Molar Enthalpy Change Research Articles

Methane and carbon dioxide solubility in 1,2-propylene glycol at temperatures ranging from 303 to 423 K and pressures up to 12 MPa

This work reports solubility data of methane and carbon dioxide in 1,2-propylene glycol and the Henry's law constant of each solute in the studied solvent at saturation pressure. The measurements were performed at 303, 323, 373, 398 and 423.15 K and pressures up to 4.5 MPa for carbon dioxide solubility and pressures up to 12.1 MPa for methane solubility. The experiments were performed in an autoclave type phase equilibrium apparatus using the total pressure method (synthetic method). All investigated systems show an increase of gas-solubility with the increase of pressure. A decrease of carbon dioxide solubility with the increase of temperature and an increase of methane solubility with the increase of temperature was observed. From the variation of solubility with temperature, partial molar enthalpy and entropy change of the solute for each mixture were calculated.

Experimental study of methane and carbon dioxide solubility in 1,4 butylene glycol at pressures up to 11 MPa and temperatures ranging from 303 to 423 K

This work reports solubility data of methane and carbon dioxide in 1,4 butylene glycol and the Henry's law constant of each solute in the studied solvent at saturation pressure. The measurements were performed at 303, 323, 373, 398 and 423.15 K and pressures up to 3.8 MPa for mixtures containing carbon dioxide and pressures up to 10.9 MPa for mixtures containing methane. The experiments were performed in an autoclave type phase equilibrium apparatus using a technique based on the total pressure method (synthetic method). All investigated systems show an increase of gas solubility with the increase of pressure. A decrease of carbon dioxide solubility with the increase of temperature and an increase of methane solubility with the increase of temperature were observed. From the variation of solubility with temperature, the partial molar enthalpy and entropy change of each mixture were calculated.

Vapor−Liquid Equilibrium Data of (Carbon Dioxide + Methyl Propionate) and (Carbon Dioxide + Propyl Propionate) at Pressures from (1.00 to 12.00) MPa and Temperatures from (313.0 to 373.0) K

High-pressure vapor-liquid equilibrium data for (methyl propionate + carbon dioxide) and (propyl propionate (PP) + carbon dioxide) were measured at pressures from (1.00 to 12.00) MPa and temperatures in the range from (313.0 to 373.0) K. Experimental results were correlated with the Peng-Robinson equation of state with the two-parameter van der Waals mixing rule. At the same time, the Henry's coefficient and partial molar enthalpy change and partial molar entropy change of CO 2 during dissolution at different temperature were also calculated.

A New Competitive Adsorption Isothermal Model of Heavy Metals in Soils

Abstract A new competitive adsorption isothermal model (CAIM) was developed for the coexistent and competitive binding of heavy metals to the soil surface. This model extended the earlier adsorption isothermal models by considering more than one kind of ion adsorption on the soil surface. It was compared with the Langmuir model using different conditions, and it was found that CAIM, which was suitable for competitive ion adsorption at the soil solid-liquid surface, had more advantages than the Langmuir model. The new competitive adsorption isothermal model was used to fit the data of heavy metal (Zn and Cd) competitive adsorption by a yellow soil at two temperatures. The results showed that CAIM was appropriate for the competitive adsorption of heavy metals on the soil surface at different temperatures. The fitted parameters of CAIM had explicit physical meaning. The model allowed for the calculation of the standard molar Gibbs free energy change, the standard molar enthalpy change, and the standard molar entropy change of the competitive adsorption of the heavy metals, Zn and Cd, by the yellow soil at two temperatures using the thermodynamic equilibrium constants.

