Vaporation characteristics of low-melting nitrocompounds by isothermal thermogravimetry

Determination of vapor pressure is critical for accurate detection of trace volatile hazardous materials, ensuring human health and environmental security. 2,4,6-Trinitrotoluene (TNT), 3,4-Dinitrofurazanfuroxan (DNTF), and 2,4-Dinitroanisole (DNAN) are an important class of volatile low-melting nitrocompounds which are widely used in military, aerospace, and defense industry. In this study, the sublimation and evaporation characteristics of these three nitrocompounds were investigated for the first time by using isothermal thermogravimetry (TG) undergoing zero order, non-activated evaporation processes, analyzed with the Antoine and Langmuir equations, and confirmed with benzoic acid as a calibration material. The Clausius–Clapeyron equations of TNT, DNTF, and DNAN at both sublimation and evaporation processes are established using the temperature dependence of vapor pressure. The enthalpies of sublimation and evaporation are determined. The results of TNT are well consistent with literatures, proving that the isothermal TG for determination of vapor pressure is eligible and accurate. This work lays the foundation for further study of the reliable trace detection of hazardous low-melting nitrocompounds.     

Combined experiments to measure low sublimation pressures and diffusion coefficients of organometallic compounds

Vapor pressures and sublimation pressures of organometallic (metalorganic) compounds are needed in several processes like chemical vapor deposition (CVD). Thermobalances at ambient pressures are often used to study the evaporation of such compounds. At least three strategies are found in the literature to evaluate the results using different theoretical approaches. In some of the frequently used approaches the diffusion out of a crucible is neglected. We present a simple theoretical approach which describes the interrelation between the observed mass transfer rate and the physical variables of typical TGA set-ups. It turns out that the mass transfer rate at a given total pressure and temperature is mainly a function of the diffusion coefficient and the vapor pressure of the sublimating substance. The vapor pressures may be determined from an independent measurement using the Knudsen cell and combined with the TGA to obtain the diffusion coefficients. Experiments have been performed with two well studied substances naphthalene and phenanthrene to check the present strategy. Further measurements were then performed for the metal organic CVD relevant compounds: ferrocene and Tris(2,2,6,6-tetramethyl-3,5-heptanedianato)cobalt III [Co(tmhd)₃].     

CRTA for the Thermoanalytical Screening of Volatile Compounds: 3. Study on thermal destruction process,taking place simultaneously with evaporation process

Quasi-equilibrium thermogravimetry (variant of CRTA) is put to use as an express method of thermoanalytical screening for volatile compounds. During the experiments for P—T relationship calculations (running with several calibrated standard sample holders) the non-volatile (polymerized) residue is formed (and is decomposed with further temperature rising). Thermogravimetric data are used for the calculation of the kinetic parameters for the polymerization reaction, taking place (concurrently with the evaporation) in the melt of the studied volatile compound.This revised version was published online in November 2005 with corrections to the Cover Date.     

Measurement of vapor pressures and heats of sublimation of dicarboxylic acids using atmospheric solids analysis probe mass spectrometry

Vapor pressures of low volatility compounds are important parameters in several atmospheric processes, including the formation of new particles and the partitioning of compounds between the gas phase and particles. Understanding these processes is critical for elucidating the impacts of aerosols on climate, visibility, and human health. Dicarboxylic acids are an important class of compounds in the atmosphere for which reported vapor pressures often vary by more than an order of magnitude. In this study, atmospheric solids analysis probe mass spectrometry (ASAP-MS), a relatively new atmospheric pressure ionization technique, is applied for the first time to the measurement of vapor pressures and heats of sublimation of a series of dicarboxylic acids. Pyrene was also studied because its vapor pressures and heat of sublimation are relatively well-known. The heats of sublimation measured using ASAP-MS were in good agreement with published values. The vapor pressures, assuming an evaporation coefficient of unity, were typically within a factor of ～3 lower than published values made at similar temperatures for most of the acids. The underestimation may be due to diffusional constraints resulting from evaporation at atmospheric pressure. However, this study establishes that ASAP-MS is a promising new technique for such measurements.     

