at any location.

Larger Intrinsic Rate Constants of Alpha-amylase is Possible if Intrinsic Forward Rate Constant is ≠ Diffusion limited Rate of Encounter

Background: Previous research has shown that the intrinsic reverse (backward) and forward rate constants are larger than the effective or apparent rate constants for the formation and dissociation of an enzyme-substrate complex (ES). It is speculated that such intrinsic rate constants could be larger if an appropriate mathematical equation was adopted for their computation. 
 Methods: Theoretical, experimental (Bernfeld method), and computational methods.
 Objectives: 1) To rederive the equations for calculating the intrinsic rate constants for forward (k1) and reverse (i.e., backward) (k2) reactions; 2) to calculate the intrinsic rate constants; and 3) to show that the probability (1/g) (or req(r)) that an enzyme is at a distance from the substrate is a variable. 
 Results and Discussion: The equations for the determination of k2 and k1 were rederived. Unlike previous findings, the intrinsic (reverse) first order rate, k2 and forward second order rate, k1 were larger than their apparent counterparts, but they were, however, very similar in magnitude. The intrinsic rate constants were much larger than previously reported values when the enzyme (E) total concentration [ET] was much less than substrate’s total concentration [ST]. The k1 and apparent forward second order rate (kf) values where [ET] is much less than [ST] were > where [ET] is less than [ST]. Therefore, the magnitude of the second order rate constant is a function of [ET]. The values of k1 and k2 where [ET] is much less than [ST] and vice-versa were respectively, 7.41 exp. (+6) L/mol. min and 81.34 exp. (+4) /min, and 15.76 exp. (+6) L/mol. min and 58.08 exp.(+4) /min. It was discovered that the probability (1/g) (or req(r))) that an enzyme is at a distance from the substrate with the possibility of mutual attraction is not constant. 
 Conclusion: If the intrinsic forward rate constant (k1) is not equal to diffusion limited rate (kD) of encounter, the k1 and k2 values could be larger than values where k1 is equal to kD. The probability (1/g) (or req(r)) that an enzyme is at a distance from the substrate with the possibility of mutual attraction has been discovered to be a variable constant dependent on the concentration of the reaction mixture components and the enzyme's affinity for the substrate, and vice versa. Future research may attempt to derive an equation for the determination of an intrinsic catalytic rate constant for the formation of a product.

A compressed logistic equation on bacteria growth: inferring time-dependent growth rate

We propose a compressed logistic model for bacterial growth by invoking a time-dependent rate instead of the intrinsic growth rate (constant), which was adopted in traditional logistic models. The new model may have a better physiological basis than the traditional ones, and it replicates experimental observations, such as the case example for E. coli, Salmonella, and Staphylococcus aureus. Stochastic colonial growth at a different rate may have a fractal-like nature, which should be an origin of the time-dependent reaction rate. The present model, from a stochastic viewpoint, is approximated as a Gaussian time evolution of bacteria (error function).

Rate, Efficiency, and Mechanisms of Electrochemical Perfluorooctanoic Acid Degradation with Boron-Doped Diamond and Plasma Electrodes.

The removal of per- or polyfluorinated alkyl substances (PFAS) has received increasing attention because of their extreme stability, our increasing awareness of their toxicity at even low levels, and scientific challenges for traditional treatment methods such as separation by activated carbon or destruction by advanced oxidation processes. Here, we performed a direct and systematic comparison of two electrified approaches that have recently shown promise for effective degradation of PFAS: plasma and conventional electrochemical degradation. We tailored a reactor configuration where one of the electrodes could be a plasma or a boron-doped diamond (BDD) electrode and operated both electrodes galvanostatically by continuous direct current. We show that while both methods achieved near-complete degradation of PFAS, the plasma was only effective as the cathode, whereas the BDD was only effective as the anode. Compared to the BDD, plasma required more than an order of magnitude higher voltage but lower current to achieve similar degradation efficiency with more rapid degradation kinetics. All these factors considered, it was noted that plasma or BDD degradation resulted in similar energy efficiencies. The BDD electrode exhibited zero-order kinetics, and thus, PFAS degradation using the conventional electrochemical method was kinetically controlled. On the contrary, analysis using a film model indicated that the plasma degradation kinetics of PFAS using plasma were mass-transfer-controlled because of the fast reaction kinetics. With the help of a simple quantitative model that incorporates mass transport, interfacial reaction, and surface accumulation, we propose that the degradation reaction kinetically follows an Eley-Rideal-type mechanism for the plasma electrode, and an intrinsic rate constant of 2.89 × 108 m4 mol-1 s-1 was obtained accordingly. The investigation shows that to realize the true kinetic potential of plasma degradation for water treatment, mass transfer to the interface must be enhanced.

