Thermochemical Measurements Research Articles

Cathodic Decomposition Electrodes as Standard Reference Electrodes for Molten Salts: Example of the Lithium Eutectic Electrode for the LiCl-KCl Eutectic

Alternatives to the widely-used standard anodic decomposition reference electrodes in molten salts are necessary to enable more easily reproduced thermochemical and electrochemical data in molten salt electrolytes. The class of standard reference electrodes called cathodic decomposition electrodes (CDEs) are easily constructed and can be used to make thermochemical measurements in molten salts more directly compared to anodic decomposition electrodes. The lithium eutectic electrode (LEE) was chosen as a sample test case for validation and was applied to thermochemical measurements of electroactive species in molten LiCl-KCl eutectic. Transient measurements were made to measure the Li+/Li reduction potential at zero current in pure LiCl-KCl eutectic relative to a Li-alloy reference electrode to validate the reference potential of the LEE. Literature-reported electromotive force measurements against Li-alloy reference electrodes were used to generate a relationship between the LEE and the standard chlorine electrode and this relationship was used to evaluate measured and reported formal potential measurements for the LiCl-KCl-GdCl3 system. This work demonstrates the general framework for defining CDEs for any molten salt system and a method for calibrating external reference electrodes against a CDE standard reference electrode, improving the ease of obtaining thermochemical and electrochemical measurements in any molten salt system.

How hot are weakly-bound cluster ions in a blackbody field? Insights from master equation modeling

Measuring accurate energetics in mass spectrometry thermochemical and calorimetry experiments depends on detailed knowledge of the energetics of the ion populations. For example, in cases where blackbody infrared radiative dissociation (BIRD) kinetics are not in the rapid energy exchange (REX) limit, threshold dissociation energies obtained directly from experiment will be too low. When ions are not in the REX limit, the ion internal energy distributions can be modeled using a master equation (ME). The ME allows evaluation of ion internal energies over time with a set of rate equations that describe the transfer of energy from one energy state to another. Here, ME modeling that accounts for the radiative absorption and emission, and dissociation rate constants is performed to determine the energetics of two model systems, M2+(H2O)n with n = 24, 55, 96, 178 and H+(AlaGly)n with n = 4, 8, 16, 32, activated by a blackbody field at temperatures between 120 and 200 K. The hydrated cluster and oligopeptide sizes are chosen such that respective ions have comparable number of internal degrees-of-freedom. The effects of blackbody temperature and inherent properties, such as frequencies and infrared (IR) intensities, molecule size, and dissociation parameters (threshold dissociation energy, E0, and high-pressure pre-exponential factor, A∞) on the resulting ion effective temperatures, steady-state energy distributions, and BIRD kinetics are explored. ME results show that at low blackbody temperatures (<∼140 K), the steady-state internal energy distributions of the ion populations resemble those of Boltzmann distributions at the blackbody temperature. At higher blackbody temperatures (>∼140 K), rapid dissociation causes the steady-state internal energy distributions to equilibrate to lower energies where absorption and emission are competitive with dissociation. This results in ion effective temperatures that deviate from and are “colder” than the blackbody temperatures. The temperature where this transition occurs depends on the competition among absorption, emission, and dissociation, and is controlled by the dissociation parameters, vibrational frequencies, and IR intensities, as illustrated for M2+(H2O)n and H+(AlaGly)n. This work shows that, under certain conditions, the ion effective temperatures can deviate significantly from those of the blackbody field temperatures. ME modeling can be used to determine the energy content of ion complexes in mass spectrometry experiments to improve the accuracy of thermochemical and calorimetry measurements of weakly-bound clusters and for more confident assignments of conformations and structures in action spectroscopy.