CRTA for the Thermoanalytical Screening of Volatile Compounds 2. The volatility of trifluoroacetylacetonates of Al,Cr and Zr studied by the method of quasi-equilibrium thermogravimetry

Quasi-equilibrium thermogravimetry (with sample holders specially calibrated for the vapour pressure) was used for thermoanalytical screening of volatile compounds. ‘p-T’ relationships (in the range 0.0006 to 0.11 at) were obtained for acetylacetonates and fluoroacetylacetonates of Al, Cr and Zr. The incorporation of the trifluoromethyl group into the acetylacetone ligand noticeably increases the volatility: the enthalpy of evaporation remains practically constant, but the evaporation entropy changes distinctly. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal properties of hafnium(IV) and zirconium(IV) β-diketonates

Five volatile hafnium(IV) and zirconium(IV) β-diketonates: hafnium(IV) acetylacetonate, hafnium(IV) trifluoroacetylacetonate, hafnium(IV) pivaloyltrifluoroacetonate, hafnium(IV) 2,2,6,6-tetramethylheptane-3,5-dionate and zirconium(IV) 2,2,6,6-tetramethylheptane-3,5-dionate were obtained, purified and identified. Thermal behavior of solid compounds was investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC) in helium atmosphere and in vacuum. DSC method was also used for definition of thermodynamic characteristics of melting processes. Using the static method with quartz membrane zero-manometer and the flow method the temperature dependencies of saturated vapor pressure for hafnium(IV) complexes was obtained. The standard thermodynamic characteristics ΔH _T⁰ and ΔS _T⁰ of sublimation and evaporation processes were calculated from the temperature dependences of saturated vapor pressure.     

Volatilisation,Evaporation and Vapour Pressure Studies Using a Thermobalance

There is considerable interest in performing volatilisation and evaporation measurements by thermogravimetry. A quick and simple method for determining vapour pressure using a conventional thermobalance and standard sample holders has been developed. These yield meaningful thermodynamic parameters such as the enthalpies of sublimation and vaporisation. Under favourable conditions the melting temperature and enthalpy of fusion of such compounds can be obtained. This technique has been used for the study of dyes, UV absorbers and plasticisers. The use of modulated- temperature programs for such work is also described. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrical processes upon the evaporation and condensation of water and ice

A mathematical model of electrical processes that take place upon the evaporation of water and sublimation of ice, as well as the condensation growth of water and ice phases from vapor, is proposed. The transfer of the main charge carriers, such as (i) protons and hydroxide ions (in ice, water, and vapor and (ii) orientational defects (in ice and water) is taken into account. Upon the evaporation of water and the sublimation of ice, the first carriers are accumulated before the phase front and cause positive charges in the surface of the water and ice, while the second carriers are depleted (their concentration becomes lower than the thermodynamic value) and impart a negative charge to water and ice. The contribution of protons and hydroxide ions dominates at a low rate of evaporation. In the condensation of vapor and relevant growth of water and ice phases, the polarity of surface charge is opposite to that observed upon evaporation. The values of interfacial current and signs of phase charges upon sublimation, evaporation, and condensation that are predicted in the model comply with experimental data.     

Attempts to explain the self-disproportionation observed in the partial sublimation of enantiomerically enriched carboxylic acids

The partial sublimation of two carboxylic acids, the mandelic acid and ibuprofen has been studied. Many (RS) + (S) samples with various enantiomeric excesses (ee) have been slowly and partially sublimed at a low temperature and the sublimates have been condensed before analysis. About 1% of the starting material was sublimed in each experiment. The results are reproducible showing that the sublimation is under control. The ee of sublimates are comparable to the ee of the eutectic but also to those obtained by mixing the sublimates of two apparatuses used to sublime separately the racemate and the enantiomer. Thus, the sublimations of both carboxylic acids could be controlled by the saturated vapor pressure of the components ((RS) and (S)) or, as usually proposed, by the formation of a gas phase with a eutectic composition. In the case of mandelic acid, a definitive answer has been given by the partial sublimation of (S) + (R) solid mixtures where sublimates with a eutectic composition have been obtained and without any indication of the sublimation of a “kinetic conglomerate”. This study paves the way for future investigations on the slow and partial sublimation of enantioenriched compounds to determine how this latter occurs.     

Thermodynamic characteristics of phase conversion of structural isomers for volatile complex of ruthenium (III) trifluoroacetylacetonate

The comprehensive analysis of volatile β-diketonate compound—ruthenium(III) trifruoroacetylacetonate (Ru(tfac)₃)—was carried out. By means of flow method in quasi-equilibrium conditions and static method the temperature dependencies of saturated vapor pressure have been measured over solid and liquid cis- and trans-modifications of Ru(tfac)₃ and isomer mixture. The thermodynamic characteristics of sublimation, evaporation, melting, and phase conversion have been calculated for structural isomers. Also by differential-scanning calorimetry the temperature meanings and the thermodynamic characteristics of melting have been determined for individual isomers of Ru(tfac)₃ and their mixtures. By XRD the structures for cis- and trans-modifications have been determined. Both structures consist of neutral molecules arranged in pseudo layers.     

