Equilibrium Solvent Research Articles

Equilibrium Solubility, Model Correlation, and Solvent Effect of Indole-3-acetic Acid in Twelve Pure Solvents

The solid–liquid solubility of indole-3-acetic acid in pure methanol, ethanol, n-propanol, isopropanol, acetone, n-butanol, acetonitrile, chloroform, ethyl acetate, 1,4-dioxane, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) was measured by using a isothermal saturation method. The determination temperature range is from 278.15 to 323.15 K. The order of solubility data from high to low was ethyl acetate > DMSO > DMF > n-butanol > acetone > isopropanol > 1,4-dioxane > n-propanol > ethanol > methanol > acetonitrile > chloroform. Two thermodynamic models (modified Apelblat equation and λh equation) were used to correlate the solubility values of indole-3-acetic acid in different monosolvents, and the two equations can correlate the experimental values very well. The values of root-mean-square deviations (RMSDs) and relative average deviations (RADs) between the experimental and calculated solubility values were not exceeding 0.27 × 10–6 and 0.75 × 10–2, respectively. In order to study the effect ...

ACTIVATION ENERGIES OF THERMO-OXIDIZED NITRILE RUBBER COMPOUNDS OF VARYING ACRYLONITRILE CONTENT

ABSTRACT The thermo-oxidative behavior of carbon black–reinforced sulfur-cured nitrile rubber compounds with varying acrylonitrile (ACN) content (18–49 wt%) was investigated. Accelerated heat aging was carried out from 40 °C to 115 °C for various aging times. Ambient aging was also included. Samples were tested for hardness, 10% tensile stress, tensile strength, elongation at break, network chain density by equilibrium solvent swell, and toluene-soluble fraction. Diffusion-limited oxidation affected data at high temperatures and was eliminated for time-temperature superposition. Linear Arrhenius kinetic behavior was confirmed throughout the whole temperature range, and calculated activation energies varied from 75 to 93 kJ/mol. Activation energies calculated through the hardness data were found to increase steadily with ACN concentration, whereas the other test responses showed less direct correlation, likely because of the influence of the underlying NBR microstructure, which changes as a function of ACN content. The high-temperature thermo-oxidative process consists of both oxidative crosslinking and chain scission reactions. Sulfur reversion and alkyl radical recombination reactions are likely prevalent at low temperatures during the buildup of hydroperoxides up to 60 °C. The shelf life of nitrile rubbers strongly depends on their ACN level, with lower ACN nitriles being more susceptible to degradation, leading to shorter shelf lives, than higher ACN-containing nitriles.

Nonideality in Silicone Network Formation via Solvent Swelling and 1H Double-Quantum NMR

The versatile cross-linking chemistry of poly(dimethylsiloxane) (PDMS)-based materials affords a large research space in which polymers with widely varying elastomeric properties may be synthesized. Parameters such as chain length, cross-link density, cross-link functionality, filler content, and chain chemistry can all be modified to produce materials with specific physical and mechanical properties. Commercial polysiloxane-based, “silicone” elastomers are generally intractable, which makes the precise characterization of their networks problematic. We report here the application of equilibrium solvent uptake analysis and 1H double-quantum nuclear magnetic resonance (1H DQ NMR) spectroscopy to determine the network topology of end-linked PDMS networks with nonideal network topology. Despite their structural complexity, we can quantify both the classical and nonclassical contributions to network structure using 1H DQ NMR which are in reasonable agreement with solvent uptake data. These findings serve as the foundation for future investigations of even more complex commercial silicones using 1H DQ NMR.

Humic acid embedded chitosan/poly (vinyl alcohol) pH-sensitive hydrogel: Synthesis, characterization, swelling kinetic and diffusion coefficient

In the present research, humic acid (HA) embedded chitosan (CHT)-poly (vinyl alcohol) (PVA) pH-sensitive hydrogels have been synthesized using glutaraldehyde as a crosslinker. The hydrogels were characterized by Fourier-transform infrared (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) analysis. The effect of HA content, pH, and temperature on the equilibrium solvent uptake has been investigated. With higher HA contents, the equilibrium swelling capacity increases while lower concentrations of acid cause a decrease in the degree of the swelling. The hydrogels have been found to undergo a number of swelling/deswelling cycles when pH of the swelling medium changes from 1.5 to 11.0. Fick’s law, first-order, and second-order kinetic models were applied to the swelling ratio of hydrogels to explain the mass transfer mechanism and specify swelling kinetic parameters. The swelling parameters such as equilibrium swelling capacity, swelling rate constant, swelling exponent, initial swelling rate, diffusion coefficient, etc., have been evaluated.