Advancing thermochemical diagnostics in kilogram-scale explosive fireballs via laser absorption spectroscopy

This article presents methodological advances in the state-of-the-art for making time-dependent, thermochemical measurements within kilogram-scale explosive post-detonation fireballs utilizing tunable laser absorption spectroscopy. This measurement capability is critical for validating multi-scale, multi-physics models of post-detonation dynamics. The technique is based on hardened gauges built around rapidly-tunable lasers and custom post-processing algorithms that provide quantitative thermochemical data interior to large and opaque explosive fireballs. The authors present a holistic overview of the technique including gauge design, the laser absorption diagnostic, and the custom data processing algorithms. Additionally, fielding high-bandwidth laser absorption probes at stand-off ranges presents new challenges in data processing that must compensate for long distance signal transmission effects. We highlight representative data from a hardened gauge measurement at 0.81 m stand-off from a 2.78 kg LX-14 explosive charge detonated in an outdoor test arena. We discuss progress in all-optical measurement of temperature, pressure, and water vapor number density at a 100 kHz repetition rate during the first 10 ms of the fireball evolution. We conclude the article with a brief discussion on our current approach for comparing hardened gauge measurements with computational fluid dynamic simulations.

Comprehensive Thermodynamic Study of Alkyl-Cyclohexanes as Liquid Organic Hydrogen Carriers Motifs

Alkyl-cyclohexanes can be considered as suitable model compounds to understand the thermochemistry of aromatic compounds and their hydrogenated counterparts discussed as Liquid Organic Hydrogen Carrier systems. Thermochemical measurements on these hydrogen-rich compounds are thwarted by complications due to the 99.9 % purity limitation and sample size specific to these methods. However, the data on vaporisation and formation enthalpies are necessary to optimize the hydrogenation/dehydrogenation processes. In this work, various empirical and theoretical methods are described to reliably assess the gas phase enthalpies of formation and vaporization enthalpies of alkyl-substituted cyclohexanes. The empirical and quantum-chemical methods have been validated against reliable literature data and provide reasonable estimates with an accuracy comparable to that of the experimental data. The liquid phase enthalpies of formation of differently shaped alkyl-cyclohexanes were derived and used to estimate the energetics of their dehydrogenation reactions. The influence of alkyl substituents on the reaction enthalpy is discussed. The vapour pressures of typical hydrogen-rich compounds at technically relevant temperatures were calculated and compared to vapour pressures of biodiesel fuels measured in this work using the static method.

Rational Design of Iron Spin-Crossover Complexes Using Heteroscorpionate Chelates.

The optical, structural, and magnetic properties of iron(II,III) sandwich complexes, Fe(Tp')2n+ (Tp' = bis(3,5-dimethylpyrazolyl)benzotriazolylborate), are described. The intensely colored FeII(Tp')2 (orange) and FeIII(Tp')2+ (purple) show strong MLCT bands. Geometric isomerism for M(Tp')2 is established crystallographically in the racemate of chiral cis-Fe(Tp')2. For the first time, paramagnetic 11B NMR describes solution-phase low-spin (LS, S = 0) to high-spin (HS, S = 2) crossover behavior in Fe(Tp')2. Thermochemical parameters for solution-phase SCO of Fe(Tp')2 demonstrate the endothermic LS to HS conversion and entropic preference of the HS state. Entropy changes for both Fe(Tp')2 isomers are significantly larger than for the majority of iron scorpionate SCO systems. Solid-state magnetic and thermochemical measurements show cis-Fe(Tp')2 to be thermally stable up to 520 K, allowing experimental investigation of a solid-state SCO magnetic hysteresis of over 45 K. A large solution vs solid-state SCO difference was observed: cis-Fe(Tp')2 shows Tc ≈ 270 K (solution) and Tc ≈ 385 K (solid), with the remarkably wide ΔTc ≈ 115 K; trans-Fe(Tp')2 shows Tc ≈ 278 K (solution) and Tc ≈ 372 K (solid). Solid-state Tc values are among the highest seen for iron(II) molecular systems. The large solution/solid ΔTc difference is explained by "anchoring" intermolecular interactions in the solid state that prevent thermal expansion of the LS iron(II) coordination sphere in its transition to the HS state. DFT calculations, validated against LS cis-Fe(Tp')2 crystallography and LS to HS SCO thermochemical parameters, demonstrate the role the benzotriazole rings play in its structural and optical properties. The Lewis basicity of M(Tp')2 is shown with the structural characterization of the air-stable tin(II) adduct [cis-Fe(Tp')2-SnCl2]; tin(II) coordination does not alter the iron(II) spin state. The Tp' chelate adds functionality (asymmetry, chirality, chemical reactivity) to the array of iron SCO materials for potential incorporation into nanoscale magnetic switches and spintronic devices.