Continuous flow nitration of 3-[2-chloro-4-(trifluoromethyl) phenoxy] benzoic acid and its chemical kinetics within droplet-based microreactors

• The synthesis of Acifluorfen by continuous flow nitration was performed within droplet-based microreactor. • The effects of operating parameters were characterized by hydrodynamics and thermodynamics. • The apparent and intrinsic kinetic model was established. Aromatic nitration still faces unmet technical challenges in relation to being heterogeneous and highly exothermic. In this work, a continuous flow aromatic nitration was developed with mixed acid within droplet-based microreactors. The effects of key operating parameters were characterized on the nitration of 3-[2-chloro-4-(trifluoromethyl)phenoxy] benzoic acid. The optimized reaction temperature, M-ratio and N/S were found to be 308 K, 1.6 and 0.57, respectively. Under these conditions, a conversion of 83.03% and selectivity of 79.52% were achieved. The addition of acetic anhydride improved both the solubility of etherate in the organic phase and the absorbability of water produced from the reaction, leading to increase in conversion, however increased by-products accordingly with reduced selectivity. Pseudo-homogeneous kinetics models were developed of apparent and intrinsic reaction rate constant, supporting the second-order reaction assumption. The calculated results based on the kinetics model were found to be in good agreement with the experimental results.

Lumped kinetic modelling of polyolefin pyrolysis: A non-isothermal method to estimate rate constants

The measurement of kinetic parameters in the pyrolysis of polyolefins requires the use of a lumped kinetic model for predicting the product distribution of wax, oil and gas yields. A non-isothermal method was established, in which a sample is heated in a tube reactor to a desired temperature at a constant rate of temperature rise. This method avoided the error present in the heating up stage which is inherent in any practical isothermal method in which reaction proceeds to a significant extent before the operating temperatures of polyolefin pyrolysis are reached, which results in challenges when defining the reaction time. The non-isothermal measurements were conducted between 450 and 550 °C for polypropylene (PP) and polyethylene (HDPE and LDPE) and the temperature and lump yields are non-linearly regressed to achieve the kinetic parameters. The measured kinetic rate constants have the same trend as those reported in the literature using the isothermal method, but are lower than the values reported at similar conditions. The kinetic parameters derived are then validated by using isothermal experimental data. The calculated data using the measured kinetic parameters are generally in agreement with the experimental data. The non-isothermal method established in this work proves to be a much faster method for the measurement of intrinsic rate constants at high temperatures.

Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association, Solvent and Kinetic H/D Isotope Effects.