Effect of reinforcement filler on vulcanization, diffusion, mechanical, and electrical properties of natural rubber

The present work investigates the properties of natural rubber (NR) filled with different contents of reinforcing carbon black (CB) and other compounding ingredients. The curing characteristics were tested at 157°C. It was found that the equilibrium solvent uptake ( Qmax) decreases with the increase of the CB concentration due to reduction of the free volume. Mechanical properties such as tensile strength and hardness of CB filled rubber composites were remarkably improved, indicating the inherent reinforcing potential of CB. The percentage of elongation at break of filled CB rubber was found to be lower than that of pure NR. The dielectric constants ( ε′), dielectric loss ( ε″), and alternating current (AC) electrical conductivity ( σAC) were investigated for all samples as a function of frequency and temperature. It is concluded that the high CB content causes the formation of channels, increasing the AC conductivity of the composites.

Equilibrium solubility, solvent effect and preferential solvation of chlorhexidine in aqueous co-solvent solutions of (methanol, ethanol, N,N-dimethylformamide and 1,4-dioxane)

The equilibrium solubilities of chlorhexidine in four solvent mixtures of methanol (1) + water (2), ethanol (1) + water (2), DMF (1) + water (2) and 1,4-dioxane (1) + w water (2) at temperature range from (278.15 to 318.15) K were reported. At the same composition of methanol, ethanol, DMF or 1,4-dioxane and temperature, the mole fraction solubility of chlorhexidine was highest in DMF (1) + w water (2) mixtures, and lowest in ethanol (1) + w water (2) mixtures. By using the Jouyban-Acree model, van’t Hoff-Jouyban-Acree model and Apelblat-Jouyban-Acree model, chlorhexidine solubility was well correlated obtaining RAD lower than 1.69% and RMSD lower than 0.87 × 10−4. The dissolution process of chlorhexidine in solvent solutions was endothermic for the dissolution process. To study the solvent effect, solubility data were correlated to KAT parameters and cavity term on the basis of linear solvation energy relationships concept. Results indicate the key effect of cavity term and π∗ in aqueous methanol and ethanol mixtures; cavity term in aqueous DMF mixtures; and cavity term, β and π∗ in aqueous 1,4-dioxan mixtures. Quantitative values for the local mole fraction of methanol (ethanol, DMF or 1,4-dioxane) and water around the chlorhexidine were computed by using the IKBI method applied to the determined solubility data. Chlorhexidine was preferentially solvated by water for the four solvent mixtures in water-rich compositions; while in intermediate and co-solvent-rich composition for ethanol (DMF, 1,4-dioxane) (1) + w water (2) mixtures, chlorhexidine is preferentially solvated by the co-solvent. The preferential solvation magnitude of chlorhexidine was highest in 1,4-dioxane mixtures than in the other three solvent mixtures.

Experimental determination of the quantity and distribution of chemical crosslinks in unaged and aged natural rubber, part 1: Peroxide vulcanization

A set of unfilled and stabilized natural rubber compounds containing different dicumyl peroxide concentrations were prepared and their chemical crosslink densities were determined using a variety of experimental techniques: stress–strain (Mooney-Rivlin), equilibrium solvent swell (Flory-Rhener) and low field time domain Nuclear Magnetic Resonance (NMR) by the Double Quantum (DQ) method. Testing was carried out on both the unaged and heat aged (96 h at 80 °C) samples. Increasing peroxide level caused a corresponding rise of crosslink density and a decrease in the chain entanglement density, while the relative crosslink distribution width remained unchanged. The molecular weight between entanglement points was in the expected range for natural rubber. Heterogeneous areas of high crosslinking (clusters) were low in concentration. Heat aging caused properties to deteriorate due to chain scission reactions that eliminated network chains creating a more heterogeneous system. Defect levels increased and more toluene soluble uncrosslinked chains were detected. The network chain and chain entanglement densities decreased with heat aging and the molar mass distribution between crosslinks substantially broadened independent of peroxide loading.

