Knudsen Effusion Technique Research Articles

Phase Transitions Equilibria of Five Dichlorinated Substituted Benzenes.

This work reports an experimental study aiming to determine the thermodynamic properties of five chlorinated compounds with environmental impact. The vapor pressures of the crystalline phases of three isomers of dichlorobenzoic acid (2,4-, 2,5-, and 2,6-) and 2,6-dichlorobenzonitrile were measured at several temperatures using the Knudsen effusion technique. Another technique (a static method based on capacitance diaphragm manometers) allowed the measurement of the vapor pressures of both the crystalline and liquid phases of 2,4-dichlorobenzonitrile between 303.0 and 380.0 K. This latter technique also enabled the measurement of sublimation vapor pressures of 2,6-dichlorobenzonitrile over a larger range interval of temperatures, T = 328.7 and 391.8 K. The standard molar enthalpy, entropy, and Gibbs energy of sublimation (for all the compounds studied) and vaporization (for 2,4-dichlorobenzonitrile) were derived, at reference temperatures, from the experimental vapor pressure results. The temperatures and enthalpies of fusion and the isobaric heat capacities of the five crystalline-substituted benzenes were determined using differential scanning calorimetry. The contributions of the three substituents (-COOH, -CN, and -Cl) to the sublimation thermodynamic properties of the compounds studied were discussed.

Thermodynamic and Kinetic Aspects of Formamidinium Lead Iodide Thermal Decomposition.

We report the results of a multi-technique study on the thermodynamics and kinetics of formamidinium lead iodide (FAPI) thermal decomposition. Thermodynamics was investigated by means of Knudsen effusion techniques. Kinetics was studied either by temperature-controlled powder X-ray diffraction or by two isoconversional treatments of differential scanning calorimetry data. FAPI appears to be much more thermally stable compared to methylammonium lead iodide, as predictable from the lower acidity of the formamidinium cation compared to methylammonium. The chemical processes responsible for its thermal degradation appear to be quite complex as highlighted by the composition of the gaseous phase evolved during the process. The apparent activation energy values of the decomposition obtained from X-ray diffraction (XRD) (112 ± 9 kJ/mol) and differential scanning calorimetry (DSC) measurements (205 ± 20 and 410 ± 20 kJ/mol, respectively, for the first and second decomposition steps identified by the deconvolution procedure) reflect the different steps of the process observed by the two techniques. The thermodynamic properties of the more important decomposition channels and the enthalpy of formation of FAPI were estimated by combining the results of Knudsen effusion measurements.

Vaporization thermodynamics of In2 O3 by Knudsen effusion mass spectrometry. The standard enthalpy of formation of In2 O(g).

In2 O3 is one of the most important semiconductor oxides in modern electronics. Vacuum deposition methods are often used for the preparation of In2 O3 -based nanomaterials. Thus, vaporization thermodynamics is of key importance for process control and optimization. Since the literature data on the vapor composition and partial pressure values for In2 O3 are contradictory, vaporization thermodynamics of In2 O3 needs to be clarified. Vaporization behavior of In2 O3 was studied using the Knudsen effusion technique in the temperature range 1400-1610 K. Quartz effusion cells were employed. A magnet mass spectrometer with an ordinary focus and a sector-type analyzer was used. Heating of samples and molecular beam ionization were performed by electron impact. The operating ionizing electron energy was 75 eV. A specially designed experiment allowed us to determine the individual mass spectrum of the In2 O molecule and, thus, to interpret the mass spectrum of the vapor registered during In2 O3 vaporization. The composition of the equilibrium vapor was quantified and the partial pressures of the vapor species were determined. On the basis of the experimental data, the standard enthalpies of some gaseous and heterogeneous reactions taking place during In2 O3 vaporization and the standard enthalpy of formation of In2 O(g) were calculated. The presence of In species in the vapor over In2 O3 was confirmed and the vapor composition was quantified. Thermodynamic characteristics of In2 O3 vaporization were obtained and a value of the standard enthalpy of formation of In2 O(g) was recommended. These data can be used for further thermodynamic calculations and for evaluating parameters for the synthesis and exploitation of In2 O3 -containing materials.

Structural and Energetic Insights on Two Dye Compounds: 1-Acetyl-2-Naphthol and 2-Acetyl-1-Naphthol.

