Enthalpies Of Exchange Research Articles

Evidence of stockpile contamination for legacy polychlorinated biphenyls and organochlorine pesticides in the urban environment of Cyprus (Eastern Mediterranean): Influence of meteorology on air level variability and gas/particle partitioning based on equilibrium and steady-state models

The present study investigated comprehensively the atmospheric occurrence and fate of an extensive range of polychlorinated biphenyls (PCBs; forty-two congeners), organochlorine pesticides (OCPs; twenty-seven emerging and legacy agrochemicals) and polycyclic aromatic hydrocarbons (PAHs; fifty parent and alkylated members, including the non USEPA-16 listed toxic ones), in both gas and particulate phase of the scarcely monitored atmosphere over Cyprus for the first time. Parent-metabolite concentration ratios suggested fresh application for dichlorodiphenyl-trichloroethanes (DDTs), dicofol, hexachlorocyclohexanes, endosulfan and chlorothalonil, particularly during spring (April-May). Regressions of logarithms of partial pressure against ambient temperature revealed that secondary recycling from contaminated terrestrial surfaces regulates the atmospheric level variability of PCBs, DDTs, aldrin, chlordane, dicofol, heptachlor and endosulfan. Enthalpies of surface-air exchange (∆HSA) calculated from Clausius-Clapeyron equations were significantly correlated to vaporization enthalpies (∆HV) determined by chromatographic techniques, corroborating presence of potential stockpile-contaminated sites around the study area. The Harner-Bidleman equilibrium model simulating urban areas, and the Li-Jia empirical model, predicted better the partitioning behavior of PAHs (<four-ring parent and alkylated members), PCBs (<hexa-chlorobiphenyls), and OCPs, respectively. For heavier PAHs and PCBs, partitioning coefficients (KP) were inadequately predicted by the Li-Ma-Yang steady-state model, probably due to local human activities and regional transport in the study area.

A first look at atmospheric concentrations and temporal trends of phthalates in distinct urban sectors of the Greater Toronto Area

Measurements of the outdoor air concentrations of phthalates (PHTs) using passive sampling media (polyurethane foam (PUF) disks) were conducted at 8 different sites across the Greater Toronto Area (GTA) during 2016–2017. Highest PHT levels were obtained at sites characterized by high levels of urban and traffic activity. The PHT profile was mainly dominated by bis(2-ethylhexyl) phthalate (DEHP), whose median air concentration across the examined sites was MDEHP = 1200 pg/m3. It was discovered that PHT levels had a strong correlation with the ambient temperature. Warmer months appeared to contribute to greater emission of PHTs at most sites. Clausius Clapeyron (CC) plots were used to investigate the presence of temperature dependent processes governing PHT emissions, such as secondary volatilization from environmental surfaces. Enthalpies of surface-air exchange calculated from CC plots were compared to respective enthalpies of vaporization for each PHT. Those values were used to examine importance of localized water-air and land-air exchanges on atmospheric levels of PHTs. The data suggested that secondary volatilizations are relevant PHT contributors and appear to depend on sampling site and PHT type. Enthalpies associated with Henry's law constant were also evaluated, providing evidence of secondary volatilization from water bodies. This work is part of an on-going study which already evaluated levels of organophosphate esters (OPEs), polybrominated diphenyl ethers (PBDEs) and polycyclic aromatic compounds (PACs) at the same sites. Ultimately, the variability in ambient concentrations of PHTs in the GTA are believed to be due to a combination of both primary and secondary sources.

Enthalpies of Anion Exchange of Chloride by Pyridinecarboxylate in AV-17-8 Anionite

Enthalpies of Cl– anion exchange for anions of nicotinic, isonicotinic, dipicolinic, and cinchomeronic acids on the strongly basic anionite AV-17-8 are measured at 298 K by means of microcalorimetry. Partial and standard enthalpies of sorption of pyridinecarboxylic acid anions on the Cl-form of AV-17-8 anionite from aqueous solutions are determined. The values of standard enthalpies lie in the range 3–14 kJ/mol, increasing in the order nicotinate, isonicotinate, dipicolinic acid anions, and cinchomeronic acid anions.