Solution Processing of Methylammonium Lead Iodide Perovskite from γ-Butyrolactone: Crystallization Mediated by Solvation Equilibrium

The chemical origin of solvents typically used for preparation of hybrid lead halide perovskites—dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and γ-butyrolactone (GBL)—strongly influences the process of perovskite crystallization because of the formation of intermediate adducts with different structures and morphology. The composition and crystal structures of the adducts depend on the coordination and binding ability of the solvents and the ratio of the precursors. New adducts of perovskite and GBL with either an unusual cluster structure, (MA)8(GBL)x[Pb18I44], or an adduct, (MA)2(GBL)2Pb3I8, similar to those observed for DMF and DMSO are described for the first time. Complex equilibriums between chemical species existing in perovskite solutions are revealed by Raman spectroscopy. As a result, new features of the perovskite crystallization through intermediate adduct phases are discussed, and effective perovskite deposition pathways are suggested.

A Semianalytical Model for Simulating Combined Electromagnetic Heating and Solvent-Assisted Gravity Drainage

Summary Solvent/thermal hybrid methods have been proposed recently to enhance heavy-oil recovery and to overcome the shortcomings that are encountered when either method is solely applied. One of the methods for this hybridization is to combine electromagnetic (EM) heating and solvent injection to facilitate heavy-oil production by gravity drainage. This approach has several advantages including reduced CO2 emissions, decreased water consumption, and appropriateness for water-hostile reservoirs. We are currently lacking any mathematical model for better understanding, designing, and optimizing this hybrid technique, which is partly attributed to this technique still being in its infancy. We propose a semianalytical model to predict the oil-flow rate resulting from the combined EM heating and solvent-assisted gravity drainage. The model first calculates the temperature distribution within the EM-excited zone caused by the radiation-dominated EM heating. Using different attenuation coefficients within and beyond the vapor chamber, the model can properly describe the corresponding temperature responses in these regions. Next, an average temperature of the chamber edge contributed by EM heating is used to estimate the temperature-dependent properties, such as vapor/liquid equilibrium ratios (K-values), heavy-oil/solvent-mixture viscosity, and solvent diffusivity. Subsequently, a 1D diffusion equation is used to calculate the solvent-concentration distribution ahead of the chamber edge. Eventually, the oil-flow rate is evaluated with the calculated temperature and solvent distributions ahead of the chamber edge. The proposed model is validated against the experimental results obtained in our previous study, and the predicted oil-flow rate agrees reasonably well with the experimental data. The proposed model can efficiently predict the oil-flow rate of this hybrid process. We conduct sensitivity analyses to examine the effect of major influential factors on the performance of this hybrid technique, including EM heating powers, solvent types, solvent-injection pressures, and initial reservoir temperatures. The modeling results demonstrate that a higher EM heating power, a heavier solvent, and a higher solvent-injection pressure could accelerate the oil-recovery rate, but tend to lower the net present value (NPV) and increase the energy consumption. In summary, the newly proposed model provides an efficient tool to understand, design, and optimize the combined technique of EM heating and solvent-assisted gravity drainage.

Excited-State Proton Transfer in 3-Cyano-7-azaindole: From Aqueous Solution to Ice.

We investigated the excited-state proton transfer (ESPT) reaction for 3-cyano-7-azaindole (3CAI) in aqueous solution and in ice. 3CAI undergoes water-catalyzed ESPT in the aqueous solution, giving normal (355 nm) and proton transfer tautomer (∼472 nm) emission bands. Detailed temperature-dependent studies showed that the values of activation free energy (Δ G‡) were similar between N-H and N-D isotopes. Therefore, water-catalyzed ESPT involves a stepwise mechanism incorporating solvation equilibrium ( Keq) to form a 1:1 (molar ratio) water:3CAI cyclic hydrogen-bonded complex as an intermediate, followed by perhaps proton tunneling reaction. In sharp contrast, 3CAI in ice undergoes entirely different photophysical properties, in which 3CAI self-organizes to form a double-hydrogen-bonded dimers at the grain boundary of the polycrystalline. Upon excitation, the dimer proceeds with a fast excited-state double proton transfer reaction, giving rise to solely a tautomer emission (∼450 nm). The distinct difference in ESPT properties between water and ice makes azaindoles feasible for the investigation of water-ice interface property.

Effect of reduced graphene oxide on the solvent transport characteristics and sorption kinetics of fluoroelastomer nanocomposites.