Helix−Coil Transition of DNA Monitored by Pressure Perturbation Calorimetry

We report the first use of pressure perturbation calorimetry (PPC) to characterize the heat-induced helix-coil transition of DNA polymers. The alternating copolymer poly[d(A-T)] was studied in aqueous solutions containing 5.2 and 18.2 mM Na+; it exhibited helix-coil transition temperatures of 33.6 and 44.7 degrees C, respectively. The transition is accompanied by a negative molar volume change, DeltaV) -2.6 and -2.1 mL/mol (base pair), respectively, and an increase in the coefficient of thermal expansion, Deltaalpha=+5x10(-4) K(-1) (at both ionic strengths). These values are consistent with a greater hydration of the coil form. The larger water-accessible surface area of the coil causes more water molecules to assume a bound, more densely packed structure that then gradually decreases with increasing temperature, leading to a larger value of R. The magnitude of the volume changes detected by PPC were larger than those deduced from high-pressure UV spectroscopy, shedding light on the effect of pressure on DeltaV. The shape of the PPC peak was nearly identical to the shape of the DSC peak, providing direct evidence for the correlation between the molar volume change and enthalpy change for the helix to coil transition of DNA.

Adsorption properties and surface chemistry of MgO

The adsorption properties of MgO, which is used as a sorbent and catalyst support, were studied using gas chromatography. The test absorbents used were n-alkanes (which show only nonspecific dispersion interactions when physisorbed on any adsorbent) and adsorbates whose molecules are capable of specific interactions with the surface reactive sites of MgO. Adsorption isotherms were measured for CHCl3, CH3NO2, CH3CN, (CH3)2CO, CH3COOC2H5, and (C2H5)2O on MgO at 50–100°C. Differential molar enthalpy changes (−ΔH), equal to molar heats of adsorption, were determined. For polar adsorbates, contributions from dispersive and specific interactions into −ΔH were determined. The electron-acceptor and electron-donor abilities of the MgO surface were estimated.

Two Three‐Dimensional Metal‐Organic Frameworks Constructed from Alkaline Earth Metal Cations (Sr and Ba) and 5‐Nitroisophthalicacid – Synthesis, Charaterization, and Thermochemistry

AbstractTwo MOFs of [SrII(5‐NO2‐BDC)(H2O)6] (1) and [BaII(5‐NO2‐BDC)(H2O)6] (2) have been synthesized in water using alkaline earth metal salts and the rigid organic ligand 5‐NO2‐H2BDC. The compounds were characterized by elemental analysis, infrared spectrum, thermal analysis, and X‐ray crystallography. Crystal structure analyses have shown that the two complexes are isostructural as evidenced by IR spectra and TG‐DTA. Both compounds present three‐dimensional frameworks built up from infinite chains of edge‐sharing twelve‐membered rings through O–H···O hydrogen bonds. The specific heat capacities of the title complexes have been determined by an improved RD496‐III microcalorimeter with the values of (109.29 ± 0.693) J mol−1 K−1 and (81.162 ± 0.858) J mol−1 K−1 at 298.15 K, and the molar enthalpy changes of the formation reactions of complexes at 298.15 K were calculated as (4.897 ± 0.008) kJ mol−1 and (2.617 ± 0.009) kJ mol−1, respectively.

Calorimetric Analysis of the 1:1 Complexes Formed between a Water-soluble Deep-cavity Cavitand, and Cyclic and Acyclic Carboxylic Acids

A water-soluble cavitand was shown to form 1:1 complexes with a series of acyclic and cyclic aliphatic carboxylic acids. Isothermal titration calorimetry was used to determine the standard molar enthalpy change (ΔH°) and binding constant (K a), and hence the Gibbs free energy (ΔG°) and entropy (ΔS°) change for the different complexes. The thermodynamic determinations were carried out from 288 to 318 K, allowing the standard molar heat capacity changes () also to be derived. Typical of the processes driven by the hydrophobic effect, was observed to be proportional to the accessible (non-polar) surface area of the guest. The cyclic and acyclic guests displayed opposite trends; the heat capacity penalty upon binding increased with longer aliphatic chains, while the opposite was observed with the cyclic guests.