A special reactor coupled with a high‐temperature mass spectrometer for the investigation of the vaporization and cracking of organometallic compounds

A special reactor coupled to a high‐temperature mass spectrometer was specifically designed for the study of vaporization and thermal cracking of organometallic precursors. This reactor has two kinds of settings. One is a single Knudsen effusion cell which enables the analysis of the composition of saturated vapors and the determination of the partial pressure of each gaseous molecule in equilibrium with its condensed phase. This cell is an evaporation/sublimation cell (operating from 243 to 473 K), which can be tightly closed – like a vacuum chamber – in order to protect organometallic compounds against moisture and atmospheric components. This cell can be independently weighed usefully to evaluate the equilibrium vapor pressures of the sample using the mass‐loss method. During experiments, the effusion aperture is externally opened for direct mass spectrometric measurements. The other setting dedicated to the study of thermal decomposition of gaseous molecules consists of a set of tandem cells: the previously described Knudsen cell and a cracking cell (operating from 293 to 973 K). Copyright © 2009 John Wiley & Sons, Ltd.     

Joint processing of experimental data on melting,evaporation, and sublimation processes

A technique and computational program for estimating thermodynamic parameters are developed for the joint processing of experimental data on the saturated vapor pressure in melting, evaporation, and sublimation processes. Computation is based on the equality of pressures over the solid and liquid phases at a melting temperature. The efficiency of the proposed technique is demonstrated by processing experimental data on the phase transitions of Sc(thd)₃.     

Determination of evaporation rates and vapor pressures of very low volatility compounds: a study of the C4-C10 and C12 dicarboxylic acids

A method for the measurement of evaporation rates and vapor pressures of low volatility compounds was developed and applied to the homologous series of C4-C10 and C12 dicarboxylic acids. Proton-transfer chemical ionization mass spectrometry was used to follow directly the temperature-dependent evaporation rates of aerosol samples collected on a cold plate that could be heated at a known rate. The vapor pressures of the deposited compounds were derived from observed evaporation rates through application of the Hertz-Knudsen equation. Temperature programmed desorption allowed for quantification of the enthalpy (DeltaHsub) and entropy (DeltaSsub) of sublimation of the diacids and is described. A strong odd-even dependence with respect to the total carbon number was observed in the derived diacid vapor pressures, consistent with previous measurements. However, the vapor pressures from this method were systematically lower than previous measurements. Though seen in the vapor pressure, no odd-even carbon chain length dependence was readily discernible in the measured values of DeltaHsub and DeltaSsub. Perhaps most importantly, these experimental results also suggest that residual solvent molecules (from the aerosol generation process) trapped in the diacid samples can have a considerable influence on the measured thermodynamic parameters and, if not properly accounted for, may give erroneous results.     

Review of Sublimation Thermodynamics of Polycyclic Aromatic Compounds and Heterocycles

This review details sublimation vapor pressure and thermodynamic data on 85 polycyclic aromatic compounds and heterocycles from the early 1900s through 2012. These data were collected using a variety of vapor pressure measurement techniques, from effusion to gas saturation to inclined‐piston manometry. A brief overview of each measurement technique is given; these methods yield reproducible sublimation vapor pressure data for low volatility organic compounds such as polycyclic aromatic compounds and heterocycles. Several conclusions can be drawn from this literature survey, specifically that there remains a dearth of data on the sublimation thermodynamics (and fusion thermodynamics) of heteroatomic high molecular weight aromatic compounds, inhibiting a holistic understanding of the effect of specific heteroatoms and substituent position on the thermodynamics of these compounds. However, we can clearly see from the data that there are a variety of potential intermolecular interactions at work that generally tend to increase the enthalpy of sublimation and decrease the vapor pressure of a substituted polycyclic aromatic compound/polycyclic heterocycles versus its parent compound.     

Thermogravimetric measurements of liquid vapor pressure

A method was developed using thermo-gravimetric analysis (TGA) to determine the vapor pressure of volatile liquids. This is achieved by measuring the rate of evaporation (mass loss) of a pure liquid contained within a cylindrical pan. The influence of factors like sample geometry and vapor diffusion on evaporation rate are discussed. The measurement can be performed across a wide range of temperature yielding reasonable results up to 10 kPa. This approach may be useful as a rapid and automatable method for measuring the volatility of flavor and fragrance raw materials.     