The chemical resistivity of a fluoroelastomer forms the basis of its applicability in chemical industries as gloves and inner linings of storage tanks; in automotive and aircraft industries as linings of fuel hoses, etc. In this context, the study of the molecular transport of industrially important chemicals is of considerable interest. The present study depicts the solvent transport characteristics of fluoroelastomer/reduced graphene oxide nanocomposites (FKM/mRGO) in a homologous series of aromatic hydrocarbons. The solvent diffusion characteristics of the nanocomposites were analyzed in terms of mRGO loading, nature of solvents used, temperature etc. The parameters such as equilibrium solvent uptake, diffusivity, sorptivity, and permeability were analyzed and found to decrease with an increase in filler content. This is mainly attributed to the decrease in free volume with increasing concentration of mRGO due to the improved filler-elastomer interactions that hinder the solvent transport. Swelling parameters such as swelling coefficient and swelling index were analyzed to study the polymer-filler interactions. Diffusion studies with respect to temperatures were used to estimate the enthalpy, entropy and free energy change of the diffusion process. The molecular mass between crosslinks and hence the crosslink densities were calculated by using the experimental data and the reinforcement effect of mRGO was confirmed by the decrease in Mc values and increase in crosslink densities. Theoretical models, namely phanton and affine, were compared with the experimental values, which showed a moderate agreement with affine network models.

Density, Viscosity, and Structure of Equilibrium Solvent Phases in Butyric Acid Extraction by Phosphonium Ionic Liquid

Density, viscosity, and dynamic light scattering (DLS) measurements have been done for quaternary phases with phosphonium ionic liquid (Cyphos IL-104 + dodecane + BA + water) from the extraction of butyric acid (BA) by a binary solvent (IL + dodecane). Density data show volume contraction in the solvent. Addition of water, BA, or dodecane considerably decreases IL viscosity, while BA decreases viscosity less than water. When increasing BA concentration, viscosity of the solvent phase in equilibrium without dodecane goes through a maximum at BA mass fraction of about 0.2. In solvents with more than 30% in mass fraction of dodecane, almost no influence of BA on viscosity was observed. Viscosity of water saturated solvents with IL concentration below 50% in mass fraction is markedly higher than in solvents containing also BA. This is due to the formation of dodecane-rich micelles in water saturated solvents which increase IL concentration in the bulk of the solvent. No large aggregates with size above 100 nm...

High mechanical properties, thermal conductivity and solvent resistance in graphene oxide/styrene-butadiene rubber nanocomposites by engineering carboxylated acrylonitrile-butadiene rubber

Abstract In this paper, graphene oxide (GO) was modified with carboxylated acrylonitrile-butadiene rubber (xNBR) to synthesize the xNBR modified GO (xNBR-GO), which was adopted to reinforce styrene-butadiene rubber (SBR) nanocomposites through latex compounding method. The interaction between GO and xNBR was verified by Fourier transform infrared spectroscopy (FTIR) and Raman spectra. The dispersion of filler and morphology of xNBR-GO/SBR nanocomposites were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The mechanical properties, dynamic mechanical properties, thermal stability, thermal conductivity as well as solvent resistance of the xNBR-GO/SBR nanocomposites were also thoroughly studied. It was confirmed that xNBR could interact with GO through hydrogen bonding. The modification would improve the dispersion and reinforcement of filler. With the incorporation of 5 phr (parts per hundred rubber) filler, the tensile strength, tear strength and thermal conductivity of xNBR-GO/SBR nanocomposite were increased by 545%, 351% and 31.7%, respectively. In addition, the equilibrium solvent uptake was decreased by 31.8%, and the thermal stability was enhanced.