The energy involved in the structural switching of acyl and hydroxyl substituents in the title compounds was evaluated combining experimental and computational studies. Combustion calorimetry and Knudsen effusion techniques were used to determine the enthalpies of formation, in the crystalline state, and of sublimation, respectively. The gas-phase enthalpy of formation of both isomers was derived combining these two experimental data. Concerning the computational study, the G3(MP2)//B3LYP composite method was used to optimize and determine the energy of the isomers in the gaseous state. From a set of hypothetical reactions it has been possible to estimate the gas-phase enthalpy of formation of the title compounds. The good agreement between the experimental and computational gas-phase enthalpies of formation of the 1-acetyl-2-naphthol and 2-acetyl-1-naphthol isomers, provided the confidence for extending the computational study to the 2-acetyl-3-naphthol isomer. The structural rearrangement of the substituents in position 1 and 2 in the naphthalene ring and the energy of the intramolecular hydrogen bond are the factors responsible for the energetic differences exhibited by the isomers. The gas phase tautomeric keto ↔ enol equilibria of the o-acetylnaphthol isomers were analyzed using the Boltzmann’s distribution.

Synthesis and Thermodynamic Properties of Cadmium Oxopivalate CdPiv2 ∙ Cd4OPiv6

The cadmium oxopivalate complex CdPiv2 ∙ Cd4OPiv6 has been synthesized and its thermodynamic characteristics have been studied using the Knudsen effusion technique with the mass spectral analysis of the gas phase. It has been found that vaporization of this complex proceeds congruently and the saturated vapor above it consists of molecules Cd2Piv4, Cd4Piv8, and Cd4OPiv6. The standard enthalpies of sublimation of these molecules have been determined; the standard enthalpy of vaporization of complex CdPiv2 ∙ Cd4OPiv6 and standard enthalpy of dissociation of the Cd4Piv8 molecule according to the second and third laws of the thermodynamics have been found to be $${{\Delta }_{S}}H_{{298}}^{^\circ }$$ = 297.0 ± 12.1 kJ/mol and $${{\Delta }_{D}}H_{{298}}^{^\circ }$$ = 133.2 ± 16.8 kJ/mol, respectively. It is shown that the oxopivalate cadmium complex can be used as a precursor in the MO CVD method for the preparation of thin oxide films and coatings.

Gas Phase Composition and Vaporization Thermodynamics of Cobalt(II) Pivalate Complexes

Vaporization of cobalt(II) pivalate [Co(Piv)2] (1, HPiv = HO2CCMe3) and cobalt(II) oxopivalate [Co4O(Piv)6] (2) complexes has been studied by Knudsen effusion technique in combination with mass-spectral analysis of gas phase composition. The congruent sublimation of the prepared compounds has occurred, it has been accompanied by partial thermal decomposition in the case of complex 1. Saturated vapor over complex 1 has been found to consist mainly of monomeric Co(Piv)2, dimeric Co2(Piv)4, and small amount of tetrameric Co4(Piv)8 molecules, as well as Co4O(Piv)6, CO2, and 2,2,4,4-tetramethylpentanone ones. Saturated vapor over complex 2 contains only Co4O(Piv)6 species. The absolute values of partial pressure and sublimation enthalpies of gas phase components have been calculated.

Vaporization behavior of an alkyl diphenyl ether and a commercial lubricant

The equilibrium vapor pressure as a function of temperature for an alkyl diphenyl ether with a vaporization coefficient significantly smaller than unity was characterized using the combined Langmuir free evaporation and mass loss Knudsen effusion technique. Results are compared using different cell designs and discrepancies, difficulties, and complications are discussed. The enthalpy and entropy of vaporization are estimated using the average temperature of the measurements.

Experimental and computational thermochemical studies of acridone and N-methylacridone

Abstract This paper reports the standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of acridone and N-methylacridone as (50.0 ± 5.0) kJ·mol−1 and (60.6 ± 4.3) kJ·mol−1, respectively. These data were obtained from experimental thermodynamic parameters, namely the standard molar enthalpies of formation, in the crystalline phase, at T = 298.15 K, derived from the standard molar enthalpies of combustion measured by static-bomb combustion calorimetry, and the standard molar enthalpies of sublimation, at T = 298.15 K, determined by Calvet microcalorimetry or the Knudsen effusion techniques. Additionally, the gas-phase enthalpies of formation were estimated by G3(MP2)//B3LYP calculations, using several hypothetical gas-phase reactions, and were compared with the corresponding ones determined experimentally. The gas-phase keto-enol tautomerization chemical equilibrium of acridone ↔ 9-acridinol was analysed using the Boltzmann’s distribution, being confirmed that the equilibrium favours the formation of the keto form. The bond dissociation enthalpies (N-H) and (C-H) and the gas-phase acidities were also determined. The electrostatic potential surfaces and the frontier molecular orbitals were determined for both compounds, allowing us to infer about their reactivity properties. Other properties studied include the HOMO-LUMO energy gap and the ionization potential.