Fusion Curves and Enthalpy and Internal Energy Changes of Benzene, Nitrobenzene, Bromobenzene, and Chlorobenzene at Pressures up to 3500 MPa

The fusion temperature as a function of pressure for benzene, nitrobenzene, bromobenzene, and chlorobenzene at pressures up to 3500 MPa has been determined. The new experimental data are an extension to higher pressures and temperatures of previous data and fitted by the equation Tfus = T0(1 + Δp/a1)a2 exp(−a3Δp). Changes of the molar enthalpy and the molar internal energy on fusion were calculated using the parameters of the fitted equation. Comparisons with data from references show that the experimental data, parameters of fitted equations, molar enthalpy changes, and molar internal energy changes are reliable.

Structure-energy map of alkali borosilicate glasses: Effects of pressure and temperature

A study of thermodynamic properties and structure of sodium borosilicate glasses with a range of pressure-temperature histories is presented. It is demonstrated how differences in the pressure-temperature path of the liquid during cooling may result in glasses with identical short-range structure and molar volume but different potential energies and, thus, different mid- or long-range structures. Rates of changes of molar volume, excess enthalpy, and boron coordination with changing fictive pressure and fictive temperature are reported. A structure-energy map is given, and paths for transitions under constant pressure, as well as between different pressures are shown. It is shown for pressures between atmospheric and $500\phantom{\rule{0.3em}{0ex}}\mathrm{MPa}$ that similar rates of cooling produce similar relative variations in fictive temperature in boron coordination environment and in the potential energy as compared to a given reference state. These results indicate that the rate of configurational entropy generation is independent of pressure in this range.

Syntheses and thermal properties of some complexes with 2-mercaptonicotinic acid

Five new complexes M(Hmna)2 [M=Mn(II) (1), Co(II) (2), Ni(II) (3), Cu(II) (4) and Zn(II) (5), H2mna=2-mercaptonicotinic acid] have been synthesized and characterized by elemental analyses, IR spectra, thermogravimetric analyses. In addition, molar specific heat capacities and enthalpy changes of reactions were determined by a microcalorimeter at 298.15 K. All the complexes exhibited similar IR spectra, the sulfur and oxygen atoms from monoanionic Hmna− ligand coordinated to M2+ in a bidentate fashion.

The perovskite to post-perovskite transition in CaIrO 3: Clapeyron slope and changes in bulk and shear moduli by density functional theory

The thermodynamic properties of perovskite (pv) and post-perovskite (ppv) of CaIrO 3 are derived from total energy calculations using density functional theory. Negative molar volume and enthalpy changes of 0.40 cm 3 mol −1 and 11.8 kJ mol −1 for the pv to ppv transition at 0 K stabilize ppv at low temperatures and high-pressures. Vibrational entropies calculated in the harmonic approximation, using the direct method, favour pv with increasing temperatures (105.5 J K −1 mol −1 for pv versus 99.2 J K −1 mol −1 for ppv at 298 K). A main reason for the lower entropy of post-perovskite compared to perovskite is probably related to constraints on certain vibrational modes imposed by edge-sharing of octahedra in post-perovskite. The Clapeyron slope of the pv–ppv phase boundary is deduced from the calculated enthalpy and volume of transition in conjunction with an experimental transition temperature. The resulting d p/d T of 18 MPa K −1 is in good agreement with experimental determinations reported in literature. The high-pressure properties were calculated from the variation of the total energy with volume using the Murnaghan (M) and the Birch–Murnaghan (B–M) equations-of-state. The two methods give the same values for the bulk modulus but somewhat different values for the pressure derivative of the bulk modulus: K 0 = 178 GPa and K ′ 0 = 2.8 ( M ) and 3.3 (B–M) for pv compared to K 0 = 164 GPa and K ′ 0 = 3.9 ( M ) and 4.0 (B–M) for ppv. By holding K ′ 0 for pv fixed at 4.0, K 0 is reduced to 172 GPa. The bulk moduli at zero pressure were also derived through calculation of the elastic constants giving K 0 = 172 GPa for pv compared to K 0 = 157 GPa for ppv. In order to compare the changes in the bulk and shear moduli across the pv–ppv phase transition, also the shear moduli were derived from the elastic constants. Whereas the bulk modulus decreases by about 9% from perovskite to post-perovskite, the shear modulus increases by about 15% across the transition. The elastic parameters obtained for the CaIrO 3 polymorphs are consistent with those of MgSiO 3 and with seismic velocity variations in the lowermost mantle. Our DFT-results demonstrate that CaIrO 3 may be a useful low-pressure analogue for studies of the general properties of the pv–ppv-transition relevant to the D″ layer since the negative and positive changes in bulk and shear moduli, respectively, across the pv- to ppv-transition boundary appear to be strongly linked to the nature of the two crystal structures.