Heats of exchange of chloride with pyridinecarboxylate ions in Dowex-1 anion exchanger

The heats of exchange of chloride with pyridinecarboxylic acids anions in Dowex-1 strongly basic anion exchanger was obtained by the microcalorimetric method at 298 K. Nicotinic acid, isonicotinic acid, dipicolinic and cinchomeronic acids as pyridinecarboxylic acids were selected. It has been revealed that the sorption of pyridinecarboxylic acid anions by Cl form of Dowex-1 anion exchanger from aqueous solutions is the endothermic process. The standard enthalpy values of the exchange of chloride by pyridinecarboxylate ions in Dowex-1 anion exchanger calculated via microcalorimetric experimental data are in the range of 3–14 kJ mol−1. The standard enthalpies of model reaction that simulated the anion exchange process were determined via quantum chemical calculations of the reagent formation enthalpies in vacuum in the framework of the MOPAC2016 program. The enthalpies of both ion exchange of chloride by pyridinecarboxylate and the model reaction increase in the sequence: nicotinate, isonicotinate, anion of dipicolinic acid, anion of cinchomeronic acid.

Calorimetric Study of Alkali Metal Ion (K+, Na+, Li+) Exchange in a Clay-Like MXene

Intercalation of ions in layered materials has been explored to improve the rate capability in Li-ion batteries and supercapacitors. This work investigates the energetics of alkali ion exchange in a clay-like MXene, Ti3C2Tx, where Tx stands for anionic surface moieties, by immersion calorimetry in aqueous solutions. The measured immersion enthalpies of clay-like Ti3C2Tx, ΔHimm, at 25 °C in 1 M KCl, 1 M NaCl, 1 M LiCl, and nanopure water are −9.19 (±0.56), −5.90 (±0.31), −1.31 (±0.20), and −1.29 (±0.13) kJ/mol of MXene, respectively. Inductively coupled plasma mass spectrometry is used to obtain the concentrations of alkali ions in the solid and aqueous phases. Using these concentrations, the enthalpies of exchange of alkali metal ions (Li+, Na+, and K+) are calculated; ΔHex in 1 M KCl, 1 M NaCl, 1 M LiCl, and nanopure water are −9.3 (±2.2), 21.0 (±0.9), −1.3 (±0.2), and 302.4 (±0.6) kJ/mol of MXene, respectively. Both immersion and exchange enthalpies are most exothermic for potassium. This suggests that ...

Tracking the Structural Dynamics of Hybrid Layered Double Hydroxides

The present study aims at investigating the structural and dynamics properties of intercalated tartrate and succinate anions in Zn2Al layered double hydroxides (LDHs). The comparison between these two anions allows the estimation of the impact of the presence of hydroxyl groups in tartrate on the dynamics, orientational, and rotational behaviors of the species in the interlayer domain. A variety of experimental techniques (X-ray diffraction, 1H−13C MAS NMR variable contact time, proton conductivity, microcalorimetry) is used in association with molecular dynamics (MD) simulations. A specific behavior is then established for the tartrate in terms of rotation around the backbone, diffusion, and organization of the water molecules in the interlayer domain. The difference in the enthalpies of exchange between the two LDHs is calculated from MD simulations and compared to that obtained by microcalorimetry experiments. This energy calculation evidences for the existence of anion−anion interactions that are more favorable for tartrate than for succinate. The combination of experimental and theoretical methods performs very well for the structural, energetic, and dynamics characterization of the interlayer species.

Acid–base properties of the alumina surface: Influence of the titration procedures on the microcalorimetric results

The enthalpy changes associated with the protonation and deprotonation of an alumina surface have been determined on the basis of microcalorimetry experiments and acid–base potentiometric titrations at 25 °C. It has been shown that the results may vary significantly according to the experimental procedure. In order to do so, the potentiometric and microcalorimetric titrations have been carried out first from an acidic pH to basic pH and second from a pH near the pH zpc of alumina to acidic or basic pH. It has been demonstrated that the p K a values deduced from the potentiometric titrations are the same whatever the experimental protocol whereas the only way to obtain meaningful enthalpies of proton exchange is to carry out microcalorimetric titrations by starting around the point of zero charge.