Vaporization in the Ga2O3−ZnO system by high temperature mass spectrometry

Vaporization thermodynamics of the Ga2O3–ZnO system was studied by the Knudsen effusion technique in combination with mass spectrometric analysis of the vapor phase in the temperature range 1360–1460K. The incongruent character of vaporization processes in the whole composition range of the Ga2O3–ZnO system was established. The vapor composition and the partial pressures of the gas phase species were determined. The p–x projection of the Ga2O3–ZnO phase diagram at 1410K was constructed. The standard molar enthalpy of formation of ZnGa2O4 at 298K was found to be −1472.8±26.6kJ·mol−1.

Energetic, structural and tautomeric analysis of 2-mercaptobenzimidazole

This paper reports an experimental and theoretical study about the energetic and structural characteristics of the 2-mercaptobenzimidazole (MBI) tautomeric forms (thione and thiol). The standard (p° = 0.1 MPa) molar enthalpy of formation, at T = 298.15 K, in the gaseous phase, for MBI was derived from its enthalpies of combustion and sublimation, obtained by rotating-bomb calorimetry and by the Knudsen effusion technique, respectively. The results are compared with the corresponding data for MBI thione/thiol tautomers calculated by the G3(MP2)//B3LYP approach that suggest the thione form as the preferred form of MBI in gaseous phase. Computationally, the molecular structures of both tautomers were established and the geometrical parameters were determined at the B3LYP/6-31G(d) level of theory. In addiction, the G3(MP2)//B3LYP tautomerization energy at 0 K for thione/thiol forms of MBI was calculated and also evidences that the tautomeric equilibrium favours the thione tautomer. The standard Gibbs energy of formation in crystalline and gaseous phases was also derived, allowing an analysis of the thermodynamic stability of MBI in comparison with other related compounds.

Enthalpies of formation of four isoxazole derivatives in the solid and gas phases: application to the study of chemical equilibria

Calorimetry provides an accurate and reliable method to determine the enthalpies of formation of organic molecules in the gas phase. From the experimental formation enthalpies and the absolute entropies, obtained by theoretical calculations, it is possible to perform Gibbs energy calculations. This thermodynamic function is widely used to describe various thermodynamic processes, such as chemical equilibrium, and allows the calculation of equilibrium constants. The specific standard combustion energies of 3,5-dimethylisoxazole-4-carboxylic acid, 5-methylisoxazole-3-carboxylic acid, 5-amino-3-methylisoxazole, and 3-amino-5-methylisoxazole were determined by using a combustion calorimeter. The sublimation enthalpies of the compounds were determined by means of the Knudsen effusion technique. From these values, the molar standard enthalpy of formation in gaseous phase of each compound was calculated. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of these compounds was carried out. The calculated enthalpies of formation are in very good agreement with the experimental values. From both the experimental and theoretical results, five gas phase chemical equilibria were studied: one of isomerization, two of CO2 loss, and two of NH3 loss. The equilibrium constants for each process were calculated, which allow prediction of the direction of the chemical process from a thermodynamic viewpoint.

Vapour pressures and phase transition properties of four substituted acetophenones

This paper reports experimental vapour pressures at different temperatures of four substituted acetophenones. Using the diaphragm manometer static method, the vapour pressures of both (crystalline and liquid) condensed phases of ortho and para acetoxyacetophenones and also of para-methoxyacetophenone were measured in the following temperatures ranges T=(313.0–398.0)K, (315.0–378.3)K and (296.2–355.7)K, respectively. This experimental technique was also used to measure the vapour pressures of the liquid phase of meta-acetoxyacetophenone through the temperature interval T=(305.1–368.5)K. The vapour pressures of the crystalline phase of meta-acetoxyacetophenone were measured using the Knudsen effusion technique over the temperature ranges, T=(297.7–307.3)K. This technique was also used for complementing the range of the vapour pressure measurements of the ortho and para acetoxyacetophenone isomers between T=(298.0–321.3)K and (298.1–309.3)K, respectively. The standard molar enthalpies, entropies and Gibbs energies of sublimation and of vaporization, at references temperatures, were derived from the experimental results. Differential scanning calorimetry was used to determine the temperatures and molar enthalpies of fusion and of the observed crystalline transitions. The contributions of the acetoxy and acetyl groups to the sublimation properties of the compounds studied were calculated.