Thermodynamic and Kinetic Characterization of Porous Graphitic Carbon in Reversed‐Phase Liquid Chromatography

The thermodynamics and kinetics of solute transfer on porous graphitic carbon (PGC) are studied for two series of aromatic hydrocarbons, alkylbenzenes and methylbenzenes. The retention behavior is characterized as a function of temperature, pressure, length of alkyl chain, and number and position of methyl substituents. In the thermodynamic studies, the retention factor (k) increases with an increase in the length of alkyl chain or number of methyl substituents. The data indicate that increases in the number of methylene and methyl substituents result in more negative changes in molar enthalpy (ΔHsm). Hence, the transition from mobile to stationary phase is a more energetically favorable, exothermic process with each methylene or methyl group added. The data also show that the change in molar volume (ΔVsm) is close to zero, which is consistent with an adsorption mechanism on PGC. Enthalpy‐entropy compensation is observed for both homologous series, and the compensation temperature suggests that the retention mechanism is distinctly different for the methylene and methyl groups. In the kinetic studies, the rate of solute transfer increases with an increasing number of methylene and methyl groups. The activation enthalpy from the stationary phase to transition state (ΔH‡s) is found to increase with an increasing number of methylene and methyl groups. But the activation enthalpy from the mobile phase to transition state (ΔH‡m) is much smaller than ΔH‡s and, at the same time, is very similar for all the solutes. The activation volumes (ΔV‡s and ΔV‡m) are very close to zero, again owing to the adsorption mechanism on PGC. These results demonstrate that the retention mechanism for PGC, which is based on adsorption, is different from that for octadecylsilica, the most widely used stationary phase in liquid chromatography.

Model-free analysis of binding at lipid membranes employing micro-calorimetric measurements

Based on universal thermodynamic principles (Schwarz in Biophys Chem 86:119-129, 2000) it is shown how measured enthalpy changes can be utilized to determine the relevant binding isotherm as well as the variation of the molar enthalpy change. This is carried out in a novel way involving multiple titration experiments whose evaluation requires no beforehand assumptions or models whatever. An appropriate specific model mechanism may be discussed afterwards and developed in view of the given experimental results. The pertinent procedure is demonstrated using micro-calorimetric data obtained in the case of the local anesthetic dibucaine as it associates with POPC liposomes. Mutual interactions of the bound ligand molecules could be described in terms of repulsive enthalpic and entropic activity coefficients. Apparently these are induced by electrostatic forces and by the finite size of binding sites, respectively.

Thermodynamics and Kinetics of Chiral Separations with β‐Cyclodextrin Stationary Phase: II. Effect of Temperature and Pressure

In this study, a series of coumarin‐based compounds is separated using a β‐cyclodextrin stationary phase with a polar‐organic mobile phase. Temperature and pressure are varied in order to observe the effects and determine important thermodynamic and kinetic parameters. Increasing the temperature decreases the retention and chiral selectivity, but increases the mass transfer rates for all chiral compounds. Increasing the pressure decreases the retention, but does not significantly affect the chiral selectivity. Van't Hoff plots of the natural logarithm of retention factor versus inverse temperature are linear with positive slopes, indicating an enthalpically favorable transfer from mobile to stationary phase. The second eluted enantiomer has a more negative change in molar enthalpy than the first, suggesting an enthalpically more favorable transfer. For all compounds, both the differential change in molar enthalpy and the differential change in molar entropy between the two enantiomers are negative. From these values, the compensation temperature is determined and is above ambient temperature, indicating an enthalpy‐driven separation. Although all the compounds have similar structures, different compensation temperatures are determined and enthalpy‐entropy compensation is not observed. This suggests that the retention mechanism is distinctly different. The change in molar volume is positive, indicating that the compounds occupy more space in the stationary phase than in the mobile phase and that inclusion in the cyclodextrin cavity does not occur to a significant extent. With regard to the kinetic behavior, the rate constants generally increase with increasing retention factor for the coumarin‐based solutes. However, the second eluted enantiomer has a surprisingly faster rate constant than the first enantiomer. The activation energy is positive, and the second eluted enantiomer always has larger activation energy than the first enantiomer. These thermodynamic and kinetic measurements provide a detailed and comprehensive view of the chiral retention mechanism.