Intercalation‐assisted massive phase transformation: The key to SHS synthesis?

AbstractSelf‐propagating high‐temperature synthesis (SHS) is the term used for a spontaneous, irreversible combustion of initial reagents when ignited. There is limited knowledge of the mechanistics of this versatile materials synthesis process for Chevrel phase compounds (CPCs), which limits its applicability. In this report, the successful and rapid processing of three classes of sulfide CPCs via SHS is demonstrated, and a mechanism is provided to explain rapid transformation rates and nanostructured products that are fully transformed. Cu4Mo6S8, Ni2Mo6S8, and Fe2Mo6S8 compounds were synthesized, and thermochemical measurements revealed that an overall reaction temperature did not surpass the melting points of the reactants. The nature of the specific SHS process is argued here to involve an intercalation‐assisted massive phase transformation facilitated by the use of an MoS2 precursor.

Gas-Phase External Solvation of Protonated Benzonitrile by Eight Methanol Molecules.

The gas-phase sequential association of methanol onto protonated benzonitrile (C6H5CNH+) and the proton-bound dimer (C6H5CN)2H+ have been examined experimentally by equilibrium thermochemical measurements and computationally by density functional theory (DFT). The bonding enthalpy (ΔH°) for the association of methanol with protonated benzonitrile (25.2 kcal mol-1) reflects the strong electrostatic interaction provided by the formation of an ionic hydrogen bond in the C6H5CNH+OHCH3 cluster in excellent agreement with a DFT-calculated binding energy of 24.9 kcal mol-1. The sequential bonding enthalpy within the (C6H5CN)H+(OHCH3)n clusters decreases from 25.2 to 10.6 kcal mol-1 for the eighth solvation step (n = 8), which remains more than 25% above the enthalpy of vaporization of liquid methanol (8.4 kcal mol-1). The nonbulk convergence of ΔH°n-1,n with eight solvent molecules is attributed to the external solvation of a benzonitrile molecule by an extended hydrogen bonding network of protonated methanol clusters H+(CH3OH)n. In the external solvation of protonated benzonitrile by methanol, the proton resides on the methanol subcluster and the neutral benzonitrile molecule remains outside and bonded to the surface of the protonated methanol cluster. The bonding enthalpy of methanol to the proton-bound benzonitrile dimer (C6H5CN)H+(NCC6H5) is measured to be 18.0 kcal mol-1, in good agreement with a DFT-calculated value of 17.1 kcal mol-1, which reflects the association of the proton with the lower proton affinity methanol molecule, thus forming a highly stable structure of protonated methanol terminated by two ionic hydrogen bonds to the two benzonitrile molecules. The external solvation of benzonitrile by methanol ices in space allows benzonitrile to remain on the ice grain surface rather than being isolated inside the ice. This could provide accessibility for reactions with incoming ions and molecules or for photochemical processes by UV irradiation, leading to the formation of complex organics on the surface of ice grains.

Hybrid constraint multi-line absorption spectroscopy for non-uniform thermochemical measurements in axisymmetric laminar and jet flames

We propose a novel strategy for the accurate determination of non-uniform thermochemical distributions in axisymmetric flames using line-of-sight laser absorption spectroscopy with the constraints of hybrid information. In this method, the computational fluid dynamics (CFD) simulation or other diagnostic method (e.g., thermocouple measurement) is used to qualitatively characterize the thermochemical properties, whereas the kinetic modelling, chemical equilibrium calculation and thermocouple measurement are used to constrain the range of unknown parameters. Experimental demonstrations were firstly performed in a standard laminar CH4/air premixed flame stabilized on a McKenna burner at different equivalence ratios (Φ = 0.8-1.2). With proper hybrid constraints, multiple solutions are avoided, and the deviation and convergence time can be simultaneously reduced comparing to the conventional method. For further demonstration, this strategy was then applied to a turbulent jet flame to recover the significant thermochemical gradient without the need for tomographic reconstruction techniques.