Solvent Transport Characteristics of Thermoplastic Elastomer Blends Based on Nylon and NBR

Transport characteristics of chlorinated hydrocarbon through Nylon/NBR (Acrylonitrile Butadiene Rubber) blend has been investigated with special reference to the effect of blend ratio, compatibilizer and effect of dynamic crosslinking. An attempt has been made to correlate the diffusion characteristics with morphology of the blend. Fine and uniform distribution of the dispersed domains with a stable morphology were obtained by dynamic vulcanization. Depending on the composition, blends show dispersed/matrix and co-continuous phase morphologies. The various transport parameters such as diffusion coefficient (D), permeation coefficient (P) and sorption coefficients (S) were evaluated at different diffusion conditions such as temperature and percentage of compatibilizer. The equilibrium solvent uptake decreases with an increase in the concentration of NBR in the blends. Finally thermodynamic parameters were evaluated from the diffusion data. POLYM. ENG. SCI., 2016. © 2016 Society of Plastics Engineers

Influence of molecular structure of blend components on the performance of thermoplastic vulcanisates prepared by electron induced reactive processing

Polypropylene (PP)/ethylene propylene diene rubber (EPDM) thermoplastic vulcanisates (TPVs) were prepared by electron induced reactive processing without use of any chemical vulcanizing agent, co-agent or other type of additive. Thus, the composition of PP/EPDM blend is not changed by this novel process and relationships between raw material, process, morphology and properties of TPVs can be studied in a simple way. Consequently, the molecular mass of PP as well as the amount and nature of diene (ethylidene norbornene or vinyl norbornene) were varied. Based on detailed mechanical, dynamic mechanical, morphological, thermal and rheological investigations as well as equilibrium solvent swelling measurements and sol–gel analyses it was found that molecular mass, amount and nature of diene influence the cross-linking efficiency. In addition, the use of low molecular mass blend components leads to PP-EPDM phase graft-links as well as increased values of elongation and tensile strength at break.

Enhanced mechanical properties and thermal conductivity of styrene–butadiene rubber reinforced with polyvinylpyrrolidone-modified graphene oxide

A facile non-covalent surface treatment method is reported in this paper to modify graphene oxide (GO) sheets with the assistance of polyvinylpyrrolidone (PVP). The PVP-modified GO (PGO) was further adopted to fabricate PGO/styrene–butadiene rubber (SBR) nano-composites through the latex compounding method. The properties of PGO were carefully investigated and interaction between GO and PVP molecules was confirmed. The mechanical properties, dynamic mechanical properties, thermal stability, thermal conductivity as well as swelling properties of the PGO/SBR nano-composites were thoroughly studied. It was confirmed that PVP molecules could have strong interaction with GO via hydrogen bond; thus, the PGO significantly improved the strength of SBR matrix, e.g., 517 and 387 % increase in tensile strength and tear strength, respectively, with the presence of only 5 phr (parts per hundred rubber) PGO in the nano-composite. The presence of PGO had also greatly reduced the glass transition temperature (Tg) and enhanced the storage modulus of SBR matrix in the nano-composites. Meanwhile, the maximum heat decomposition temperature (Tmax) was increased by 23.6 °C; equilibrium solvent uptake in toluene was reduced by 41 % and thermal conductivity was increased by 30 %. All the observations indicated that PVP modification of GO can achieve excellent exfoliation and dispersion of GO in the SBR matrix. These findings were further supported by X-ray diffraction and scanning electron microscopy measurements.

Symmetry-Breaking in Cationic Polymethine Dyes: Part 2. Shape of Electronic Absorption Bands Explained by the Thermal Fluctuations of the Solvent Reaction Field.

The electronic absorption spectra of the symmetric cyanines exhibit dramatic dependence on the conjugated chain length: whereas short-chain homologues are characterized by the narrow and sharp absorption bands of high intensity, the long-chain homologues demonstrate very broad, structureless bands of low intensity. Spectra of the intermediate homologues combine both features. These broad bands are often explained using spontaneous symmetry-breaking and charge localization at one of the termini, and the combination of broad and sharp features was interpreted as coexistence of symmetric and asymmetric species in solution. These explanations were not supported by the first principle simulations until now. Here, we employ a combination of time-dependent density functional theory, a polarizable continuum model, and Franck-Condon (FC) approximation to predict the absorption line shapes for the series of 2-azaazulene and 1-methylpyridine-4-substituted polymethine dyes. To simulate inhomogeneous broadening by the solvent, the molecular structures are optimized in the presence of a finite electric field of various strengths. The calculated FC line shapes, averaged with the Boltzmann weights of different field strengths, reproduce the experimentally observed spectra closely. Although the polarizable continuum model accounts for the equilibrium solvent reaction field at absolute zero, the finite field accounts for the thermal fluctuations in the solvent, which break the symmetry of the solute molecule. This model of inhomogeneous broadening opens the possibility for computational studies of thermochromism. The choice of the global hybrid exchange-correlation functional SOGGA11-X, including 40% of the exact exchange, plays the critical role in the success of our model.