Thermodynamics of reactions catalyzed by d-hydantoinase and N-carbamoyl- d-amino acid hydrolase

Microcalorimetry and high-performance liquid chromatography have been used to conduct a thermodynamic investigation of several biochemical reactions catalyzed by the enzymes d-hydantoinase and N-carbamoyl- d-amino acid hydrolase. Values of apparent equilibrium constants K′ and calorimetrically determined molar enthalpies of reaction Δ r H m(cal) were measured for the following d-hydantoinase catalyzed reactions: hydantoin(aq) + H 2O(l) = ureidoacetic acid(aq); 5,6-dihydrouracil(aq) + H 2O(l) = 3-ureidopropanoic acid(aq); 5-phenylhydantoin(aq) + H 2O(l) = 5-phenyl d-carbamoylate(aq); and 5-(4-hydroxyphenyl)hydantoin(aq) + H 2O(l) = 5-(4-hydroxyphenyl) d-carbamoylate(aq). Calorimetric measurements, in which both d-hydantoinase and N-carbamoyl- d-amino acid hydrolase were present, were performed on the following reactions: 5-phenylhydantoin(aq) + 3 H 2O(l) = d-phenylglycine(aq) + carbon dioxide(aq) + ammonia(aq); and (4-hydroxy)-phenylhydantoin(aq) + 3H 2O(l) = d-(4-hydroxyphenyl)glycine(aq) + carbon dioxide(aq) + ammonia(aq). A chemical equilibrium model was then used to calculate thermodynamic property values (equilibrium constants K, standard molar enthalpies of reaction Δ r H m ∘ , standard molar Gibbs free energies of reaction Δ r G m ∘ , and standard molar entropies of reaction Δ r S m ∘ ) for chemical reference reactions that correspond to the overall biochemical reactions that were studied experimentally. In several cases, we also provided estimated property values based on data for structurally similar reactions. The results can be used together with the chemical equilibrium model to calculate values of K′, the standard apparent molar Gibbs free energy change Δ r G m ′ ∘ , the standard apparent molar enthalpy change Δ r H m ′ ∘ , changes in binding of the proton Δ r N(H +), and the position of equilibrium for the several overall biochemical reactions considered in this study over a reasonably wide range of temperature, pH, and ionic strength.

Calorimetric and volumetric data of salting of albumin from human serum using NaCl solutions

Apparent molar volume and enthalpy changes for mixing NaCl (aq.) with albumin from human serum (aq.) are experimentally determined (25°C). Calorimetric experiments were carried out in an LKB 10700-2 calorimeter, whereas volumetric measurements were realized using an Anton Paar 60/602 densimeter. The density measurements were made after 1 and 24 h of the dissolution in the buffer (pH 4.2). The relation between the changes of the enthalpy and apparent molar volumes vs. molality of NaCl were determined. The obtained data are discussed together with data obtained previously for bovine albumin and hen egg lysozyme solutions with NaCl, Li2SO4 and (NH4)2SO4 salts of various concentration. As results correlations between the changes of enthalpy of salting and apparent molar volumes vs. molality of salts were made.