Sensitivity estimation for calculated phase equilibria

The development of a consistent framework for Calphad model sensitivity is necessary for the rational reduction of uncertainty via new models and experiments. In the present work, a sensitivity theory for Calphad was developed, and a closed‐form expression for the log‐likelihood gradient and Hessian of a multi‐phase equilibrium measurement was presented. The inherent locality of the defined sensitivity metric was mitigated through the use of Monte Carlo averaging. A case study of the Cr–Ni system was used to demonstrate visualizations and analyses enabled by the developed theory. Criteria based on the classical Cramér–Rao bound were shown to be a useful diagnostic in assessing the accuracy of parameter covariance estimates from Markov Chain Monte Carlo. The developed sensitivity framework was applied to estimate the statistical value of phase equilibria measurements in comparison with thermochemical measurements, with implications for Calphad model uncertainty reduction.

Critical thermodynamic evaluation of the binary sub-systems of the core sulphate system Na2SO4–K2SO4–MgSO4–CaSO4

The binary subsystems of the quaternary system Na2SO4–K2SO4–MgSO4–CaSO4 were thermodynamically assessed using the available experimental information including own thermochemical and structure measurements. Phase equilibria and thermodynamic properties of all six binary systems were properly considered. The modified associate species model was successfully applied for the description of the liquid phase while the mutual solubility between various sulphates in the solid state was treated with appropriate multi-sublattice models. The resulting new databank being compatible with the general oxide database was used for the representation of the thermodynamic properties in the binary systems including phase equilibria, activity and mixing data. The calculations are in good agreement with the experimental data.

Sensitivity estimation for calculated phase equilibria

Abstract

Formation of the oxonium phenol ion in the stepwise hydration of the phenyl cation in the gas phase

This study provides the first evidence for the formation of the oxonium phenol ion and its hydrated clusters by the sequential addition of water molecules onto the phenylium ion in the gas phase. The oxonium phenol ion exhibits thermal stability at higher temperatures up to nearly 575 K while the hydrated phenyl cation dissociates by the loss of water at temperatures below 400 K. A second water molecule is attached reversibly to each of the oxonium phenol and the hydrated phenyl ions and equilibrium thermochemical measurements in the low temperature range yield an average −ΔH° of 15.5 kcal/mol reflecting 35% contribution from the hydrated oxonium phenol C6H5OH2+(H2O) and 65% contribution from the second hydration step of the phenyl cation C6H5+(H2O)2. In the high temperature range (373–423 K), the measured average −ΔH° of 18.7 kcal/mol reflects 45% and 55% contributions from the hydrated oxonium phenol and the second hydration step of the phenyl cation, respectively. DFT calculations at the B3LYP/6-311++G** level show that the sequential hydration of the oxonium phenol ion results in the formation of externally hydrated clusters C6H5OH2+(H2O)n where the incoming water molecules form a hydrogen bonding network attached to the oxonium site, and the sequential hydration energy decreases from 25.9 kcal/mol for n = 1 to 11.5 kcal/mol for n = 5. For the hydrated phenyl cation clusters C6H5+(H2O)n, the water molecules are attached at two CHδ+ sites of the phenyl cation, and the sequential hydration energy decreases slowly from 11.3 kcal/mol for n = 1 to 8.1 kcal/mol for n = 5.

Determining Proton-Coupled Standard Potentials and X–H Bond Dissociation Free Energies in Nonaqueous Solvents Using Open-Circuit Potential Measurements

Proton-coupled electron transfer (PCET) reactions are increasingly being studied in nonaqueous conditions, where the thermochemistry of PCET substrates is largely unknown. Herein, we report a method to obtain electrochemical standard potentials and calculate the corresponding bond dissociation free energies (BDFEs) of stable PCET reagents in nonaqueous solvents, using open-circuit potential (OCP) measurements. With this method, we measure PCET thermochemistry in acetonitrile and tetrahydrofuran for substrates with O-H and N-H bonds that undergo 1e-/1H+ and 2e-/2H+ redox processes. We also report corrected thermochemical values for the 1/2H2(g)/H•1M and H+/H• (CG) couples in several organic solvents. For 2e-/2H+ couples, OCP measurements provide the multielectron/multiproton standard potential and the average of the two X-H BDFEs. In contrast to traditional approaches for calculating BDFEs from electrochemical measurements, the OCP method directly measures the overall PCET reaction thermodynamics and avoids the need for a pKa scale in the solvent of interest. Consequently, the OCP approach yields more accurate thermochemical values and should be general to any solvent mixture compatible with electrochemical measurements. The longer time scale of OCP measurements enables accurate thermochemical measurements for redox couples with irreversible or distorted electrochemical responses by cyclic voltammetry, provided the PCET reaction is chemically reversible. Recommendations for successful OCP measurements and limitations of the approach are discussed, including the current inability to measure processes involving C-H bonds. As a straightforward and robust technique to determine nonaqueous PCET thermochemistry, these OCP measurements will be broadly valuable, with applications ranging from fundamental reactivity studies to device development.