Plasticizer effect on network structure and hydrolytic degradation

The hydrolytic degradation of fully cured polyester–urethane networks polymerized in the presence of several weight ratios of triacetin was monitored by the residual concentration in elastically active chains obtained from modulus and equilibrium solvent swelling measurements. The presence of triacetin does not change the water uptake but induces a lower rate of degradation. Comparisons were performed with networks in which triacetin was removed before ageing, and with networks in which polyester–urethane was first polymerized and then impregnated by triacetin. Data suggest that the presence of triacetin during polymerization induces the presence of elastically inactive chains such as dangling chains and loops, the hydrolysis of which does not change the elastic properties of the network. This explanation was checked from relaxation measurements by NMR and DMA, and by the analysis of the soluble fraction generated by hydrolysis.

Novel methods for predicting gas–particle partitioning during the formation of secondary organic aerosol

Abstract. Several methods have been presented in the literature to predict an organic chemical's equilibrium partitioning between the water insoluble organic matter (WIOM) component of aerosol and the gas phase, Ki,WIOM, as a function of temperature. They include (i) polyparameter linear free energy relationships calibrated with empirical aerosol sorption data, as well as (ii) the solvation models implemented in SPARC and (iii) the quantum-chemical software COSMOtherm, which predict solvation equilibria from molecular structure alone. We demonstrate that these methods can be used to predict Ki,WIOM for large numbers of individual molecules implicated in secondary organic aerosol (SOA) formation, including those with multiple functional groups. Although very different in their theoretical foundations, these methods give remarkably consistent results for the products of the reaction of normal alkanes with OH, i.e. their partition coefficients Ki,WIOM generally agree within one order of magnitude over a range of more than ten orders of magnitude. This level of agreement is much better than that achieved by different vapour pressure estimation methods that are more commonly used in the SOA community. Also, in contrast to the agreement between vapour pressure estimates, the agreement between the Ki,WIOM estimates does not deteriorate with increasing number of functional groups. Furthermore, these partitioning coefficients Ki,WIOM predicted SOA mass yields in agreement with those measured in chamber experiments of the oxidation of normal alkanes. If a Ki,WIOM prediction method was based on one or more surrogate molecules representing the solvation properties of the mixed OM phase of SOA, the choice of those molecule(s) was found to have a relatively minor effect on the predicted Ki,WIOM, as long as the molecule(s) are not very polar. This suggests that a single surrogate molecule, such as 1-octanol or a hypothetical SOA structure proposed by Kalberer et al. (2004), may often be sufficient to represent the WIOM component of the SOA phase, greatly simplifying the prediction. The presented methods could substitute for vapour-pressure-based methods in studies such as the explicit modelling of SOA formation from single precursor molecules in chamber experiments.

The rate of second electron transfer to QB− in bacterial reaction center of impaired proton delivery shows hydrogen-isotope effect

The 2nd electron transfer in reaction center of photosynthetic bacterium Rhodobacter sphaeroides is a two step process in which protonation of QB− precedes interquinone electron transfer. The thermal activation and pH dependence of the overall rate constants of different RC variants were measured and compared in solvents of water (H2O) and heavy water (D2O). The electron transfer variants where the electron transfer is rate limiting (wild type and M17DN, L210DN and H173EQ mutants) do not show solvent isotope effect and the significant decrease of the rate constant of the second electron transfer in these mutants is due to lowering the operational pKa of QB−/QBH: 4.5 (native), 3.9 (L210DN), 3.7 (M17DN) and 3.1 (H173EQ) at pH7. On the other hand, the proton transfer variants where the proton transfer is rate limiting demonstrate solvent isotope effect of pH-independent moderate magnitude (2.11±0.26 (WT+Ni2+), 2.16±0.35 (WT+Cd2+) and 2.34±0.44 (L210DN/M17DN)) or pH-dependent large magnitude (5.7 at pH4 (L213DN)). Upon deuteration, the free energy and the enthalpy of activation increase in all proton transfer variants by about 1kcal/mol and the entropy of activation becomes negligible in L210DN/M17DN mutant. The results are interpreted as manifestation of equilibrium and kinetic solvent isotope effects and the structural, energetic and kinetic possibility of alternate proton delivery pathways are discussed.