The differential enthalpy of ion exchange on selective ionites

The problem of the experimental determination of the differential enthalpy of ion exchange is considered. The high errors in the enthalpies of ion exchange based on experimental corrected equilibrium coefficients substantially decrease the value of the parameters obtained this way. A method is suggested for finding the differential enthalpies of ion exchange on selective ionites on the basis of an analysis of only the composition of solutions in equilibrium systems at two temperatures. This allows the enthalpies to be determined with substantially lower errors than when traditional techniques are used.

Performances under representative pressure and temperature conditions of the gas chromatography–mass spectrometry space experiment to investigate Titan's atmospheric composition

In the frame of the calibration of the aerosol collector and pyrolyser, and gas chromatography–mass spectrometry experiments of the Huygens probe arrived at Titan, systematic experimental studies were led to estimate the influence of the operating conditions on the analyses that should have been achieved in the Titan's atmosphere. The primary objective of this study was to estimate the influence of operating conditions variations induced by (i) instrumental modifications made shortly before the probe launch which can have changed the operating pressures; (ii) the change of the probe environmental conditions (pressure, temperature) during its descent in the atmosphere; (iii) a possible deviation of pressure and temperature regulations from their nominal values because of the long journey of the instrument in space, or of other external events. The secondary objective of this work was to create an analytical database that can be used as a reference to treat the chromatograms obtained in situ, and help to identify chromatographically the analyzed species, complementary to mass spectrometry. Beyond the application to a specific instrument, this work was also useful to experimentally estimate the fundamental evolution of the separation as a function of the changes of operating conditions with time. The obtained results show (i) the significant influence of inlet and outlet pressure variation on the time of analysis, but not on the separation power. It thus enables to significantly shorten the analysis duration, and thus to analyze more compounds within the fixed time of analysis of the instrument; (ii) the significant influence of temperature on the retention. In this frame, the enthalpies of exchange between the gas phase and the stationary phase of the species were determined to be used to retrieve the analyzed species in case of deviation of the operating temperature; (iii) that the possible aging of the columns does not have influence on the columns efficiency and separation power; (iv) the analytical capabilities of the gas chromatography–mass spectrometry experiment within operating conditions representative of those encountered in situ. Finally, in spite of possible operating condition changes, it is shown that results coming from the gas chromatograph–mass spectrometer experiment, which are currently under analysis, could bring important information on the Titan's atmosphere and its history.

Thermodynamics of Potassium–Magnesium Exchange in Two Alfisols of Northern Greece

The exchange behavior of potassium (K) in soils has been extensively studied relative to a dominant cation, which in most cases was considered to be calcium (Ca). Magnesium (Mg) was tacitly assumed that existed in minor quantities or that it had the same exchange behavior with Ca. However, under certain conditions and practices (Mg-rich soils, greenhouse crops) Mg can become an important component of the soil exchange complex participating drastically in the exchange reactions. Consequently, the exchange behavior of the pair K–Mg has merit and in this study the thermodynamics of exchange for this pair were evaluated in two Alfisols with different mineralogical composition of the clay fraction. The exchange reaction was studied in both directions, namely, Mgsoil→Ksoil and Ksoil→Mgsoil. Comparison of the experimental exchange isotherms to the non-preference isotherms revealed that both soils exhibited preference for K relative to Mg. Standard free energies of exchange (ΔG0) for the reaction Mgsoil→Ksoil were negative (−5.7 to −6.7 kJ mol−1) indicating the spontaneity of the reaction and that the formation of a K-soil is favored. Also, ΔG0 values for the reaction Ksoil→Mgsoil were positive (7.3 to 7.5 kJ mol−1) which indicated the reluctance of K-soil to be converted to Mg-soil, thus corroborating the previous findings. Standard enthalpies of exchange (ΔH0) were negative (−8.7 to −13.5 kJ mol−1) for the reaction Mgsoil→Ksoil and positive (13.8 to 15.1 kJ mol−1) for the backward reaction suggesting a strong binding of K with some sites of the soils. Vanselow selectivity coefficients (KV) showed a stronger preference for K in the soil where mica-type minerals dominated the clay fraction. Selectivity for K, as judged by the KV values, was also strongly K-saturation dependent; the lower the degree of K saturation the higher the selectivity for K. Under the conditions of the experiment the exchange reaction of the pair K–Mg was not reversible but exhibited hysteresis, the magnitude of which was more pronounced in the soil where mica-type minerals were prevalent in the clay fraction.