Probing the conformational landscape and thermochemistry of DNA dinucleotide anions via helium nanodroplet infrared action spectroscopy.

Isolation of biomolecules in vacuum facilitates characterization of the intramolecular interactions that determine three-dimensional structure, but experimental quantification of conformer thermochemistry remains challenging. Infrared spectroscopy of molecules trapped in helium nanodroplets is a promising methodology for the measurement of thermochemical parameters. When molecules are captured in a helium nanodroplet, the rate of cooling to an equilibrium temperature of ca. 0.4 K is generally faster than the rate of isomerization, resulting in "shock-freezing" that kinetically traps molecules in local conformational minima. This unique property enables the study of temperature-dependent conformational equilibria via infrared spectroscopy at 0.4 K, thereby avoiding the deleterious effects of spectral broadening at higher temperatures. Herein, we demonstrate the first application of this approach to ionic species by coupling electrospray ionization mass spectrometry (ESI-MS) with helium nanodroplet infrared action spectroscopy to probe the structure and thermochemistry of deprotonated DNA dinucleotides. Dinucleotide anions were generated by ESI, confined in an ion trap at temperatures between 90 and 350 K, and entrained in traversing helium nanodroplets. The infrared action spectra of the entrained ions show a strong dependence on pre-pickup ion temperature, consistent with the preservation of conformer population upon cooling to 0.4 K. Non-negative matrix factorization was utilized to identify component conformer infrared spectra and determine temperature-dependent conformer populations. Relative enthalpies and entropies of conformers were subsequently obtained from a van't Hoff analysis. IR spectra and conformer thermochemistry are compared to results from ion mobility spectrometry (IMS) and electronic structure methods. The implementation of ESI-MS as a source of dopant molecules expands the diversity of molecules accessible for thermochemical measurements, enabling the study of larger, non-volatile species.

Equimolecular cocrystal of cis- and trans-coordinated N,N′-dimethylglycolurils: Some standard thermodynamic properties in the aqueous solution between 278.15 K and 318.15 K

In this paper, we have discussed the results of densimetric and calorimetric studies of dissolution process in water for the (1:1) cocrystal consisting of two heterocyclic isomers being cis- and trans-coordinated dimethylglycolurils (2,8-DMGU and 2,6-DMGU, respectively) in the temperature range from (278.15 to 318.15) K at the ambient pressure. The standard (apparent or partial at infinite dilution) molar volumes and isobaric expansibilities of cocrystal as a solute in water were derived from the experimental data on solution density. Using thermochemical measurements, the standard molar enthalpies and isobaric heat capacities of the dissolution of cocrystal in water were also determined at the temperatures of interest. Based on the previously obtained similar results for aqueous 2,6-DMGU and 2,8-DMGU, a comparative analysis of packing and interaction changes in the hydration complex induced by the cocrystallization process has been carried out. It was found that the formation of cocrystal leads to a peculiar “synergism” in both volume and heat capacity effect of N,N′-dimethylglycoluril hydration at all temperatures studied.

Neutron Spectroscopic and Thermochemical Characterization of Lithium–Aluminum-Layered Double Hydroxide Chloride: Implications for Lithium Recovery

Lithium–aluminum-layered double hydroxide chloride (LDH) has been recently demonstrated to be a promising sorbent material for selective lithium extraction and recovery from geothermal brine. In this research, LDH samples synthesized from two different starting materials (alumina and gibbsite) and with two post-drying conditions (ambient-dried and oven-dried) as well as and Fe-doped LDH are studied using neutron vibrational spectroscopy. The results reveal differences in the ordering of interlayer water molecules. Thermochemical measurements of LDHs show that a higher interlayer ordering of water is directly related to the stability of gibbsite- and alumina-derived LDH. This corroborates with the previous work on increasing the stabilization of LDH via partial substitution of aluminum with iron. These findings on the properties of LDH have implications on their lithium extraction abilities and subsequent recovery.