Thermochemical studies on two alkyl-bulky substituted xanthene derivatives: 9,9-dimethylxanthene and 2,7-di-tert-butyl-9,9-dimethylxanthene

Thermodynamic properties of 9,9-dimethylxanthene and 2,7-di-tert-butyl-9,9-dimethylxanthene for the condensed and gas states were derived from experimental and computational studies. Static-bomb combustion calorimetry, vacuum drop microcalorimetry and the Knudsen effusion techniques were used. Computational calculations of the enthalpies of hypothetical reactions in the gaseous phase, using the G3(MP2)//B3LYP composite method, were performed for the two xanthene derivatives. Natural bond orbital (NBO) calculations were also performed to ascertain the structure and reactivity of these compounds.The energetic effects caused by replacing hydrogen atoms in the xanthene moiety by methyl and tert-butyl groups yielding 9,9-dimethylxanthene and 2,7-di-tert-butyl-9,9-dimethylxanthene species were determined from direct comparison of their standard (po=0.1MPa) molar enthalpies of formation in the gaseous phase, at T=298.15K.

Thermodynamic properties of alkyl 1H-indole carboxylate derivatives: A combined experimental and computational study

The standard (po=0.1MPa) molar enthalpies of formation, in the crystalline phase, of methyl 1H-indole-3-carboxylate and ethyl 1H-indole-2-carboxylate, at T=298.15K, were derived from measurements of the standard massic energies of combustion using a static bomb combustion calorimeter. The Knudsen effusion technique was used to measure the vapour pressures as a function of the temperature, which allowed determining the standard molar enthalpies of sublimation of these compounds. The standard (po=0.1MPa) molar enthalpies of formation, in the gaseous phase, at T=298.15K, were calculated by combining, for each compound, the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation, yielding −(207.6±3.6)kJ·mol−1 and −(234.4±2.4)kJ·mol−1, for methyl 1H-indole-3-carboxylate and ethyl 1H-indole-2-carboxylate, respectively.Quantum chemical studies were also conducted, in order to complement the experimental study. The gas-phase enthalpies of formation were estimated from high level ab initio molecular orbital calculations, at the G3(MP2) level, for the compounds studied experimentally, extending the study to the methyl 1H-indole-2-carboxylate and ethyl 1H-indole-3-carboxylate. The results obtained were compared with the experimental data and were also analysed in terms of structural enthalpic group contributions.

Energetics of neutral and deprotonated (Z)-cinnamic acid

We have performed a study of structural, thermochemical and thermophysical properties of the (Z)-cinnamic acid neutral molecule and its corresponding oxyanion (formed by deprotonation of the carboxylic group). The thermophysical study (heat capacities, temperature and enthalpy of fusion) was made by DSC. The following intrinsic (gas-phase) thermochemical magnitudes have been experimentally determined: (i) standard enthalpy of formation, at T=298.15K, of the neutral molecule, ΔfHm0(g)=(−215.5±3.2)kJ·mol−1, by combustion calorimetry and by the Knudsen effusion technique, (ii) deprotonation enthalpy, ΔacidH0(g)=(1416.4±8.8)kJ·mol−1 and acidity, GA=(1386.7±8.8)kJ·mol−1, by the EKM method using ESI-TQ Mass Spectrometry. From these results we have also derived the enthalpy of formation of the oxyanion, ΔfHm0(oxyanion, g)=(−303.5±9.4)kJ·mol−1. A computational study, through density functional calculations at the B3LYP/6-311++G(d,p) level of theory, was used to check the good consistency of the experimental results. The global results show that (Z)-cinnamic acid is significantly less stable than the corresponding (E)-isomer, which can be related to the greater acidity of the (Z)-form found in both the gas and aqueous solution phases.