Using dynamic liquid-state NMR spectroscopy a degenerate double proton tautomerism was detected in tetramethyl reductic acid (TMRA) dissolved in toluene-d8 and in CD2Cl2. Similar to vitamin C, TMRA belongs to the class of reductones of biologically important compounds. The tautomerism involves an intramolecular HH transfer that interconverts the peripheric and the central positions of the two OH groups. It is slow in the NMR time scale around 200 K and fast at room temperature. Pseudo-first-order rate constants of the HH transfer and of the HD transfer after suitable deuteration were obtained by line shape analyses. Interestingly, the chemical shifts were found to be temperature dependent carrying information about an equilibrium between a hydrogen bonded dimer and a monomer forming two weak intramolecular hydrogen bonds. The structures of the monomer and the dimer are discussed. The latter may consist of several rapidly interconverting hydrogen-bonded associates. A way was found to obtain the enthalpies and entropies of dissociation, which allowed us to convert the pseudo-first-order rate constants of the reaction mixture into first-order rate constants of the tautomerization of the monomer. Surprisingly, these intrinsic rate constants were the same for toluene-d8 and CD2Cl2, but in the latter solvent more monomer is formed. This finding is attributed to the dipole moment of the TMRA monomer, compensated in the dimer, and to the larger dielectric constant of CD2Cl2. Within the margin of error, the kinetic HH/HD isotope effects were found to be of the order of 3 but independent of temperature. That finding indicates a stepwise HH transfer involving a tunnel mechanism along a double barrier pathway. The Arrhenius curves were described in terms of the Bell–Limbach tunneling model.

Energy analysis and determination of controlling mechanisms of dissociation for hydrate reservoirs via tracking of dissociation front

A number of dissociation techniques like depressurization, thermal stimulation, CO2 exchange, etc. have been reported and investigated for natural gas hydrates in literature. However, dissociation of hydrate reservoirs is a complex process that is actually governed by several parameters. Hence, when inspected closely, the dissociation characteristics are a reflection of how these parameters are interlinked with each other. The present study is designed to numerically simulate a matrix of 21 different cases, which comprises an in-depth analysis for thermal stimulation, drawdown pressure, permeability, intrinsic rate constant, and thermal conductivity. The focus of the current study is motivated by practical interest, which aims at deciphering the associated mechanisms with hydrate dissociation. Over the years, different researchers have outlined the role of mechanisms such as heat transfer (HT), fluid flow (FF), and dissociation (D). The governing mechanisms are deduced through mathematical fitting and plotting of thermodynamic parameters for deciphering associated mechanisms. A “characteristic exponent” is determined for each of the cases to identify and delineate the different mechanisms for each of the 21 cases. Further, energy conversion from hydrate and energy quantification has been described in detail. As such, this information pool about hydrate dissociation is vital and necessary for efficient energy harvesting from hydrate reservoirs from varied geological settings and composition.

Phosphate has dual roles in cross-bridge kinetics in rabbit psoas single myofibrils.

In this study, we aimed to study the role of inorganic phosphate (Pi) in the production of oscillatory work and cross-bridge (CB) kinetics of striated muscle. We applied small-amplitude sinusoidal length oscillations to rabbit psoas single myofibrils and muscle fibers, and the resulting force responses were analyzed during maximal Ca2+ activation (pCa 4.65) at 15°C. Three exponential processes, A, B, and C, were identified from the tension transients, which were studied as functions of Pi concentration ([Pi]). In myofibrils, we found that process C, corresponding to phase 2 of step analysis during isometric contraction, is almost a perfect single exponential function compared with skinned fibers, which exhibit distributed rate constants, as described previously. The [Pi] dependence of the apparent rate constants 2πb and 2πc, and that of isometric tension, was studied to characterize the force generation and Pi release steps in the CB cycle, as well as the inhibitory effect of Pi. In contrast to skinned fibers, Pi does not accumulate in the core of myofibrils, allowing sinusoidal analysis to be performed nearly at [Pi] = 0. Process B disappeared as [Pi] approached 0 mM in myofibrils, indicating the significance of the role of Pi rebinding to CBs in the production of oscillatory work (process B). Our results also suggest that Pi competitively inhibits ATP binding to CBs, with an inhibitory dissociation constant of ∼2.6 mM. Finally, we found that the sinusoidal waveform of tension is mostly distorted by second harmonics and that this distortion is closely correlated with production of oscillatory work, indicating that the mechanism of generating force is intrinsically nonlinear. A nonlinear force generation mechanism suggests that the length-dependent intrinsic rate constant is asymmetric upon stretch and release and that there may be a ratchet mechanism involved in the CB cycle.