Understanding the solvation process and solute-solvent interactions of imidazole compounds in three different solvents through solution calorimetry and 1H NMR

The enthalpies of solvation of imidazole, 2-methyl-, 2-ethyl- and 2-isopropyl-imidazole in water, methanol and acetonitrile were determined through thermochemical measurements of the corresponding enthalpies of sublimation and solution. The Pierotti equation, which incorporates the enthalpy of cavity formation and the solvent reorganization for each solute in each solvent, was used to obtain the corresponding enthalpies of interaction. Additionally, the 1H NMR spectra of all solutes, in the corresponding deuterated solvents, were also obtained. The discussion of the solvation process and enthalpies of interaction of the imidazole compounds is based on the specific intermolecular interactions, like hydrogen bonds, H − π and non-specific interactions from London forces nature, supported by the features of the 1H NMR spectra of each solute in each solvent.

Mechanochemical Synthesis, Accelerated Aging, and Thermodynamic Stability of the Organic Mineral Paceite and Its Cadmium Analogue

We demonstrate the use of ball milling mechanochemistry for rapid, simple, and materials-efficient synthesis of the organic mineral paceite CaCu(OAc)4·6H2O (where OAc– is the acetate ion), composed of coordination polymer chains containing alternating Ca2+ and Cu2+ ions, as well as its cadmium-based analogue CaCd(OAc)4·6H2O. While the synthesis of paceite in aqueous solutions requires a high excess of the copper precursor, mechanochemistry permits the use of stoichiometric amounts of reagents, as well as the use of poorly soluble and readily accessible calcium carbonate or hydroxide reactants. As established by thermochemical measurements, enthalpies of formation of both synthetic paceite and its cadmium analogue relevant to the mechanochemical reactions are highly exothermic. Reactions can also be conducted using accelerated aging, a synthetic technique that mimics geological processes of mineral weathering. Accelerated aging reactivity involving copper(II) acetate monohydrate (hoganite) and calcium carbonate (calcite) provides a potential explanation of how complex organic minerals like paceite could form in a geological environment.

Excess functions of chlorite solid solutions and neoformation of Fe-chlorites: Some implications of recent thermochemical measurements

Excess functions of chlorite solid solutions and neoformation of Fe-chlorites: Some implications of recent thermochemical measurements

Temperature gradients for thermophysical and thermochemical property measurements to 3000 °C for an aerodynamically levitated spheroid.

This study examines thermal gradients in ceramic oxide spheroids being aerodynamically levitated in a conical nozzle levitator (CNL) system equipped with a CO2 laser (10.6 µm wavelength). The CNL system is a versatile piece of equipment that can easily be coupled with advanced thermophysical and thermochemical measuring devices, such as diffraction/scattering (X-ray and neutron), nuclear magnetic resonance, and calorimetry, for the analysis of bulk spheroidal solids and liquids. The thermal gradients of a series of single crystal, polycrystalline solids, and liquid spheroids have been measured spatially in the CNL system, by means of a disappearing filament pyrometer (800-3000 °C) and by X-ray diffraction with reference to an internal standard (Pt: 800-1600 °C). The thermal gradient in a levitated sample being heated by a laser from the top can be minimized by: (i) maximizing the sphericity, (ii) maximizing the density, and (iii) minimizing microstructural features. A spheroid with these properties can be manufactured via machining a perfect sphere from a highly dense, chemically and phase pure pellet. These properties promote rotation of the sample about multiple axes in the air stream, enabling homogeneous heating. This homogeneous heating is the dominant factor in reducing thermal gradients in solid state samples. It was found that the thermal gradient in an ∼3 mm diameter solid sample could be reduced from 1000 °C to 30 °C, by having a perfectly spherical shape that could rotate on multiple axes in a high velocity gas stream (∼1500-2000 cm3/min). These findings will allow accurate thermophysical and thermochemical property measurements of solids in situ at high temperatures, using the CNL system.