Thermodynamic study of (anthracene + phenanthrene) solid state mixtures

Polycyclic aromatic hydrocarbons (PAH) are common components of many materials, such as petroleum and various types of tars. They are generally present in mixtures, occurring both naturally and as byproducts of fuel processing operations. It is important to understand the thermodynamic properties of such mixtures in order to understand better and predict their behavior (i.e., fate and transport) in the environment and in industrial operations. To characterize better the thermodynamic behavior of PAH mixtures, the phase behavior of a binary (anthracene+phenanthrene) system was studied by differential scanning calorimetry, X-ray diffraction, and the Knudsen effusion technique. Mixtures of (anthracene+phenanthrene) exhibit non-ideal mixture behavior. They form a lower-melting, phenanthrene-rich phase with an initial melting temperature of 372K (identical to the melting temperature of pure phenanthrene) and a vapor pressure of roughly lnP/Pa=−2.38. The phenanthrene-rich phase coexists with an anthracene-rich phase when the mole fraction of phenanthrene (xP) in the mixture is less than or equal to 0.80. Mixtures initially at xP=0.90 consist entirely of the phenanthrene-rich phase and sublime at nearly constant vapor pressure and composition, consistent with azeotrope-like behavior. Quasi-azeotropy was also observed for very high-content anthracene mixtures (2.5<xP<5) indicating that anthracene may accommodate very low levels of phenanthrene in its crystal structure.

Experimental and computational energetic study of 1-R-2-phenylindole (R = H, CH3, C2H5)

The standard (po=0.1MPa) molar energies of combustion, ΔcUmo, of 2-phenylindole, 1-methyl-2-phenylindole and 1-ethyl-2-phenylindole, in the crystalline state, were determined, at T=298.15K, using a static bomb combustion calorimeter. The vapour pressures as a function of the temperature for these compounds were measured by the Knudsen effusion technique and the standard molar enthalpies of sublimation were derived. From the experimental results, the standard (po=0.1MPa) molar enthalpies of formation in the condensed and gaseous phases, at T=298.15K, of 2-phenylindole, 1-methyl-2-phenylindole and 1-ethyl-2-phenylindole were calculated.Additionally, the gas-phase enthalpies of formation of these compounds and also for 1-ethylindole were estimated by G3(MP2) calculations. Enthalpies of formation, obtained using appropriate working reactions, were calculated and compared with the experimental data. The results were also analysed in terms of structural enthalpic group contributions.

Thermochemistry of 6-propyl-2-thiouracil: An experimental and computational study

The standard (po = 0.1 MPa) molar enthalpy of formation of 6-propyl-2-thiouracil was derived from its standard molar energy of combustion, in oxygen, to yield CO2 (g), N2 (g) and H2SO4·115H2O (l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The vapor pressures as function of temperature were measured by the Knudsen effusion technique and the standard molar enthalpy of sublimation, ΔcrgHmo, at T = 298.15 K, was derived by the Clausius–Clapeyron equation. These two thermodynamic parameters yielded the standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K: −(142.5 ± 1.9) kJ mol−1. This value was compared with estimates obtained from very accurate computational calculations using the G3 and G4 composite methods.

Thermodynamic study of 2-aminothiazole and 2-aminobenzothiazole: Experimental and computational approaches

This work reports an experimental and computational thermochemical study of two aminothiazole derivatives, namely 2-aminothiazole and 2-aminobenzothiazole.The standard (p°=0.1MPa) molar energies of combustion of these compounds were measured by rotating bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T=298.15K, were derived from the temperature dependence of the vapor pressures of these compounds, measured by the Knudsen-effusion technique and from high temperature Calvet microcalorimetry. The conjugation of these experimental results enabled the calculation of the standard molar enthalpies of formation in the gaseous state, at T=298.15K, for the compounds studied. The corresponding standard Gibbs free energies of formation in crystalline and gaseous phases were also derived, allowing the analysis of their stability, in these phases.We have also estimated the gas-phase enthalpies of formation from high-level molecular orbital calculations at the G3(MP2)//B3LYP level of theory, the estimates revealing very good agreement with the experimental ones.The importance of some stabilizing electronic interactions occurring in the title molecules has been studied and quantitatively evaluated through Natural Bonding Orbital (NBO) of the corresponding wavefunctions and their Nucleus Independent Chemical Shifts (NICS) parameters have been calculated in order to rationalize the effect of electronic delocalization upon stability.

Vaporization thermodynamics of the ZnO–SnO2 system

Vaporization processes in the ZnO–SnO2 system have been studied by the Knudsen effusion technique in combination with mass spectrometric analysis of the vapor phase in the temperature range 1360K to 1460K. It has been identified by studying the ZnO–SnO2 system in quartz cells that its gas phase mainly consists of Zn(g), O2 and SnO(g).The partial pressures of the vapor species were determined at 1450K. The incongruent character of the Zn2SnO4 vaporization was stated, and the p–x projection of the ZnO–SnO2 phase diagram at 1450K was constructed. The standard molar enthalpy of formation of the mixed oxide Zn2SnO4 at 298K was found to be −1296.2±7.5kJ·mol−1.