Thermodynamic properties of dimeric molecules of lanthanum and lanthanide trichlorides Ln2Cl6(gas)

The partial pressures of dimeric molecules Ln₂Cl₆ in the saturated vapor over lanthanum and lanthanide trichlorides LnCl₃ (Ln = La, ..., Nd, Sm, Gd, ..., Lu) have been determined by high-temperature mass spectrometry. From these data, the enthalpies of the gas-phase reaction Ln₂Cl₆ = 2LnCl₃ and the enthalpies of sublimation of the compounds under consideration in the form of Ln₂Cl₆ dimers have been calculated by the third law. Analogous characteristics have also been calculated by the second and third laws from the available literature data on the partial pressures of Ln₂Cl₆ in the course of sublimation (evaporation) of LnCl₃. Taking into account typical tendencies in the standard thermodynamic characteristics of lanthanum and lanthanide compounds, a set of recommended D ₂₉₈ ⁰ (LnCl₃-LnCl₃) values has been determined. This set has been used for calculating the enthalpies of atomization Δ_at H ₂₉₈ ⁰ (Ln₂Cl₆), where Ln = La, ..., Lu.     

Addition of internal standards to particulate sample matrices for routine trace analyses of semivolatile organic compounds: A source of systematical and random errors

This article is a criticism of the strategy of adding (isotope labelled) internal standards of semi volatile hydrophobic organic compounds directly on the surface of particulate samples matrix such as sediment, soil and fly ash, etc. in a small aliquot (mL) of solvent, before trace level analysis. The use of the internal standard is intended to compensate for incomplete extractions, clean-up losses, dilution errors and instrument variations. However, direct addition of internal standards to sample matrices creates two possibilities for inaccurate results by processes only affecting the internal standard: First, evaporation losses of standard from the sample matrix during evaporation of the carrier solvent. Second, the native analyte and internal standard sorb to the sample matrix with differing force. Both processes can introduce systematic and random error to the result. A systematic error of 74% due to evaporation losses of tetra chlorinated dibenzo-p-dioxins is observed, while the corresponding error for octa chlorinated dioxin is 0%. The associated random error is 45% for tetra down to 1–4% relative standard deviations for hepta and octa chlorinated dioxins. For laboratory staff the evaporation losses of standard (and native) compounds causes, besides dust, an additional risk of inhalation exposure. The internal standard should instead be added to the extraction solvent after the extraction. Smaller systematical errors (10–20%) and associated random errors due to irreversible sorption are discussed. Received: 4 September 1997 / Revised: 26 January 1998 / Accepted: 31 January 1998     

Sample container temperature gradient influence on the BET specific surface area

Differences between BET specific surface area (BET SSA) values exist due to data collected in stainless steel and less thermally conductive sample holders. Not accounting for the temperature gradient along stainless steel sample holders during manometric gas adsorption measurements at cryogenic temperatures leads to errors of up to 3.2% in the BET SSA values with a relative combined standard uncertainty (RCSU) of 0.63%. A unidimensional heat flow model accurately accounts for the temperature gradient, leading to an agreement of 0.16% between the BET SSA values for both sample holder units.     

Estimation of solid vapor pressures of pure compounds at different temperatures using a multilayer network with particle swarm algorithm

Solid vapor pressures (P^S) of pure compounds have been estimated at several temperatures using a hybrid model that includes an artificial neural network with particle swarm optimization and a group contribution method. A total of 700 data points of solid vapor pressure versus temperature, corresponding to 70 substances, have been used to train the neural network developed using Matlab. The following properties were considered as input parameters: 36 structural groups, molecular mass, dipole moment, temperature and pressure in the triple point (upper limit of the sublimation curve), and the limiting value P^S → 0 as T → 0 (lower limit of the sublimation curve). Then, the solid vapor pressures of 28 other solids (280 data points) have been predicted and results compared to experimental data from the literature. The study shows that the proposed method represents an excellent alternative for the prediction of solid vapor pressures from the knowledge of some other available properties and from the structure of the molecule.     

Thermodynamics and vapor formation features of zinc(II) and tin(II) pivalates

Vapor formation of tin(II) and zinc(II) pivalates were studied using the Knudsen effusion method and mass spectrometry analysis of the gas phase. Sublimation of the tin complex is shown to be accompanied by polymerization of the condensed phase, changing its thermodynamic parameters. The vapor formation of zinc pivalate in the presence of trace amounts of water is accompanied by partial hydrolysis of the condensed phase and sublimation of the sample in the form of Zn₄O(piv)₆ and Zn₂piv₄. The absolute partial pressures and heats of sublimation of the components of the gas phase above tin and zinc complexes are calculated.