Intercalation of dicarboxylate anions into a Zn–Al–Cl layered double hydroxide: microcalorimetric determination of the enthalpies of anion exchange

The enthalpies of anion exchange in a chloride-intercalated zinc aluminium layered double hydroxide (LDH) in suspension in water have been studied at 25 °C. The heat effects associated with the exchange of the chloride anions for dicarboxylate anions (oxalate, succinate, adipate and tartrate) have been determined by titration microcalorimetry for different amounts of LDH. Except for tartrate, the exchange processes are endothermic, in contrast to the exchange processes involving only inorganic anions. The enthalpies of exchange vary linearly with the number of carbon atoms, suggesting that the intercalated dicarboxylate anions are perpendicular to the layers. The energetic parameters show, for the first time, hydrophobic interactions between the CH2 groups of the parallel succinate or adipate anions and hydrogen bonding between the OH groups of the parallel tartrate anions.

Thermodynamics of Ion Exchange. An Alternative Methodology

The thermodynamics of ion exchange is usually presented in terms of ion exchange equilibrium constants. This formalism is somewhat cumbersome, especially when more than two ionic species are involved in the exchange equilibria. It is shown that an alternative formulation using a notation developed previously by the author simplifies matters. Clausius−Clapeyron equations are derived for enthalpies of exchange under the practically convenient condition of constant solvent activity rather than constant solvent content of the exchanger phase. A definition is forwarded of thermodynamically ideal behavior in the exchanger phase which corresponds as closely as possible with the thermodynamic terms in the Nernst−Planck equations describing ideal ion transport behavior. When Kirkwood−Buff solution theory is applied to fixed charge exchangers, it turns out that all terms explicitly involving the fixed charge distribution are zero.

Enthalpies of ligand exchange for the complexes [(NH3)5Co-L]3+ (L = water, dimethyl sulfoxide, N,N-dimethylformamide, and pyridine)

The enthalpies of ligand exchange for monosubstituted complexes of pentaamminecobalt (III) have been determined by using solution calorimetry in the solid phase and in aqueous solution. It is found that stabilities toward ligand exchange for the compounds [(NH 3 ) 5 Co−L] 3+ increase in the following order: L=DMSO<water<N,N-DMF<pyridine=ammonia

NMR study of environment modulated proton tautomerism in crystalline and amorphous phthalocyanine

Using high resolution 15N and 13C CPMAS NMR spectroscopy (CP - cross polarization, MAS - magic angle spinning) we have detected fast thermally activated proton transfer processes in solid 15N enriched phthalocyanine (Pc). The synthesis of the latter is described. NMR experiments were performed on the crystalline α- and β-modifications, as well as on a novel amorphous modification (am-Pc). In order to extract thermodynamic and kinetic data from the NMR spectra an appropriate lineshape theory of bistable molecules in ordered and disordered matrices is developed. Bistable molecules are subject to exchange between at least two molecular states. The lineshape theory includes the possibility of site dependent perturbations of the rate and equilibrium constants of state exchange, as well as of exchange between different sites. For disordered environments bi-Gaussian distributions of the reaction enthalpies of the state exchange and of the enthalpies of activation are proposed. Different possibilities, including Marcus theory, of reducing this two-dimensional site distribution function to a one-dimensional distribution are discussed. The analysis of the NMR spectra gave the following results. Whereas the proton potential in α-Pc is quasisymmetric, the degeneracy of the tautomers in the β modification is lifted because of a subtle interplay between inter- and intramolecular interactions. The amorphous modification is characterized by a broad distribution of differently perturbed asymmetric double-minimum potentials, as expected for a disordered environment. Rotation of Pc in either of these modifications can be excluded. Proton transfer in the β phase is faster than in the α phase due to smaller energy of activation. This finding is interpreted with a different geometric arrangement of the inner nitrogen atoms in both phases. In addition, the proton transfer in the β phase is characterized by a smaller pre-exponential factor than in the α phase. This effect indicates substantial but different tunnel contributions to the reaction rates in both phases. The implications of the environment modulated proton transfer processes in Pc for the mechanism of hydrogen transfer reactions in liquids is discussed.