Application of thermal electrochemical equation to metal-hydride half-cell system

Application of thermal electrochemical equation to metal-hydride half-cell system was investigated, and the influence of state of charge on the thermal electrochemical performance of hydrogen storage materials was studied. The results show that both the absolute value of the molar enthalpy change and the internal resistance of evolution hydrogen reaction are less than that of absorption hydrogen reaction at the same state of charge. The molar reaction enthalpy change of absorption and evolution of hydride electrode change contrarily with the enhancement of filling degree of hydrogen in hydride electrode. The relation curve of molar reaction enthalpy change to state of charge, both absorption and evolution hydrogen reaction, is close to a constant when the state of charge is 10%-60%, and during state of charge below 10% or state of charge above 60%, the molar reaction enthalpy change varies sharply. Meanwhile, the internal resistance of electrode reaction has an ascending trend with the enhancement on filling degree of hydrogen in hydride electrode in both absorption and evolution hydrogen reaction.

Selective Binding Thermodynamics of Bile Acids by Oligo (ethylenediamino)-β-Cyclodextrins and Their Copper (II) Complexes

Complex stability constants (KS), standard molar enthalpy changes (DH) and entropy changes (TDS) for the inclusion complexation of native b-cyclodextrin (b-CD) (1) and some modified b-CDs, i.e., mono(6-ethylenediamino-6-deoxy)-b-CD (3), mono[6-diethylenetriamino-6-deoxy]-b-CD (4) and their corresponding copper complexes 5 and 6, with four representative bile acid guests, i.e., cholate (CA), deoxycholate (DCA), glycocholate (GCA) and taurocholate (TCA), were determined at 25 C in aqueous phosphate buffer solution (pH 7.20) by means of isothermal titration microcalorimetry (ITC). The stoichiometry of resulting inclusion complexes between CDs and bile acids was demonstrated by UV and conductivity as well as ITC experiments, showing 1:1 binding model upon all inclusion complexation except for metal-mediated dimer 5. The complex stability constants for modified b-CD 2-4 are dramatically magnified with the extended length of amino tether. As compared with 3 and 4, copper(II) complexes 5 and 6 significantly enhance not only binding ability but also molecular selectivity toward bile guest molecule CA through multipoint recognition, but decreased complexes stability toward TCA could be attributed to the decreased hydrophobic microenvironment of CDs cavity due to the introduction of copper(II) coordination center. Thermodynamically, the resulting complexes between hosts and bile guests are driven absolutely by enthalpy, accompanied by entropy gain or loss. Using the present data and those previously reported for mono(6-amino-6-deoxy)-b-CD (2), thermodynamic behavior and enhanced molecular selectivity could be discussed from the viewpoint of hydrophobic interactions, electrostatic cooperation and van der Waals between the hosts and guests.

A thermodynamic study of ketoreductase-catalyzed reactions 3. Reduction of 1-phenyl-1-alkanones in non-aqueous solvents

The equilibrium constants K for the reactions (1-phenyl-1-alkanone + 2-propanol = 1-phenyl-1-alkanol + acetone) in the solvents n-pentane and n-hexane have been determined by using gas chromatography at the temperature 298.15 K. The 1-phenyl-1-alkaones included in this study were: 1-phenyl-1-ethanone, 1-phenyl-1-propanone, 1-phenyl-1-butanone, 1-phenyl-1-pentanone, 1-phenyl-1-hexanone, and 1-phenyl-1-heptanone. The equilibrium constants for the reaction involving 1-phenyl-1-ethanone were measured in the solvent n-hexane as a function of temperature (288 K to 308 K). The calculated thermodynamic quantities for the 1-phenyl-1-ethanone reaction at T = 298.15 K are: K = 0.2177 ± 0.0018; the standard molar Gibbs free energy change, Δ r G m ∘ = ( 3.78 ± 0.02 ) kJ · mol - 1 , the standard molar enthalpy change, Δ r H m ∘ = ( 4.53 ± 0.87 ) kJ · mol - 1 , and the standard molar entropy change, Δ r S m ∘ = ( 2.5 ± 2.9 ) J · K - 1 · mol - 1 . The equilibrium constants of 1-phenyl-1-alkanone with an odd number of carbons in alkyl side chain are higher than the equilibrium constants of the preceding 1-phenyl-1-alkanone having an even number of carbons in the side chain and follow a zig-zag pattern with increasing carbon number in the alkyl side chain.