Photoacoustic calorimetry studies of ligand-exchange of aliphatic amines with Cr(CO) 6: Steric effects and hydrogen bonding

The exchange of aliphatic amines with Cr(CO) 6 in heptane occurs in two steps: a CO is lost forming a solvent-coordinated complex, Cr(CO) 5(heptane), and then heptane is lost to form Cr(CO) 5(amine). The enthalpies of exchange of aliphatic amines (piperidine, n-butylamine, di- n-butylamine and tri- n-butylamine) with Cr(CO) 6 have been determined in heptane at ambient temperatures. The enthalpies of reaction determined by photoacoustic calorimetry (PAC) include contributions from hydrogen-bond formation between free amine and the pentacarbonylamine chromium, 1. The enthalpy of hydrogen-bond formation was found to be about 4 kcal mol −1 and was determined from the temperature and concentration dependence of the amine NH chemical shifts of 1. The PAC values were corrected for hydrogen-bond formation contributions to obtain only the enthalpy of ligand-exchange (Δ H 12). The Δ H 12 was more endothermic for bulky amines, and this steric effect can be attributed to inhibition of CRN bond formation. These results, along with those in the literature, provide a detailed description of the energetics of each step of the ligand-exchange in both directions. The results suggest there is a continuum from an associative to a dissociative process for the displacement of heptane by amine that depends on the steric bulk of the amine.

Substituent effects on acetylene stability. A comparison of STO-3G, 6-31G, 6-31G**, and 6-311G** calculations

Abinitio calculations using STO-3G, 6-31G, 6-31G**, and 6-311G** basis sets and standard geometries, and 6-31G and 6-311G** basis sets with 6-31G optimized geometries have been performed on a series of acetylene derivatives (H—C≡C—R; –R is —CH3, —Li, —C≡N, [Formula: see text] —NH2,—C≡CH, —CH—CH2, and —F). Substituent effects on stability have been studied using isodesmic methyl exchange reactions (H—C≡C—R + CH4 → H—C≡C—H + CH3R). STO-3G and 6-31G calculations have been carried out for methyl exchange reactions involving several large substituents including phenyl, and for the tert-butyl exchange reactions of 10 substituents (H—C≡C—R + Me3CH → H—C≡C—H + Me3CR).Geometry optimization has an appreciable effect only on the π donor substituents OCH3 and NH2. None of the smaller basis sets reproduces the orderof stabilities or the absolute values of the 6-311G**//6-31G calculations. The latter are in satisfactory agreement with the very few experimentally based enthalpies of methyl exchange.

Adsorbed ion activities and other thermodynamic parameters of ion exchange defined by mole or equivalent fractions

SummaryThe ‘Gaines and Thomas’ method for calculating the thermodynamic parameters of a cation exchange process has been criticized because equivalent rather than mole fractions were used in the definition of adsorbed ion activity coefficients. Although such coefficients are thus not theoretically correct, comparison with directly measured enthalpies of exchange shows that they indicate heterogeneity in the exchange process correctly. They also differ little from coefficients defined in terms of mole fractions. Such a re‐definition alters values of the thermodynamic equilibrium constant, and free energy and entropy of exchange by a constant factor, independent of cations and exchanger. Comparisons of series of cations, soils or clays using these parameters are therefore not affected.

Thermochemistry of ion exchange and particle interaction in clay suspensions

The measurement of enthalpies of ion exchange for a number of homovalent and heterovalent exchange pairs by titration calorimetry is described. A self-consistent addition to previous measurements on such systems is reported. In addition to the enthalpies of ion exchange the heats of tactoid formation and breakdown were observed. The apparent heat of tactoid formation for the Na–Ca exchange pair is estimated to be −4.9 kJ per equivalent of ion exchange.